An Introduction to the Design of Multihull Sailing Craft By R o b e r t B. H a r r i s '

Interest in multihull sailing craft has been steadily increasing since the end of World W a r II. Early postwar development was centered mostly around day sailers and racers, but during the past ten years considerable numbers of cruising multihulls have been built-mostly by amateurs. Many of the designs also were prepared by amateurs, resulting in numerous failures both in structure and performance. On the other hand, there have been so many successful, long-distance ocean passages, in both trimaran and catamaran, that these craft deserve closer examination by professional designers. This paper is a review of current practices in the design, construction, performance, and habitability, of all types of multihull sailing craft, including several comparisons with monohull craft.

Introduction

T~E CKrA~,~AaAN, trimaran, and proa designs, collectively called multihull craft [1 ], 2 are basically unballasted sailing craft stabilized by an equal division of their displacement; b y two widely spaced hulls for the catamaran, by two floats and a main hull for the trimaran, and b y a single float and a main hull for the proa. Although the question often arises, "Which is best: trimaran, catamaran, or pr()a? ~', no general conclusion is intended or implied. Advantages and disadvantages frequently overlap. If this paper serves as a basis for making the proper choice of type and size of multihull to fulfill specific service requirements, it will have further succeeded in its purpose. Unlike Eskimo kayaks, which have a certain amount of b e a m making them stable enough to paddle, the hollowed-out logs of the Pacific Islanders are comparatively unstable and cannot even be paddled, let alone sailed, without some form of stabilization. T h a t ancient. sailing versions of these outriggers and catamarans were extremely fast, as reported by Pacific sailing ship masters, was probably taken as a m a t t e r of course or considered a delightful and useful complement, but to those responsible for the revival of multihull sailing craft, speed was Naval Architect, Great Neck, N. Y. 2 Numbers in brackets designate References at end of paper. Presented at the Small Boat Symposium, New York Metropolitan Section, February 1968, of T ~ Socim, y oF Naw~L A e e m TI~CTS AND MARINE ENGINEEI~S.

Fig. 1

OCTOBER 1968

paramount. However, the sophistication of the hull forms developed in outrigger craft for the exploration and settlement of' the Pacific Islands was discovered and adapted to modern multihu]I design. The Malibu Outrigger, a name given to a popular class of sailing proas centered on the West Coast of the United States, strongly resembles the Microuesian canoe except that on one tack the float is to windward and on the other to leeward, whereas in the former it was always to windward. Hull shapes vary little from the earlier versions. The first Western experiments in mull\hull sailing yachts useful for modern designers were conducted by Nathanial Herreshoff [2] between 1876 and 1879. The lines illustrated in Fig. 1, typical of' the c a t a m a r a n hull Herreshoff built i.e., long and narrow, rounded midsection, and double-ended--are remark~bly similar to those of the 18-It, single-handed catamaran, which was recommended to the International Y a c h t Racing Union as the prototype of a new official International c a t a m a r a n class [3], Fig. 2. Turner's paper, presented to the Hawaiian Section of the Society in September 1955, serves as an excellent s u m m a r y of c a t a m a r a n development after World W a r I f [4]. Hull Forms

NO~rE: Unless otherwise noted, comments in this section are applicable to all mull\hulls, whether the main hull or float of a trimaran or proa, or the hulls of a

Hull lines typical o f N. G. Herreshoff catamarans

347

Fig. 2 A Cat II winner of IYRU Selection Trials, 1967

catamaran. S y m m e t r y and a s y m m e t r y refer to equal or unequal division, respectively, of the volume of a hull or float about its theoretical centerline plane. Displacement length ratio A/(L/100)3 refers to the total displacement and the length of the craft, unless specifically noted. The displacement/length ratio for one hull of a catamaran may be determined by dividing the number by two.

Two diverse sets of experiments in sailing multihull development were begun simultaneously in early 1947 by two separate teams of designers and builders at points halfway around the world from each other. The results ereated two schools of thought regarding multihull forms. In Honolulu, Woodbridge Brown was working with asymmetric hulls without centerboards. His experiments cuhninated in the successful daysailer, Ma~u Kai, a 40ft catamaran marconi-rigged sloop used as a pleasure craft to carry passengers short distances offshore from Waikiki Beach. Meanwhile, Roland and Francis Prout's efforts in England were concentrated on a series of small, day-racing catamaran sloops with symmetric hulls and eenterboards. Their boat, the 16-ft Shearwater, is a development class with a world-wide membership of over eight thousand owners. Several modifications of symmetric and asymmetric hull forms were subsequently tried in various experiments, m a n y of which are continuing as of this writing. While individual claims have been made regarding certain advantages held by one or the other of these modifications, no reliable full-size or model test series has been conducted to substantiate any such claims. Many theories abound, and the results of various mixed multi348

hull races seem to point toward definite conclusions-though at no time has there been sufficient qualitative data collected to support these conclusions. Furthermore, none of the model tank tests t h a t have been eonducted to date on multihulls have gone beyond the investigation of upright resistance in smooth water. While the information from such tests is of some value for downwind performance, it is useless for determining sailing multihull speed made good to windward. Also, little testing has been done at speed/length ratios over 2.8. Daysailers and Racers

Examination of day-racing results reveal only minor variations in hull forms. Any eoneIusive evaluation of comparison would have to be broadly expanded in a standard model test series based on a parent model. Sufficient money for controlled tests has not been made available because there has been no time for supporting interest in multihull racing to grow. Thus, until that interest develops, progress will depend largely on trial and error. For example, the day racers which have won the most races have the following similarities in hull form, including Wildwind, which is considered the fastest sailing craft in the world today, Fig. 3 : 1 Fine entry with included angles from 10 to 14 deg. 2 Semicircular below-water sections with maximum area positioned slightly forward of amidships. 3 B e a m / d r a f t ratios of approximately 2.0 (minimum wetted area sections). MARINE TECHNOLOGY

4 Gently swept up buttocks, usually terminating at the light waterline plane. 5 Transom sterns slightly immersed at full static displacement. 6 Twin centerboards and rudders. Both of the new catamaran prototypes, selected in the 1967 trials at Sheppey, England by the International Yacht Racing Union for promotion to an official International class and Olympic status, conformed to all of the points, the exception being that the smaller, one-man cat was double-ended (sharp waterplane entry and exit), Fig. 2, while the larger, two-man eat had transom sterns. Rigs, displacements, and helmsmanship varied widely in the trials, and selection was based on the individual performanees of the craft exhibited under the existing conditions. To put it more technically, the best-performing day racers operate at displacement/length ratios in the region of 18 to 25, at overall length/extreme beam ratios of from 1.39 to 2.25 for catamarans, hull and float waterline length/beam ratios of 13.5 and 18.0 respectively for trimarans, and from 15 to 18 for catamarans, with beam/draft ratios averaging 2.00. At such ratios the largest portion of the resistance is frictional throughout the speed range, and with hull spacings equal to or greater than half the length of the waterline, it has been shown that there will be no wave reinforcement between the hulls [5]. Completely symmetric, minimum wetted area forms seem to be the fastest for the day racers operating at extremely low displacement/length ratios, where the majority of the total resistance is skin friction. However, it is possible that if each section of such a form were aligned along longitudinal planes, for instance, so as to eause all of the longitudinal curvature to be inboard or outboard, some reduction of residual resistance might be obtained. However, in view of the magnitude of hull separations and other considerations set forth in the sueeeeding paragraphs, it does not seem likely that such variations on the very light displacements attainable in day racers willprove beneficial to higher speed. In a few instances diversions have been chosen to simplify construction and reduee first eosts; for example, the use of sheet material versus formed pieces. But, the performances of such craft were so far less satisfactory that few day sailers are so compromised any more. Hull sections other than semicircular have been tried with the idea of increasing the average speed around a triangular course, such as planing forms. Attempts at planing multihull sailing forms for this purpose have so far been unsuccessful for any one, or all, of the following reasons: :

4 Adverse variations of bottom loadings due to shift in displacement from one hull to the other when heeling, in addition to change in angle of attack. 5 Reductions in maneuverability. 6 Loss of speed due to slamming in a head sea oeeasioned by full forward sections of low deadrise desired for planing at low speed.

1 Lack of sustained wind to maintain planing speeds. 2 Ineffieieney of planing forms except at planing speeds, particularly true when going to windward. 3 Difficulty of maintaining ideal trim angles due to variation of wind forees acting longitudinally through the center of effort of the sails.

As can be seen from :Fig. 2, day-racing catamarans on the wind often sail with the weather hull flying; if the hulls are designed for planing on one while the other is flying, then the proportions will be incorrect for sailing on both hulls, and vice versa. While the effieieney of planing catamarans may be improved, and greater sustained sail

OCTOBER 1968

Fig. 3

~Vildwind

349

t

U Fig. 4

T y p i c a l lines of a m o d e r n , d a y - r a c i n g catamaran

power developed from better rigs, it seems unlikely that the all-around performance will match that of the types shown in Figs. 2 and 3, both of which have underbodies simih~r to the lines in Fig. 4. As a matter of fact, the 2,5ft sloop Beverl~t, designed in the author's office and winner of the 1963 One-of-a-Kind race, still holds the highest recorded average speed/length ratio around a triangular course of 2.260 [6]. Her best average speed was 10.4 knots, but her top speed was unofficially stated as 22.5 knots in trials. The lines shown are typical of the world's most popular day sailing and racing classes. Considerable controversy remains over the treatment of the sterns, whether they should be transom or canoe type, but from performances to date there appears to be little to choose between them. Spray steps are often seen incorporated into the lines as shown in Fig. 5 on the 52-ft Stra,tger. T h e y might better be referred to as displacement steps, because their real purpose is to give sharp increase in reserve buoyancy forward. This is done to maintain a finer entry at low and moderate speeds, while tending to counteract the tendency in such fine-bowed craft to bury the lee low in heavy winds and higher speeds. As such they are effective, but also respond from dynamic lift to reduce pitching, as well as reducing side wetting and spray. Possible advantages in asymmetric configurations are the elimination of centerboards and daggers needed in most symmetric arrangements for lateral plane. This would be true only for day sailers, where speed is not the primary object. Savings in cost and weight are thus obtained, and the problem of beach objects jamming a centerboard are eliminated--an important consideration for those sailing off stony beaches. Since the majority of the hulls on day racers and sailers, be the)" catamaran, trimaran, or pros., are connected by 350

tubing or other open beams stiffened by wire trusses, and because the separations are so large in comparison to the beam of the individual forms, no particular advantage is gained in asymmetry by flaming the insides of the hulls to reduce the connecting beam lengths. Furthermore, doing so adds weight, a most critical area in day-racing design. Proponents of asymmetry a ()nee claimed that substantial windward lift was gained from heeling a catamaran, so that an asymmetric hull on the heeled side would act like a lifting foil, while the windward hull would be nearly out of water, causing little opposition. This was found to have four disadvantages, resulting in configurations which were less maneuverable, less safe, and slower than symmetric arrangements namely: 1 The low aspect ratio of the foil attained in this manher produces excessive drag. 2 Narrower, deeper, and chined hull sections associated with asymmetric forms contain more wetted area which is permanently fixed. 3 If intentional heeling of a catamaran is specified, as is necessary for proper action of aysmmetric forms, the hull spacing will have to be reduced to reduce the stability, and this increases the drag due to reinforced a William R. MehMTey, a consulting engineer from Chicago, II1., experimented in ~958 with a 30-ft catamaran sloop with asymmetric hulls, in which, at the predetermined angle of heel of 10 to 15 deg in 20-mph winds, approximately 1400 lb of lift, to windward was attained from tile lee hull, at which point the hull speed was approximately 12 mph. Her displacement/length ratio was approximately 44 and Iengt,h/extreme beam ratio 2.5. Since this experiment, length/extreme beam ratios for catamarans of this size and displacement have decreased to about 1.85, with proportionately higher initial stability. Racing results have shown that the asymmetric-hulled catamarans can point higher, but have so much more total drag that their speed made good to windward symmetric, round-bottom centerboarders.

is lower than

MARINE TECHNOLOGY

Fig. 5 Stranger reveals much in hull form of modern, cruising, sailing c a t a m a r a n

w a v e f o r m a t i o n s b e t w e e n t h e hulls. W a t e r pile-up is increased because the greatest longitudinal curvature must be on t h e inside of t h e hulls. 4 T o reduce t h e b e a m is to r e d u c e s t a b i l i t y , safety, a n d sail drive.

G= ~ 117..-I"~

Offshore Cruisers and Ocean Racers 4

T o a far g r e a t e r e x t e n t t h a n monohulls, t h e shapes of i n d i v i d u a l c a t a m a r a n , t r i m a r a n , a n d p r o a hulls a n d floats are i n d e p e n d e n t of t r a n s v e r s e s t a b i l i t y a n d s u b j e c t to less c h a n g e of flow b e c a u s e of r e l a t i v e l y low a v e r a g e angles of heel. A n infinite v a r i e t y of hull sections is p o s s i b l e which c a n be as b e a m y a n d shallow, or deep a n d fine, as t h e d e s i g n e r m a y select. T y p i c a l sections d r a w n to t h e s a m e scale of e q u a l a r e a w i t h g i r t h s p r o p o r t i o n a l to t h a t of t h e s e m i c i r c u l a r s e c t i o n are s h o w n in Fig. 6. W h i l e m o s t designers agree on t h e m i n i m u m - w e t t e d a r e a f o r m as b e i n g b e s t for d a y sailers a n d racers, opinions v a r y as to w h a t is b e s t for t h e offshore cruisers a n d ocean racers. F i r s t of all, a d i s t i n c t i o n m u s t be m a d e b e t w e e n d a y r a c e r s a n d c r u i s i n g m u l t i h u l l s on t h e basis of t h e w a y t h e y a r e sailed. T h e former, o p e r a t i n g o v e r closed, p r o r e t t e d courses, o f t e n sail on one hull w i t h a d v a n t a g e , Fig. 2. Since t h e erew w e i g h t m a y r u n as high as 50 p e r c e n t of 4 Cruising multihulls may be defined as having accommodation for sleeping, eating, and in general for living aboard for short or extended periods. Some cruisers will be lightly equipped like ocean racers and others more heavily, tending to create broad differences in displacement/length ratios and Froude numbers. Hull forms vary considerably more in cruisers than in day sailers and racers because considerable differences of opinion exist on what the best form is to satisfy these variations. OCTOBER 1968

G

(o. "Z~,

G = (o.4-0 8/H= .44

G[= (o.5"7

~= ~Jt4 •

(0.88 I •6o

c~.= -/. 14 BI~ = 5 . 5 " /

Fig. 6 T y p i c a l s e c t i o n s o f e q u a l area 351

the total displacement, this attitude m a y be controlled by rapidly shifting crew weight and adjusting the sails. Failure to so so m a y mean capsizing, but assist~,,nce under these conditions is usually nearby. On the other hand, cruising boats must be designed for, and svdl in, the oceans -beyond the areas of timely assistance. Also, it is too fatiguing for the crew to remain in a ballasting position for long periods of time, and it is not practical for the watch to be constantly adjusting sail. Great care is taken, even on ocean racers, to avoid lifting a h u l l - especially in heavy seas, where the risk of capsizing is greater. Therefore, in d a y racers, where hull characteristics m a y be determined on the basis of one hull supporting from 60 to 100 percent of the total displacement, cruising catanmrans will be proportioned more nearly on a 50-50 basis. In the trimaran, where the floats are acting as stabilizers and the main hull supports from 90 to 100 percent of the displacement, individual hull forms will be determined according to average sMling attitudes, which in turn are a function of hull spacing, weight, length, sail area, and so forth. In any case, as compv~red with day racers--with the possible exception of trimaran and proa floats displacement/length ratios will be higher. B y the addition of cruising boat equipment, Froude numbers will be lower, freeboards increased for safety and comfort, and hull forms varied for reasons of seakeeping, dr'fit, speed, and internM arrangement. Perh'~ps to a greater extent on catamarans than on trim~mms, a distinction between offshore cruisers and ocean racers nmst also be made when discussing hull forms, because of the substantial differences in displacem e n t / l e n g l h r'~tios. (The same differences apply to monohulls, except t h a t they are smaller on account of the addition or subtraction of ballast, which in turn is compromised by rating rules and adjustments in sail area.) The same distinction m a y be made between light and het~vy cruisers. I t arises because the displacement/length ratio of a h e a v y cruiser m a y be twice t h a t of a light cruiser. Because light cruisers m a y be equipped with one or two outboard motors, a small light auxiliary generator, no mechanical refrigeration, the very minimum of fuel and water, untrimmed plywood joiner work of the lightest possible scantling, few lockers, and the lightest of transistorized solid-state electronic equipment, the total displacement is low enough to substantially reduce the mass m o m e n t of inertia, with subsequent reduction of hull scantlings. On the other extreme, heavy c a t a m a r a n cruisers of the same length m a y be equipped with twin diesels, a diesel auxiliary generator, enough fuel for a 1000-mile cruise under power, enough water to supply four heads with showers and the galley for an extended period, deep-freeze and chill boxes, air conditioning and heating, hi-fi, radar, loran, and innumerable other luxuries not found on the ocean racer. On ocean racers the designer's primary objective is to produce a hull t h a t will permit the yacht to get from point A to point B in the least amount of time. This will be true for whatever displacement/length ratio he m a y have to work to, since most handicap rules are set up to 352

take weight differences into account. On heavy cruisers where there is no intention to race, the owner would still like his yacht to be as fast as possible. However, he will often allow serious compromises in speed to achieve some particular advantage. He m a y want very shallow draft, a high proportion of auxiliary power, and perhaps complete protection for propellers and shafts with skegs and balanced rudders, all of which tend to add to the resistance. I n any event, the best, designers will struggle to produce the form of least resistance and most efficiency to perform the service intended, but it is in the ocean racer t h a t the greatest attention is given to hull forms in terms of speed. The addition of accommodations immediately raises the displacement/length ratio, calling for more waterline beam and more draft, which results in proportionately higher wetted area attd righting moment. Since stability is a function of weight times the distance between the center of" buoyancy of one of a c a t a m a r a n ' s hulls or a trimaran's floats attd the center of gravity of the yacht, plus the added righting m o m e n t of the center hull of a trimaran created in the usual manner when heeled, Fig. 11, it is obvious that the hull spacings m a y be reduced. As a practical matter, in most cases hull separations of cruising multihulls are insufficient to avoid wave interaction. Concurrently, the displacement/length ratios have increased to the point where the residual resistance accounts for a substantially larger portion of' the total resistance than on extremely light day racers. These conditions give rise to three important basic decisions in terms of huli forms. The first concerns lateral platte; the second, wave interference between the hulls; and the third (closely related with the second), the individual hull or float characteristics. I t is between these factors that most of the controversy has arisen among the leading designers over the most effective hull forms for ocean racers. I f there is insufficient evidence on which to describe the ideal racing form, there is even less for the ocean racer, because so m a n y more variables can affect the outcome of long-distance, ocean-crossing races. However, the following general observations m a y shed some light on this simulated approach to multihull design. While the symmetric, m i n i m u m wetted area, fullrounded sections with centerboards have proved the most efficient for the day racer, there is some evidence t h a t the same m a y not be true for the ocean racer. I t is simply t h a t the full-rounded sections do not provide adequate deadrise in the faster ocean racers to permit the maintenance of high speed in the seas, which are normally created b y winds in which those speeds m a y be obtained. The pounding t h a t results from sections of too low deadrise can be destructive to the hull and rigging and can produce severe crew discomfort, preventing sleep and making footing difficult, if not dangerous. I n the d a y racer, any amount of discomfort can be tolerated for short durations of time, but crew efficiency would soon dissipate in the face of prolonged discomfort in the cruiser. M A R I N E TECHNOLOGY'

Although displacement/length ratios of ocean racers ~re higher than those of the day racers, they are still considerably lower than those of ballasted monohulls. At the same time, freeboards are proportionately higher and drafts proportionately less than monohulls. The net result is that to have sutficient lateral plane, very l~rge centerboards must be employed a n d / o r what lateral plane the hulls offer must be shaped to produce the most resistance to side motion as is possible at the lowest resistance. Large centerboards have the disadvantages of adding substantial weight, higher original cost, and difficult and costly maintenance, while often interfering with the living quarters of many mediumsized craft. On the other hand, use of low-aspect-ratio fixed fins or chine shapes in the hulls in association with hull asymmetry can induce more resistance and offer greater resistance to turning. The latter point in ocean races is, however, not as serious as it would be in day racing, where much more maneuvering is required, and where at sea directional stability becomes more significant, especially in carrying spinnakers, running, and reaching. Because the wetted area of such combinations is greater at all times than on the minimum-wetted-area forms, fixed fin, deep keel, chined a n d / o r asymmetric forms will be slower in light air and smooth sea. The reader must bear in mind that the argument in process herewith is confined to ocean racers. As already mentioned, radical compromises are often made in cruiser hull forms for special services which could minimize or even negate higher speed-producing factors. In reference [7], Choy argues strongly in favor of ~symmetric catamaran hulls with the maximum curvature inboard and hard chine outboard and, for all practical purposes, a canoe or double-ended stern, similar to the lines shown in Fig. 7(a), as being the best solution to the problems stated for hull forms of ocean racers, tie notes the following advantages of the configurations: 1 No centerboards are required for best speed made good to windward in wind speeds over 8 knots. 2 Less wavemaking resistance because of finer sections, entries, and exits. 3 Reduction in leeway due to wave action on the bows--less lateral drift. 4 Less induced drag by means of' reduction in lateral wave formations. 5 More seakindliness and seaworthiness because the prescribed hulls ease through the wave formations with minimum fuss. Bearing in mind that ocean-racing catamarans sail between 85 and 90 percent of' the time with the displacement evenly distributed between the hulls, and during the remaining time with the leeward hull rarely bearing more than 55 or 60 percent of the total displacement, it might appear thai, there is no reason why the hulls could not be reversed, thus putting the maximum longitudinal curvature outboard, and the chine inboard. Since for all practical purposes catamarans are being sailed in an essentially upright position, it might, as a matter of fact, OCTOBER 1968

[

®

Fig. 7

Catamaran midsection

reduce the resistance. Recent tank tests on the upright. resistance of catamaran forms have shown reductions in resistance where the curvature has been outboard and the inboard sides relatively straight [8], and where wave interaction was present. On the other hand, in view of the fact that the effect of symmetry versus asymmetry m a y constitute such a minor effect by comparison to sail area, weight, and length, it may not m a t t e r if the chine is oi1 the centerline, as Meyers [9] used in his mathematical approach to the development of catamaran hull forms, Fig. 7(b). Choy's forms have the advantage, however, of placing the hull's center of buoyancy as far outboard as possible in relation to overall beam, which helps in keeping deck areas and weight to a minimum while attaining maximmn stability. Also, by having most of the sectional slope inboard, the span of transverse beams connecting the hulls is kept to a minimum. In both cases the chines contribute to lateral resistance and permit the use of smaller centerboards for racing to windward, by comparison with the semicircular hull section forms. (On recent 12-meter design and several other modern ocean-racing monohulls, leeway angles have been reduced by veeing the keel bottoms to a sharp edge at the centerline, indicating a total advantage in speed made good to windward in spite of the slightly higher induced drag.) Glass Slipper [10], the winner on corrected time of the 1966 Transpacific Race from Los Angeles to Hawaii is a~ 50-ft-tength-overall, canoe-stern catamaran sloop, 40 ft6 in. on the water, 20 ft extreme beam with midsection similar to Fig. 7(a), hull LWL/B ratio of 6.67 and ~ hull 353

hull is basically bearing the total displacement and the floats are acting as stabilizers. As argued b y Piver, it is necessary under these conditions to employ deep-vee float. midsections of approximately 65 deg of included angle. This will avoid an unbearable snap-roll, which can become so violent t h a t the course m u s t be altered. Piver also claims t h a t the deep-vee float form supplies sufficient lateral plane to m a k e the use of centerboards unnecessary. An additional benefit is claimed by reason of the floats acting as low-aspect-ratio hydrofoils [13]. Of the two leading trimarans to complete the 1966 Round Britain Race [12], the ~t2-ft Toria had semicircular sections on main hull and floats and the 40-ft Vietress had a v e e - b o t t o m main hull and deep-vee float sections. (Both had centerboards; the Toria having two daggers, one in each float.) Toria placed fourth on corrected time after three catamarans, and Victress placed fifth. Piver, designer of Victress, claims t h a t Startled Fawn, a much more recent example of his work (and a near copy of Stilletto, in which he crossed the Atlantic in 1967 to race in England), which placed sixth, could have easily beaten the older Victress with better handling. Therefore, her characteristics, along with Toria's, are presented in the following table as typical of the t r i m a r a n ocean-racing types. Toria was designed by Derek Kelsall, who, like Piver, has twice crossed the Atlantic in trimarans, Fig. 8.

Fig. 8 All three hulls of Toria, elapsed-time winner of the 1966 R o u n d Britain Race

b e a m / d r a f t ratio of 1.20. The displacement/length ratio of each hull, consistent with the ratios just given is 56. By contrast, Iroquois [11], corrected-time winner of the 1965 Round Britain Race [12], is a 30-ft-length-overall, transom-stern sloop with ovular midsection, 26 ft-3 in. L W L , 13 ft-6 in. extreme beam, L W L / B ratio of 7.66, and B / H draft, ratio of 3.72. Her displacement/length ratio is 62. Neither of the rules under which these two catamarans raced factored wetted area, waterline beam, absence or presence of centerboards or propellers. When a trimaran is heeled and sailing with a float in the water, she is, insofar as water flow is concerned, v e r y similar to a c a t a m a r a n with hulls of unequal length and displacement. The same m a y be said of a proa, but the overall action of the trimaran in a sea is markedly different, which can account for substantial differences in hull forms. However, most of the remarks concerning catam a r a n hull forms covered in this section apply to trimarans, with the following exceptions. Because there is an even distribution of displacement between the hulls on the catamaran, there is m u c h less tendency utlder certain sea conditions for t h e m to break clean of the surface t h a n in the trimaran, where the center 354

Startled Fawn 32.0 (approx) 3.4 20.0 5.33 1.63 3.50 3000 1.392

Characteristics LI,VL, ft . . . . . . . . . . . . . . . . . . . . . . LWL beam (main hull), ft . . . . . . Extreme beam, ft . . . . . . . . . . . . . . . Beam (main hull), ft . . . . . . . . . . . . Draft (main hull), ft . . . . . . . . . . . . Float beam, ft . . . . . . . . . . . . . . . . . . Displacement, lb . . . . . . . . . . . . . . . Float beam LWL, ft . . . . . . . . . . . . . .

Toria 40 (approx) about 3.5 22.4 4.0 1.83

Displ/length

46.7'

41

1.90

2.06

ratio .............

B/H . . . . . . . . . . . . . . . . . . . . . . . . . .

6000"

Toria and Startled Fawn also represent two other schools of thought concerning the vertical position of the floats. Since the designers of both boats have had extensive offshore sailing, it is interesting to note t h a t Toria's floats were several inches above the water at rest, while Startled Fawn's were several inches immersed. There is insufficient evidence to point to the most desirable position for best speed or comfort. With floats in the high position the trimaran immediately heels more, thus losing drive from the sails and adding windage. However, it has the advantage of reducing wetted area b y getting the windward float out of water sooner and h a y ing it remain higher over the water and less subject to wetting. The opposite would be true of a trimaran with floats in the lower position. To conclude this section, a wide v a r i e t y of hull forms will continue to appear. B y comparison with monohull yacht design, multihull development is still in its infancy. With an ever-increasing degree of competent M A R I N E TECHNOLOGY

research, there is room for substantial advances in all three types of multihulls. The 1968 single-handed Transatlantic race will see a great variety of single, double, and triple-hulled yachts, including all multihull types, a 65-ft. trimaran and a 40-ft proa. Although the race will be of little value for comparing any one performance characteristic, it will help in forming an overall opinion of lnultihull versus monohull as ocean-crossing, single-handed sailing craft. Since this seems a most unsuitable and unfriendly way to cross an ocean, it in itself is of doubtful value. Sails and Rigging

Because of the higher potential speeds attainable, particularly in the day-racing (:lasses of both catamarans and trimarans, combined with higher stability and comparatively lower angles of heel than monohulls, considerable sail and rig development is now in progress, mostly by amateurs in Great Britain, Western Europe, Scandinavia, Canada, Australia, and the United States. Since a catamaran won "Yachting" magazine's One-of-a-I(ind Race [14] in 1959, a rash of experiments has been carried on, which have increased in the past, five years. M u c h of this new work has been recorded in the publications of the A m a t e u r Yacht Research Society by its editor and founder, John Morwood [15]. Major problems in multihull sail and rigging plans are similar to those of monohulls, but are compounded by a speed range more than double that of a 12-meter yacht. Present rating rules place few restrictions on m a s t and sail combinations even for some ocean racers and permit everything from conventional soft sails and fixed rigs, full batten sails in combination with rotating masts, to completely solid wings. Inspired by sailing iceboat rigs, the quest for greater speed under sail has brought forth m a n y unusual rig combinations. With the stability t h a t multihulls offer but with much greater resistance t h a n iceboats, particular attention has been given to developing higher sail lift/drag ratios using wing masts. To date these have taken the form of symmetrical airfoils which are stayed from a single point. This permits them to rotate up to about 60 or 70 deg off the centerline, so t h a t when set at the proper angle of attack they closely resemble the wing of a plane. Wing masts have now grown ~;o such a large percentage of the total sail area t h a t the remaining area t h a t is sail essentially performs the function of a flap in a wing. The wind masts have been built mostly of thin plywood, fiberglass-sheathed to protect and strengthen the extreme fibers, and braced and sparred internally, similar to airplane wing construction. The stays put large compression loads on the masts, however, and when strong enough to resist buckling with suitable factors of safety are strong enough to resist any other dynamic loads. The rotation of the masts is sometimes automatically performed by wind forces in the sail, but assisted in light air by use of a tiller or block and tackle at the base of the mas~s. Further details of these spars OCTOBER 1968

and their scientific development are presented later herein. Most of the new rigs resulting from such experiments are confined to day racers, but as time passes and more experience is gained in handling these rigs, particularly as means are found to control t h e m under severe wind and sea conditions, more of the innovations will be used oil ocean racers and offshore cruisers. Wide sheeting bases and high initial stability of multihull craft have caused considerable changes in thought regarding sheeting arrangements, workingof headsails and sphmakers, sail construction and weights, and m a s t staying. For example, the sheeting base, particularly on trimarans, is quite often wide enough to set a spinnaker without the aid of a pole. I n the future, spinnakers cut especially to take advantage of this condition could completely eliminate the necessity for a pole and greatly facilitate their handling, particularly when jibbing. In the smaller jib-headed cruisers, chainplates for a t t a c h m e n t of the shrouds to the hull are often positioned well inboard in order to keep the athwartship distance between them in the same proportion to the base of the fore triangle as is common to similarly rigged monohulls. Thus, overlapping headsails (genoas) can be sheeted near enough to the centerline of the ship for best closehauled trim to go to windward. As yacht size increases it becomes desirable to locate the chainplates on the outsides of the hulls to create more favorable staying angles from the mast, reduce compression loads, and permit smaller, lighter masts ~md supporting beams. With extreme beams ordinarily double, and righting moments five to eight times greater than most monohulls, it becomes important to mhfimize the strains wherever possible. Due to the c a t a m a r a n ' s high initial stability, standing and running rigging, sail weights, and spars have to be increased over those of monohulls of equal length in proportion to the righting moment. Unfortunately, it is the larger, heavier nmRihulI cruisers whose wetted surfaces are proportionately larger t h a n monohulls t h a t could use the added drive of large overlapping headsails going to windward, Fig. 9. A-frame bowsprits and bumpkins are being attempted on catamarans to get added sail area, and on trimar~ms the same as is done on large single-hull yachts, but lengthening the hulls is preferred. I t is a common struggle with monohulls and multihulls alike to keep displacement/length ratios low. However, the consequences in multihulls of not doing so ~re more severe, where widening the hulls increases the resistance as twice the square of the beam, and further resistance is invited from reinforced wave drag between the hulls. If the hulls are spaced further apart, to reduce interference, the weight of the beams connecting the hulls goes up as the fourth power, and if the hulls are deepened, too nmch wetted area is added. So, on "cats" and "tris" where there is ample stability, it is more beneficial in all airs to add length to get more sail area. Generally, owners will settle for bowsprits and bumpkins because it is cheaper t h a n adding length to the hulls, but it is better to avoid them by keeping the yachts 355

Fig. 9 52-ft Stranger

light. A good rule is to keep a generous third of the total length of a hull free of any accommodations or stowage. Rigging problems are less severe in trimarans than in catamarans. One is able to see from Fig. 11 that the catamaran is at its maximum stability when upright whereas the trimaran heels somewhat before reaching maximum stability. With whatever stability there is in the main hull and the gradual depression of the lee float, both acting to absorb the shock from a sudden blast of wind in the sails, less initial load is imposed on the mast and rigging, and some wind is spilled due to heeling. Owing to the presence of a main hull with more width than either hull of a catamaran (Fig. 6), it is often convenient to bolt ehainplates for the transverse rigging through the sides and framing, a position inboard enough to allow setting overlapping headsails. Cost and weight of construction is less in the trimaran, again because the main hull is there to support the mast. Masts of larger trimarans may be stepped down onto the keel of the main hull for better columnar end restriction. On the smaller, lighter "tris," masts are stepped on deck as in smaller monohulls to reduce interference with the arrangement below and to lessen the chance of damage to the mast and hull in ease of rigging failure. The most prominent evidence of advanced rigs taking 356

advantage of the stability and speed of which multihulls are capable m a y be found in International Yacht Racing Union's g00 and 500-sq ft sail area, C and D class catamarans. Capable of speed/length ratios of 5 and better, these craft are currently raced with full-length batten sails and rotating "wing" masts, whose surface area divided by 2 may be as high as 50 percent of the total actual sail area. (In size and proportion the wing masts bear little resemblance to those of iceboats, with which they are often compared, because the fastest multihulls operate at ratios of boat/wind speed of 1.20 to 1.25, while iceboat ratios m a y be three to four times higher.) Wing theory predicts that solid-wing symmetrical airfoils with large trailing-edge flaps will produce the highest lift to the least drag. Designers are moving rapidly in this direction, though they have been hindered by lack of time, money, and testing facilities. There appears to be a problem of weight, however. Present 60-50 rigs m a y be put up at about ~ psf of allowable sail area in the g00-sq ft size. Preliminary investigations show that only with highly expensive aircraft construction will the same weight per square foot be maintained in a 100-percent solid-wing sail. Hope of weight reductions may be expressed by saying that it does not m a t t e r what shape or material is used, as long as it provides the same or more lift than the eompetitor's rig. The extreme length/beam ratios of day-racing multihulls--vim, up to 1.78--render decks large enough on which to mount a variety of wing shapes with complete closure at the deck if it becomes desirable. There is some evidence that creating an end plate for the base of the wing in this manner will effectively inere se the aspect ratio, which will result in more lift with no increase in heeling moment. Racing sail and wing mast combinations in 1967 had aspect ratios of 4.3 (span squared divided by the area). Modern glider aircraft wings, designed also for maximum lift and minimum drag, have equivalent aspect ratios of 12.5. This is twice that recommended for optimum performance b y the Eiffel experiments on rectangular airfoils without thickness (like a regular sail and small mast combination), as noted by Morwood in reference [16]. Morwood suggests, however, t h a t the maximum coefficient of thrust over several points of sailing and varying wind speed can be achieved using the USATS 10 airfoil section based on a reaetangular profile with aspect ratio 6:1, which would give up to four times the thrust of a conventional Bermudian soft sail plan (mainsail and jib). I t is only because of the added stability of a day-racing catamaran over lightly ballasted or unballasted singlehull craft t h a t the use of wing masts and full batten sail rigs with aspect ratios of 4.3 are feasible. Unless the aspect ratios can be raised without adding to the heeling moment, it seems unlikely that more efficiency will be sought in this direction. Increased aspect ratios, however, are if anything more valuable in light airs, and means to increase them during light air and decrease in heavy air m a y be useful for optimum all-around performanee. M A R I N E TECHNOLOGY

One thing that the wide bases of trimarans and catamarans make possible are segments of circular sheet tracks for roller slides, which when connected to the boom with a r a n g maintain rigid vertical restraint in the sail. Wide beams permit maintaining prescribed shapes of sails in this manner over 90 percent of the courses sailed. Most of the high-speed rigs used on the day racers are unsuitable for cruisers. Ocean racers are beginning to use full batten sails and swiveling masts where rules permit, but for the heavier cruisers and private charter yachts the racing rigs which have just been reviewed lack flexibility required for safe handling at sea.

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Stability

The stability of multihull craft is a function of hull spacings, weight, and height of center of gravity, whieh is assumed to l i e on the eenterline of ship, except in small craft where shifting crew weight to weather is a large enough proportion of the total weight to be considered. The righting moment created by the disposition of these factors equals the heeling forces of the wind to create a state of equilibrium. Generally, any righting arm created by a shift of center of buoyancy in a catamaran's hulls or the center hull or floats of a trimaran is so small t h a t it is neglected. However, the beam of some trim a r a n hulls is large enough to consider adding it to the righting arm of the float.. Catamarans receive their stability under sail by reason of the division of displacement equally into the two hulls set well apart, with maximum stability at zero angle of heel. Theoretically, this indicates that because the stability decreases continuously with increased angle of heel, a steady wind force capable of raising the windward hull will capsize a catamaran unless additional righting moment, is applied or the heeling force is reduced by l u ~ n g , or sail is reduced, or course is changed. In small eats, luffing and adding righting moment may be done simultaneously. The crew hikes further out to windward to increase the right moment a n d / o r the sheet is slackened to reduce the effective sail area. In the large cats where there is often not enough crew weight to provide a sufficient additional righting moment, wind forces are reduced by running off downwind, accompanied by slacking the sheets. Again in smaller eats, by playing the sheet and shifting weight, the weather hull may be kept slightly oscillating at any height off the water short of the point of negative stability. I n the hands of a skilled cat sailor, capsize is unlikely. The crew would not have to hold the position any longer than he pleased, and if he should capsize it would usually be in an area where there was assistance nearby. On the other hand, since no such chance of capsize may be taken at sea, great care is taken not to raise the weather hull to the water's surface. Any sign of this is regarded as the point at which sail must be shortened if it is found that one must constantly run off or slack sheets in order to maintain the proper angle of heel. Course and sail settings will be determined by the strength and duration of the new wind. I n ocean racing, OCTOBER 1968

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The righting inoments given above for trinlaran and catamaran are for the static condition when heeled in smooth water. In order to obtain the maximum rigging loads, it must be imagined that one hull of the catamaran and one pontoon of the trimaran are supporting the entire weight of the craft on the crest of a wave and that the opposite hull of the catamaran and the main hull and pontoon of the trinlaran are in the space of a wave trough held in equilibrium by the wind forces in the sails. For the catamaran, the heeling force would be approximately 18 percent greater than the maximum static righting moment, and 14 percent greater in the trimaI'al-l.

Fig. 10 Typical stability curves of a catamaran, trimaran, and monohull

larger numbers of crew will be available for standby on deck, but, unless they are overzealous, inexperienced or just plain stunt seekers, they will not permit the hull to come out of water. The stability condition of a catamaran through several angles of heel is shown in Fig. 10. Fig. 11 compares maximum stability attitudes.

Weights of multihull craft of the same length may vary widely, just as they may in a monohull, depending upon type of construction and material, amount of fuel, water, power, auxiliaries and equipment. In monohulls of similar type and length, where it is assumed that beam and draft vary l i t t l e , the amount of ballast will be varied for stability. This will be supplemented by righting moment due to the normal weight of the craft coupled about the heeled center of buoyancy to maintain equilibrium with the sail force. I n the multihull, on the other hand, the hull spacings are varied to accomplish the same thing in lieu of the ballast. While some designers have used ballast from 10 to 15 percent of the total displacement, usually in fin keels of small light multihulls, the idea has generally been abandoned in favor of increased beams. A problem arises on larger multihulls, particularly catamarans which are more heavily loaded, where 357

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(a)

the hull spacing for sufficient stability m a y be so small as to cause serious wave interferences between the hulls, attended by large increases in resistance. Arbitrary increase of spacings to reduce interference causes a sharp increase in total weight, because the size of the connecting beams varies as the m a s t loads on the beams and the square of their lengths. A stand-off between the added resistance due to additional weight and t h a t saved b y reducing wave interferences is soon reached. At this point the craft is so stable t h a t m a s t and rigging sizes have increased to the point where they seem disproportionately large for the length of the yacht. If' they are arbitrarily reduced to keep their appearance in proportion to other craft of equal length, the possibility of having the rig go over the side or the sails blow out m a y occur in a sudden blast of wind. Square riggers, heavily laden and extremely stable with certain types of cargo, faced the same problem. I n h e a v y squalls, if the sails did not blow out, the hulls were strained, seams opened up, and m a n y foundered and sunk. More often than not with cotton sails, the sails let go first or spars and rigging failed, thus relieving the loads on the hulls and minimizing the danger of capsizing, but they had no auxiliary power and were left helpless until jury rigs could be made. Conditions of extreme stability were encountered in the design of the 72-ft catamaran, in conjunction with suitable hull spacing, which when fully loaded has righting m o m e n t of 1,500,000 ft-lb versus the 72-ft monohull with a righting m o m e n t of only 220,000 ft-lb, Fig. 18. Since it was felt t h a t dacron sails of weights suitable for normal craft use might not blow out even under extreme conditions, this m e a n t t h a t rigging or spars would have to fail before the m a x i m u m righting m o m e n t was realized, if their size was to be kept within reasonable limits. I n the end, both spars and rigging strengths were allowed to. fall from 10 to 15 percent below t h a t actually required to raise the weather hull, on the basis t h a t the crew would have had to reduce sail well before the wind reached the point of imposing parting loads on the rigging. T h e

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Comparison of heel angle and G Z for maximum stability of trimaran, catamaran, and monohull MARINE TECHNOLOGY"

maximum wind velocity required to raise the windward hull of the 72-footer with full sail is approximately 60 mph. It is assumed that structural strains in the yacht at potential hull speed in the heavy seas which such wind would raise could not be tolerated, nor could the crew stand the mental and physical strains such wild actions would produce. The trimaran [1] is basically a narrow monohull with float stabilization. Essentially the main hull supports the entire weight of the craft in a static condition. Those who favor designing trimarans with floats out of water at rest m a y do so with the possibility in mind of balancing both floats free of the surface in ghosting to light airs by shifting crew weight to keep the wetted surface to a minimum. ARernatively, keeping the windward float higher off the surface of the water reduces wetting and pounding. Generally, however, when the floats arc designed to be in the water at rest, they just pierce the surface, taking only 10 to 15 percent of the static displacement. Those who prefer the latter arrangement point out that at anchor, when someone is walking from side to side, there is sufficient float buoyaney acting immediately to prevent rolling, and that the sailing angle of heel is lower, thus reducing windage and improving the efficiency of the sails [17]. Fig. 11 illustrates these points, as well as the transverse stability of the trimaran. Most trimaran floats of recent design contain enough volume to support the entire weight of the craft, with sufficient additional buoyancy to prevent the deck from becoming awash when hard pressed in a seaway. Obviously, by increasing the spacing of the floats from the main hull, they can be theoretically finer, smaller, and lighter, thus causing less total resistance. There is a practical limit, the determination of which is still being widely experimented. The limit is governed, to a large extent, by the action of the lee float in steep seas. A combination of yawing, pitching, and rolling for the trimaran at sea can bury the bow of the lee float, with too little freeboard and too fine an entry, well under the surface. At high speeds this can cause severe structural strains in the connecting members, or sudden capsizing. The Indonesians understood this danger perfectly. In their single outriggers (proas) they always had the outrigger to windward by shiRing the rudder front one end of the main hull to the other, thus making the bow the stern and what was the stern, the bow. The outrigger in fact, was merely to prevent the craft from capsizing to windward in a sudden lull when the crew was perched out on the connecting members. Used only in this manner, the outrigger could be kept very fine. The ratios of float volume/main hull volume among trimaran designs, which have crossed most of the oceans of the world and have been entered in the longest offshore races, vary between 30 and 40 percent, and float centerline to float centerline between 50 and 60 percent of the length of the main hull. (Float proportions of these designs are discussed under Hull Forms.) When either a catamaran or a trimaran heels, more profile area is presented to the wind, just as if freeboard OCTOBER 1968

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were added, Fig. 11. At low angles of heel the additional heeling force from wind acting on the heeled profile or added wind resistance is negligible, except to racing multihull yachts. At larger angles of heel, permissible in small catamarans and customary in trimarans with high floats, the additional heeling force acting on the underside of the connecting structure, in addition to the side force and drag created by the windage, becomes a significant factor in the overall stability and performance. (This point is discussed further in the following section.) For this reason, m a n y recent offshore racing trimarans are being built with the area between the main hull floats and connecting members left entirely open, except for safety netting. Wind forces acting upon raised hulls and wings at large angles of heel are substantial, especially in conjunction with the light displacements possible in offshore racing craft. This should be taken into considerations of stability comparisons. Some eats and tris have been fitted with various flotation devices, either fixed or inflatable, at the masthead. It is generally agreed that the fixed devices are unsightly and add weight and wind resistance high up where it is most unwanted. Therefore, the inflatable type is getting more attention. Such a device was fitted to the earn359

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maran that won both the first Round Britain and Crystal T r o p h y races from England in 1966 and 1967, respectively [18]. Fig. 11 is included for comparative studies of transverse stability with a typical monohull keel eenterboarder. Comparative righting moment curves for a trimaran, catamaran, and monohull of equal displacements and similar sail plans are shown in Fig. 10. These are based on displacements demanded of the craft equipped in like manner in the way of cruising gear, accommodations, refrigeration, outfit, plumbing, electrical and electronic equipment. It is assumed that the catamaran would be fitted with two small engines on account of her twin hulls, where there is one for the trimaran and monohull. Very little has been done with liquid ballasting systems on multihull craft, because of the added complexity, original cost, and maintenance of the required pumps, tanks, and other equipment--not to mention the disadvantage of the additional weight. 5/~ost multihull owners prefer to rely on the inherent stability of the design and their ability to control the angle of heel with change of course and sail changes, and to avoid the possibility of turning upside down in the event of capsizing by the addition of masthead flotation. The 40-ft catamaran proa shown in Fig. 14 has no rigging, thus allowing a complete luff in any direction. She was designed by Dick Newiek of Chirstiansted, St. Croix, especially to compete in the 1968 Transatlantic Single-Handed Ocean Race.

Fig. 14

N e w l c k ' s 40-ft cat-proa n o t yet l o a d e d to transatlantic race

Habitability

Multihulls present their own design problems and solutions pertaining to comfort aboard. As in monohulls, a compromise must be arrived at that is satisfactory both in terms of habitability, seaworthiness, and speed. The solutions are different in each size range for both catamarans and trimarans. Of particular interest and importance is the problem of headroom versus wing height in catamarans, which is discussed in detail in this section. Individually, practices in ventilation, headroom, lighting color scheme, berth length, seating, and general access currently acceptable in monohull craft are being incorporated in carefully planned multihulls. On the smaller catamarans, narrow hulls create special problems in the general arrangement of living quarters. Passage fore and aft in the narrow hulls is restricted by protruding berths and joiner work. Headroom is difficult to obtain while keeping a low outboard profile, workable deck, and at the same time trying to create a protected lounge area. A partial solution for the small catamaran appears in the arrangement shown in Fig. 13, where standing headroom is provided by raising a trunk over each hull. Berths do not interfere with access to the galley or head, and a protected lounge area with sitting headroom is created by raising a dacron hood over the U-shaped seating area installed between the trunks. With a removable table between the seats, the area may be used for eating, the seats converted to berths, and with the hood down is a useful extension of the OCTOBER 1968

cockpit. Also, with the hood down when underway, movement fore and aft is not difficult if one steps up to the deck over the seats, and there is no interference with the working of the sails. In smaller catamarans, for overnighting the trunks may be dispensed with and only the U-shaped seating area kept with collapsible hood. Several small catamaran designs from abroad are in production with deckhouses, which nearly span the full beam of the craft to provide sitting headroom between the hulls and protected access from one hull to the other. Deckhouses with standing headroom raised between the hulls in catamarans of less than 50 ft in length appear disproportionately high in comparison to a normal freeboard and have the further disadvantage of raising the main boom to awkward heights. Any attempt to lower the floor of the wing structure to reduce the height will result in pounding a n d / o r increased frictional resistance from frequent wetting. I t is quite obvious that continuing to lower the wing will result in a barge form. In 1962 an unoffeial 47-ft catamaran entry in the Storm Trysail Club's Block Island Race, upon reentering the Sound from rounding the Island, at which point she was over three hours ahead of the first yacht in the monohull fleet, a 72-footer, suffered the breaking away of a large plywood panel in the forward portion of the underside of the wing while proceeding to windward in a steep, short chop. In all fairness it should be noted that over half of the 86 boat fleet dropped out of the race with gear failures and d i s 361

Fig. 15 Trice has twice sailed from St. Thomas to New England

mastings on account of the severity of the weather. Had the wing been higher by summarily reducing headroom of the deckhouse, attended by an increase in wing scantlings, she would have avoided structural failure. The connecting structure between the hulls should be kept as high as possible in relation to the freeboard and required strength, but, based on a percentage of the clear width between the hulls, the suggested minimum for a 15 to 20-ft-waterline craft would be 16 to 20 percent and 25 percent for larger craft. Large fillets at the hull and wing joint and deep longitudinal external stiffeners are recommended to reduce the amount of area of zero deadrise. Solutions to the same problems in small trimarans are somewhat easier. The total costs are less, and the profile can be kept lower than in catamarans of comparable length. This is because when a trunk is raised for headroom, it is done over the center hull, which, being beamier than a catamaran hull, is large enough for one-sided seating area. By extending the trunk out over the floats, wide, comfortable berths are provided, with ample side deck remaining for movement fore and aft. Newick's Trice has twice passaged from St. T h o m a s to New England for charter and returned, Fig. 15. Piver has twice crossed the Atlantic in small trimarans of his design, and once crossed the Pacific [19, 20]. His 30-ft Stiletto, in which he made the second Atlantic crossing in the summer of 1967 and successfully competed in the Crystal T r o p h y Race, is entered in the 1968 single hander's race from England to the United States. She exemplifies most of the small cruising trimarans in use today. Bernard Rodriguez won the first multihull race from the United States to B e r m u d a in Amistad, a 25-ft trimaran. The floats of trimarans under 45 ft are generally not large enough for living quarters. Some designers, how362

ever, utilize the floats for staterooms in the 40-ft range by bringing the cabin trunk out over the floats. Such practice raises the displacement/length ratio to such high levels that maximum speed is reduced considerably and a sluggish feeling is noticed. Many existing trimarans and catamarans are overloaded, and this is manifested to a large extent by their going deeply overdraft and bringing wings too close to the water. For the most part, sufficient human comforts commensurate with the time spent aboard can be achieved without jeopardizing performance as long as the number of persons to be accommodated is kept within the limits normally seen on monohull craft of equal length. Owners are tempted to use all of the available space so much more in evidence on multihull craft. A good rule of thumb is to keep 1/~ of the total volume of the hulls empty and to keep only the ]ightest of stowage in trimaran floats. There are considerably more cruising trimarans than cruising catamarans in the 40 to 60-ft range in use as private yachts, the ratio being about four to one in the world. Many multihulls in this range now ply the West Indies and Bahamas as private charter yachts, but whether or not they are used for charter the following comparisons of trimarans and catamarans in this size range is of interest. Building and maintenance costs of trimarans can be lower than catamarans, based on the fact that they are usually fitted with one auxiliary engine, one centerboard (if any), one rudder, and one set of fuel and water tanks, whereas the catamarans are customarily arranged with two auxiliary engines, two rudders, two centerboards, two sets of fuel and water tanks, and so on. This explains the reason for greater popularity of the cruising tris in all sizes. However, from the standpoint of habitability, trimarans in the stated foot range exhibit certain disadvantages, both as private yachts and for charter use. A comparison of the arrangements of the 60-ft trimaran in Fig. 16 and the 52-ft catamaran in Fig. 17 shows that the distribution of living quarters, lounging, eating, crew, and working spaces of the catamaran provide greater convenience, and more privacy for the owner's party. In the trimaran of the size where the floats can be utilized for sleeping quarters, there is no inside passage to them. Trimaran floats are usually smaller than either of the two hulls of a catamaran of corresponding size, resulting in less space for the guests. The galley in the trimaran is adjacent to the dining area, affording the owner's party no privacy at meal time or when lounging about inside during inclement weather. The floor space in the lounge and dining area of the trimaran will be limited by the beam of the main hull. On the other hand, in the catamaran of similar length, as noted in the 52-footer, the deckhouse spans the inside edges of each of the hulls. This produces a single area nearly double that of the tri, which at meal time may be devoted entirely to the owner's party, and at other times for lounging. Ladders leading clown into each hull provide sheltered access to the quarters in both hulls. The M A R I N E TECHNOLOGY

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5 O

Fig. 17 Arrangement of Stranger

t

J

/ / /

tn e.l

z'

t~

, , f l y

0 e,l

,,j

c~

7X 0

ta

V~ Fig. 18 Comparison of speed diagrams: 72-ft catamaran, 72-ft monohnll

hulls, when arranged like those of Stranger" with the crew quarters and galley in one hull and the owner's p a r t y in the other hull, provide distant separation for maximum privacy and noise reduction. The auxiliary engines in the catamaran are placed aft of all the living and lounging area,s, separated by heavy structural bulkheads, with sep~brate access for the crew, whereas in the trimaran the main auxiliaries are immediately below the lounging ares,. lqTowever, except for the trimaran operated as a ehal~ter yacht or one of the more formally run private yachts with professionals, the disadvantages .iust mentioned may be entirely dismissed, especially in the interest of reducing first costs and keeping operating costs at a minimum. There are those who claim that the trimaran's motion in rough seas is less violent than either the eat~bmaran or the monohull. This quality is so illusive, and so few persons have sailed sufficiently in all three types, that rendering any clear-cut comparison is beyond the scope of this paper. I t is suggested that a study of the comparative motions of the multihull types with each other and the monohulls would make an excellent subjeer for a future paper for the Society. Perhaps the best way to terminate a discussion of multihull amenities is to mention two long-distance voyages. First, the voyage of the Galinule [21], a 40-ft trimaran which passaged from Mombasa, East Africa, to Wellington, New Zealand, 11,500 miles, one of whose crew was 93-year-old Grannie Cole. She was reported to have been in excellent health on her arrival in Wellington and she said she was comfortable throughout the trip. Second, the voyage of the R e h n Monna, a 42-ft catamaran cutter in which David Lewis, with his wife and two daughters, just returned to his native England OCTOBER 1968

after completing a westbound circumnavigation via the Straits of Magellan° He sailed the first leg alone as an entry in the single-handed Transatlantic Race of 1964 [22]. He claims he will have a catamaran for his next trip at sea. Of particular interest is Table 1, which compares a 72ft welded aluminum monohull ketch with a 72-ft wooden catamaran ketch, both of which were designed by MacLear & Harris. The catamaran is not yet launched, and speed estimates are extrapolated from theory by Meyers [23 ]. Since no actual measurements of speed at various headings have been taken aboard the monohull, tank test data are used for the construction of her speed curve in Fig. 18, which compares it with t h a t of the catamaran° A comparison of arrangements at equal scale is presented in Figs. 19 through 22. Construction

The majority of sailing multihull cruisers in use or under construction today are custom built, planked with sheet or cold-molded plywood and strip plank wood, sheathed o~l the outside with fiber glass-reinforced plastic and framed with plywood, sawn, or laminated wood. Framing is largely longitudinal with plywood web frames, wood stringers, plank floors, and bulkheads. Heavy reliance is placed on glue bonds of faying surfaces, with numerous light fastenings, like ribbed nails and staples, to obtain gluing pressure rather than mechanical connection. Hull connections are generally composed of plywood box beams, wood trussed and flanged, with extensions of the webs into the hulls in the form of transverse bulkheads to distribute the loads over the depth of the hulls. M@)r longitudinal strength members such as 365

Table 1

Comparison of Dimensions: 72-ft Monohull and 72-ft Catamaran Jubilee I l l

Quickstep I I

Monohull 73-3

Catamaran 72-3}

80-0

80-5

Load waterline length, ft-in . . . . . . .

50-0

60-0

Beam, extreme, ft-in . . . . . . . . . . . . . .

16-4

30-0

Hull beam at shoulder height, ft-in

16-0

8-3

Beam load waterline, ft-in . . . . . . . . .

15-0

6-9½

Draft, centerboards up, ft-in . . . . . . .

8-0

8-6

Draft, centerboards down, ft-in . . . .

14-1

11-6

Freeboard forward, ft-in . . . . . . . . . .

6-3

7-6

Freeboard amidships, ft-in . . . . . . . .

4-9

6-9

Freeboard aft, ft-in . . . . . . . . . . . . . . .

4-6

5-6

°°o

l)epth of hull, ft-in . . . . . . . . . . . . . . . Height of main boom above L W L , ft-in . . . . . . . . . . . . . . B/H ............................

12-7

10-6

° ° .

11-0 1.87

15-0 1.24

L/B ............................

3.32

8.84

Displacement, full load, Ib . . . . . . . .

95,000

136,000

Displacement, H, half load, lb . . . . .

93,000

123,000

Dimensions Length overall, ft-in . . . . . . . . . . . . . . Length w/bowsprit and bumpkin, ft-in . . . . . . . . . . . . . . . . .

Displacement, light ship, lb . . . . . . .

89,000

114,000

A / ( L / I O 0 ) a. . . . . . . . . . . . . . . . . . . . . .

388

282

2535

3082

Wetted area boards down . . . . . . . . . Wetted area boards up . . . . . . . . . . . . Sail area w/l()O~, fore, sq ft . . . . . . .

l~emarks

I)ifference

(No bumpkin required on cat to reach end of miz. boom /Ca~ 20% 1 longer

fCat 84% °

.

.

°

,

o

wider Cat hull 48% narrower do. 55% 'Cat 31% less draft do.

t 18%

...

Cat 20% more freeboard fk do. t

42%

"Of each t~(111fer eat \ at D W L Same as above (Full fuel, water, t stores, crew and effects Half fuel, half water, I stores, crew and t effects No fuel, water, t stores, eI'ew~ or effects 141 for each hull of cat 34% greater than I monohull at full t displacement 26% greater than I monohull at full [ displacement

J do. 122% fCat 16~, / less fCat 36% \ higher Cat--34% fCat--63% proport'ly

Cat/wood

[ 43% plus

( do. t 32.4%

/ plus Cat/wood 28% plus Cat 27% less

i

Cat 22% r n e r e

keels, shelves, clamps and bilge stringers are usually laminated wood or plywood and sawn wood. The basic concepts and techniques of the type of wooden construction just described were introduced during World War II in the construction of air/sea rescue craft, P T boats, high-speed patrol craft, and landing craft. The system was first successfully employed in muttihull craft by Woodbridge Brown [17] in Hawaii in 1947 and used on Wailcitci Surf, the first modern catamax'an to passage between Hawaii and California, built by Brown and Kumulae. Since then the same structural s y s t e m has spread t h r o u g h o u t the world. As m u l t i h u l l craft grew in p o p u l a r i t y after W o r l d W a r I I , a n d small racing classes, p a r t i c u l a r l y of c a t a m a r a n s , were introduced, molded fiber glass-reinforced plastic was 366

used for their p r o d u c t i o n , while in the larger craft, a l u m i n u m a n d steel were being tried. Light a l u m i n u m alloy e x t r u d e d t u b i n g has become the m o s t p o p u l a r m a t e rim a n d shape for c o n n e c t i n g the hulls a n d floats of t h e d a y racers, Fig. 3. E n t i r e a l u m i n u m alloy welded hulls a n d hull c o n n e c t i o n s are being designed for the larger m u l t i h u l l yachts. T o date, steel has been used for the m o s t p a r t on cornmerciM power c a t a m a r a n s for fishing a n d oceanographic research. I n 1951, C o p u l a , a steel c a t a m a r a n of 48-ft, designed a n d b u i l t in F r a n c e b y C a p t a i n Christian, crossed the A t l a n t i c to N e w York. T w e n t y - f o u r - f o o t - h i g h seas were m e t d u r i n g the 3 1 - d a y crossing, eight days of which she lay b e c a l m e d off t h e Azores. T h e designer reported t h a t due to her h e a v y steel c o n s t r u c t i o n she was sluggish a n d p o u n d e d b a d l y on MARINE TECHNOLOGY

Table

1

(cont)

Jubilee IZI

Qui&step i [

Monohull

Catamaran

Wind resistance area, sq ft . . . . . . . .

113

286

Max. righting moment, ft-lb . . . . . . .

214,000

1,500,000

Area lateral plane, sq ft . . . . . . . . . . .

343

564

Lateral plane coefficient . . . . . . . . . . .

0.75

0.71

Main deck area (inelud. cockpit), sq ft

232

1950

Dimensions

Remarks 9 ~ less for cat

SA/WA ..............................

Cabin sole area, sq ft . . . . . . . . . . . . .

386

15.4 20.9 5210

Heads w/showers . . . . . . . . . . . . . . . . Auxiliary horsepower. . . . . . . . . . . . .

3

140

4 280

Engines . . . . . . . . . . . . . . . . . . . . . . . . .

1

3

Speed u n d e r power, knots . . . . . . . . .

9

12

Cruising range under power, miles..

607

1700

Fuel capacity, gal . . . . . . . . . . . . . . . .

211

1200

Fresh-water capacity, gaI . . . . . . . . .

568

1200

Deep freeze vol, cuft . . . . . . . . . . . . . Living space, vol, cuft . . . . . . . . . . .

Includes dacron net area between hulls i of cat, fore and aft [Total of both hulls

/MoG/~un

( 60% less jCat 7 times \ greater Cat 1.6 tlnles

more

rcZ 8.5

I

{ times

[

Cat 1.86 t times more

720

15 5 1815

Refrigerator vol, cuft ............

Ctrbds. down, inclu. rudders Of each hull for cat

Difference

and deck house for cat

more

fCat 4" times t larger Cat 2.86 t times more

fCat, twin screws with diesel generator

Auxiliary generator . . . . . . . . . . . . . . .

6.4:kw/32v 14 kw/32v

Air conditioning and heating . . . . . .

none

yes

Electronics . . . . . . . . . . . . . . . . . . . . . .

equal

equal

Cost of construction . . . . . . . . . . . . . .

$250,000

8250,000

Beachability . . . . . . . . . . . . . . . . . . . . .

none

yes

the u n d e r s i d e of the commcting s t r u c t u r e b e t w e e n the hulls. I n 1965, R a b b i t , a 33-R steel m o n o h u l i sloop, designed b y R i c h a r d Carter, won the F a s t n e t ocean race. A l t h o u g h b o t h craft h a d 1/{-in. shell plate, t h e c a t a m a r a n suffered because of the large a m o u n t of shell area. While with different lines a n d less deck area, the steel weight of a c a t a m a r a n of this size can be red u c e d b y a small percentage, the t o t a l s t r u c t u r a l weight would be greater t h a n the m o n o h u l l because it is i m p r a c tical to weld shell plate t h i n n e r t h a n ~/{-in. (The a u t h o r s r e c e n t l y heard of one welder who claims t h a t t h i n n e r plate can be successfully welded w i t h o u t excessive dist o r t i o n or b u r n - o u t b y u s i n g a x~ater s p r a y b e h i n d t h e welder a n d a d e q u a t e stiffening, b u t we h a v e seen no

yachts produced in this manner.) Generally, in multiOCTOBER 1968

Maximum rated

~iaximum

fCat 2 times t more Monohull 66% less fCat 33% I faster Cat 2.8

i

times more t range Cat approx 6 times more Cat 2 times [

nlore

'Alternators on roche-

hull-diesel gen. on

Cat 2 times

more cat ']Reverse cycle Heating: 34,500 Btu Cooling: 32,400 Bm fExeept cat's radar larger (Ca~'s cost, estimated IComparable I finished, based on I yards, cWu wood, built in ~[ 25-30% l China rnore

hulls, welded steel hulls are practical a b o v e 75 ft, b u t even t h e n lighter plywood fiber glass-covered decks a n d wood or a l u m i n u m deekhouses would be r e c o m m e n d e d . On the other h a n d , all-welded a l u m i n u m alloy is well suited for m u l t i h u l l s with shell plate thickness d o w n to }{ in. Below 5{ in., shell plate d i s t o r t i o n is excessive a n d the p l a t e is n o t s t r o n g e n o u g h to resist p u n c t u r e b y local i m p a c t as from docking or coming alongside a n o t h e r craft. W i t h f u r t h e r reference to the 52-ft c a t a m a r a n described in the previous section a n d i l l u s t r a t e d i n Fig. 17, i t should be n o t e d t h a t it has a n i d e n t i c a l sister yacht, except t h a t one is all-wood c o n s t r u c t i o n a n d t h e other h a s all-welded a l u m i n u m alloy hulls, framing, a n d connecting: beams, with plywood fiber glass-covered decks a n d deckhouse. T h e shell p l a t e of the a l u m i n u m y a c h t below t h e 367

0~'~ ST*,ER~

COCKPlX

~

Fig. 19 Xl

m -4 m ('1 "IZ

o

Arrangement plan of a 73-ft monohull

i

D G

HOUSE

Fig. 20

D e c k plan of a 73-ft monohull

turn of the bilge is ~ in. and a~6 in. on the topsides, whereas the wooden hulls were strip-planked on the bottom with ~-in.-thick mahogany covered with several layers of fiber glass up to the waterline and doubleplanked on the topsides. Beams connecting the hulls of the wooden boat are wood and decks and deekhouse of fiber glass-covered plywood, the same as in the aluminum boat. Transverse framing systems were used in each craft as illustrated in Fig. 23. The aluminum boat was approximately 20 percent lighter before outfit. The advantage of aluminum alloy's favorable weight/strength ratio is readily appreciated, for which there is much supporting evidence by its extensive use in construction of monohull yachts, high-speed power boats, and large-ship superstructures. However, its greater advantage in multihull construction by comparison with monohull construction may be less obvious, especially with the catamaran. First, on both trimaran and catamaran, there are two and three times as many points of abrupt change in direction of exterior surfaces as on a monohull, at which points the redundancy of wooden eonnections makes strength analysis and joint performanee uncertain. Higher factors of safety introduced by using larger members, more bracketing, and doubling result in further weight. Filleted and rounded joints using laminated wood members can improve such ioints eonsiderably, but rely heavily on good gluing and careful workmanship. Both end in higher construction costs. The foundations of the catamaran for twin centerboard boxes, twin rudders, twin engines, and duplicate separate tanks are usually planned for metal construction on a wooden boat. Because of the conglomeration of types of metal used in wooden hull structure, which could raise serious problems in electrolysis, metals other than aluminum are usually specified for these parts, such as stainless steel, galvanized mild steel, monel and silicon bronze. All of these, in addition to their separate natures from the basic structure, are materially and structurally heavier than aluminum. In comparison with the monohull, proportionately higher weight savings in the use of aluminum alloy for multihulls are realized, again especially in the catamaran, because of the beams required to connect the hulls. The large compression loads of the masts are concentrated in the midspan of the main transverse beams which connect OCTOBER 1968

the hulls. In order to maintain deflections at tolerablc levels, wood members must be quite large and are heavier than aluminum by the ratio of their weight, strength, and stiffness. Molded fiber glass construction is being used in multihulls, both catamaran and trimaran, up to between 40 and 50 ft, with basically the same techniques as are now practiced in monohulls; viz., molded hulls, plywood bulkheads as transverse stiffening, and fiber glass sandwith decks, usually with end-grain balsa cores. Because of the higher percentage of surface area, total structural weights of molded fiber glass multihulls above 30 ft wilI be higher than monohulls of equal length. Total displacements are lower than monohulls because of the ballast required for the stability of the latter. Fibec glass-covered plywood construction is generally lightm~ than molded fiber glass, and laminated cold-molded wood is the lightest practicable construction. To prevent fracturing thin laminated wood skins, one or two layers of very light fiber glass-reinforced plastic is placed over the outside surfaces. Considerable experimentation with fiber glass-reinforced, plastic-faced sandwieh construction using foam plastic cores, particularly the polyvinyl chlorides, is in progress on multihulls. Application of heat below boiling temperatures, which can be practically applied, allow rigid sheets of polyvinyl chloride to bend to any desired curvature in a plastic state, but return to their former rigidity after cooling, while maintaining the new shape. Because it is composed of a noninterconneeting cellutar structure with no open volume between the cells to prevent water absorption, and with considerably higher shear, peel resistance, and tensile strengths than former foam plastics, much more extensive use of it in boats is predicted, especially in multihulls, where it is particularly desirable to reduce the weight per square foot of their large exterior surfaces. In the absence of ballast, multihull cruising craft may be kept extremely light in displacement, especially if they are auxiliary powered with outboards and have little fuel and water. In fact, it often happens that scantlings must be arbitrarily increased to withstand local damage. The. author recalls a 22-ft catamaran day racer designed in his', office in which the outer skin of the plastic sandwich construction was so light that one could practically pu~ his 369

[

. f - -

~ ~ ~.

L .

._ . _ . v

~

r

aa.--

i/j

\. o

,,

o, o, o.

Fig. 21

A r r a n g e m e n t o f a 72-ft c a t a m a r a n

I ~

C

)

j Fig. 22

D e c k h o u s e a r r a n g e m e n t o f a 72-ft c a t a m a r a n

finger through it, yet it was amply strong to resist water forces arid the other loads normally imposed while sailing. However, with newly developed PVC cores with higher impact strength of between 7 and 10-1b density, higher loads are spread over a greater area, reducing the possibility of puncture, and, due to the closed cellular' structure of the core, if puncture should occur, no leaking will ensue. Core thicknesses thus far tried vary considerably. T h a t used in Glass Slipper -~as 2½ in. thick as against in. thick on Toria. (See previous subsection, "Offshore Cruisers and Ocean Racers.") In some instances, in order to reduce further structuraI weight, dacron canvas and netting has been substituted for deck areas between the hulls and floats and the transverse beams. Considerable weight savings can be made ir~ this manner. It, also reduces the amount of 370

.

solid structure exposed to w~ve slapping between the hulls and flo~ts. Mechanical Systems

The following is a brief discussion of' some special problems in steering, powering, and electrical and plumbing systems of multihull cruisers. Because single centerline arrangements of these systems are possible and customary on cruising trimarans, the problems assoo g l e d with them are similar to those of the monohull in most respects. However, catamarans require dual propulsion, plumbing, and steering systems, making them more costly in construction and maintenance, and heavier and more complex than trimarans. Because of the higher speeds possible in the lighter multihull cruisers, it is desirable to have retractable MARINE TECHNOLOGY

Fig. 23 Aluminum 52-ft catamaran in frame

aluminum housings does not permit long life and durability in the toughest conditions of a marine environment, that of being half in and half out of salt water. Manufacturers are reluctant to change their inboardoutboard units to suit custom installations in catamarans. These units are usually short-shafted to the engine and must be hung on the transom. Furthermore, the manufacturers will not sell their inboard-outboard units separately from the engines, which places further restrictions on their use. Their weight and proper arrangement at the transom calls for submerged transoms, which retard turning under sail, are not particularly attractive, and create water noise. The center of gravity of the engine so far aft increases the mass radius of gyration, raising the pitching angles. Increased pitching reduces sail drive, can be uncomfortable to the crew, and causes pounding on the underside of the wing structure. On the basis of the foregoing, standard, permanent propulsion systems with fixed propeller and shaft fitted with folding or fixed two-bladed propellers, and variable: pitch and controllable-pitch propellers, are being reconsidered. The necessary penalty of parasitic resistance is being accepted in favor of reliability and flexibility in choice and arrangement of engines. This is particularly true where diesel engines are becoming increasingly popular because of the lower volatility and cheapness of diesel fuel. V-drives and belt drives provide adequate flexibility in arrangement. Engine compartments widely separated in catamaran hulls require special attention to the handling and distribution of fuel, especially since one hull may have an variety of dissimilar metals used in construction in cast auxiliary motor-driven unit in terms of transverse trim.

propellers and shafting in order to reduce parasitic resistance, which at 20 knots in a catamaran with dual systems can approach 15 percent of the total resistance. The smaller cruisers and ocean racers have for the most part solved this by using outboard motors in various arrangements which can be swung up clear of the water when under sail. A more sophisticated and costly solution is the use of inboard-outboard stern drives which may be rotated 180 deg out of water when under sail. Outboard motors arranged to retract in self-closing wells just forward of the rudders afford some added efficiency for maneuvering, protection from the elements, noise isolation, and slightly reduce the chance of propeller emergence when pitching over the usual transom-hung units. Generally, the methods for retraction, opening, and closing of the through hull port, and general mechanical arrangement, have not been sophisticated enough to operate in a trouble-free manner. Since auxiliary power is often used in emergency, the unreliability of inadequately engineered and designed installations cannot be tolerated. 2\.Iost stock outboards and inboard-outboards have no means of propeller rpm reduction, which means there will be considerable loss of efficiency, particularly in the larger heavier auxiliary cats and iris. Such inefficiency means higher fuel consumption per mile, and dangerous loss of power in emergency. The complexity of extension and retraction of either inboard-outboards or outboards and the arrangement of their mutual action and location for maneuverability and propulsion make them less attractive and reliable. A

OCTOBER 1968

371

I t is sometimes possible to put auxiliary generating units in one hull and batteries in the other, while refrigeration and air-conditioning units m a y be divided up according to their respective weights. However, fuel transfer pumps must be provided to maintain proper distribution, and fresh-water transfer p u m p s should be provided for the same reasons and emergency use. Because of the difficulty of mounting single rudders and their vulnerability between the hulls, catamarans are usually fitted with two rudders. I t is also preferable to have a rudder in association with each engine in each hull. Normally, twin rudders in monohull craft are turned in unison u ith tillers and a crossbar, but in a c a t a m a r a n it is not possible to do so unless she is decked over all the way aft. The system used on the 72-ft catamaran which was not decked over all the way aft, described earlier, included a quadrant for each rudder interconnected through dual steering wheels with flexible cable. A hydraulic autopilot consisting of two rams was installed in conjunction with one rudder stock, which actuated the other rudder in unison through the cables. This was done in preference to mounting the hydraulic rams in line with the transverse cable run between the rudders to assure t h a t there would always be one rudder operating in ease of a cable failure, and in view of the fact t h a t one rudder acting as a slave in follow-up of the ram-operated rudder would be easier to maintMn in similar angular movement. I n view of the large beams of multihulls, the general desirability of reliable and adequate auxiliary power for use in entering and leaving increasingly crowded ports and in emergency situations puts a greater responsibility on the designers to m a k e the necessary compromise in cost and sailing performance to satisfy this demand. Failure to so so will slow progress in popularizing multihulls. Windward Performance

Considerable criticism has been leveled at multihulls for their inability to sail to windward as well as can monohulls of similar size and sail area. I n the author's opinion, much of this is deserved, because in m a n y instances insufficient lateral plane has been provided and more often poorly distributed. Most multihulls are lighter, draw less water, and have more windage because of higher freeboards and larger exposed area above water than monohulls. Therefore, more lateral plane is required. The amount varies with speed, sail area, hull form, and appendages. As has been stated in this paper for other performance criteria, insufficient qualitative test data exist from which to choose the best combinations when t a k e n in conjunction with other parameters. Meyers [23] suggests that, for semicircular, symmetric, ocean-racing c a t a m a r a n hulls with twin centerboards and rudders, the lateral area of each centerboard or skeg should be 1 percent of the sail area; this in conjunction with twin rudders where the lateral area of each rudder should be 8 to 10 percent of the total lateral plane, l i e also notes t h a t the board areas of asymmetric hulls can be much smaller, but t h a t the extra hull wetted area more 372

than compensates for the smaller boards. General practice seems to follow Meyer's rule. However; in the author's opinion, an increase of eenterboard area to1 } percent of the sail area would result in better speed made good to windward for fuller bodied, heavier cruisers. With similar hull forms and proportions in a trimaran fitted with one centerboard, the area could be 65 to 70 percent of the total area of the two c a t a m a r a n boards, on the basis t h a t a single board creates less drag than two. On the other hand, the trimaran has more windage than the catamaran. Therefore, a better percentage of board area/sail for trimarans would be from 1±2 t o 2 percent. Rudder areas m a y be reduced in a trimaran to 6 to 8 percent of the total lateral plane, because they offer proportionately less resistance to turning. One-of-a-kind races, in which monohulls have raeed with nmltihulls, show t h a t if the angle between the direction of the true wind and the actual course sailed is increased when sailing to windward, approximately from 4 to 7 deg over t h a t of the monohull, there will be a sufficient increase in speed to more than compensate for the extra distanee traveled. Also, the average speed made good to windward will be greater. Although with comparatively lighter hulls and substantially higher initial stability, cats and tris of considerably less length will best larger monohulls to windward in breezes above 12 knots in relatively smooth seas. Experience has shown t h a t a 17-ft catamaran, capable of 15 knots on the reach, cannot beat a 12-meter yacht to windward, because there is simply too much difference between the former's speed to windward and reaching. On the other hand, the windward speed of multihulls increases rapidly with length, with proportionately lower increase in pounds of boat per square foot of sail than the ballasted monohull. So, for this reason, a 32-ft catamaran like that shown in Fig. 3 could make an average speed made good to windward of 1.2 to 1.5 times t h a t of a 12-meter in protected water. Conclusion

Multihulls are fulfilling an ever bro~t.der range of requirements as concepts are improved with actual sailing experience. The surface has just been scratched in this fascinating area of yacht design. Future development should be most exciting and rewarding, especially if the need for improved methods of comparison and competent research is heeded. References

1 The Complete Boating Encyclopedia, M. Weeks, Jr. editor, Golden Press, New York, N. Y., 1964, p. 366. 2 L . F . Herreshoff, Common Sense of Yacht Design, vol. 2, l~udder Publishing, New York, N.Y., 1948, p. 123. 3 " R e p o r t on the I Y R U Trials at Sheppey;" One Design and Off Shore Magazine, December 1967. 4 R. F. Turner, " C a t a m a r a n s , Past, Present, and Future," S N A M E , Hawaiian Section, September 1955. 5 Charles Henry, Gregory Robillard, and Jesus Villaflor, "Effects of Wave Interference of C a t a m a r a n MARINE TECHNOLOGY

Hulls," Thesis, MIT, Cambridge, Mass., May 1955. 6 "Pacific Multihull Association Ratings," published by Pacific _\tultihull Association, Los Angeles, Calif., March 1967. 7 Rudy Choy, "Catamarans and Common Sense," Motoboating 3Iagazine, March 1963. 8 "Feasibility Design Study of a Catam~mn Oceanographic Vessel for the Marine Laboratory of the U. of Miami," Friede and Goldman, Inc., Naval Architects & Marine Engineers, New Orleans, La. 9 H.A. Meyers, "Mathematical Yacht Hull Lines," SNALXIE, April 1966, Fig. 5. 10 "Catamaran and Trimaran International News," July 1966, p. 174. 11 Yaehtin~/ World 3/[agazine, May 1965. 12 Catamaran and T'ri~naran Ir~te'rnational News, September 1966. 13 E. Bruce, "Designing for Speed to Windward," Amateur Yacid Research Society Publication, no. 61. 14 R.B. Ha.rris, Modern Sailing Catamarans, Charles Scribner Sons, circa 1960, p. 13.

OCTOBER 1968

15 "Publications of the Amateur Yacht Research Society," Wood Acres, Hythe, Kent, Engbmd. 16 John Morwood, Sailing Aerodynamics, published by Morwood, 1953. 17 P~'i'neiples of Naval Architecture, Rosselle and Chapman, editors, vol. 1, p. 124. 18 BP Shield Magazine, British Petroleum Co., Ltd. London, England, August 1967. (Advertisement) 19 Arthur Piver, "Handling the Trimaran in Storm Conditions," from International Book of Catamarans and T)~bnarans, by E. F. Carter, circa 1966. 20 Arthur Piver, Transatlantic Trimaran, Pi-craft, 1961; also Transpacific Trimaran, Pi-craft, 1963. 21 "Catamaran and T~'ima~an International News," Letters, April 1967, p. 797. 22 Yachts & Yachting Magazine, Yachting Press, Ltd., Essex, England, October 16, 1964. 23 H. Meyers, "Theory of Sailing With Application to Modern Catamarans," presented to SNAME, Southern California Section Meeting, 1964.

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