Sailing Ship Intact Stability Criteria

Marine Technology, Vol. 33, No. 3, July 1996, pp. 218-232 Sailing Ship Intact Stability Criteria Chris Cleary 1, John C. Daidola 2 and Christopher J....
Author: Priscilla Perry
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Marine Technology, Vol. 33, No. 3, July 1996, pp. 218-232

Sailing Ship Intact Stability Criteria Chris Cleary 1, John C. Daidola 2 and Christopher J. Reyling 2 The comeback in recent years to the world's oceans of large displacement sailing vessels has increased the interest in design requirements. At the same time, there have been a number of accidents and capsizings of sailing vessels that investigators have attributed to a lack of adequate intact stability. In an attempt to identify improved stability criteria for large sailing vessels, using the U.S. Coast Guard training barque Eagle as a model, the authors have conducted a worldwide search of the literature and other sources to gather a body of criteriafor comparison with each other and then with the existing standards for the Eagle. The results of this research are presented and illustrated, including the domain of interaction between ship motion and sail forces. The methods presented herein could be a useful contribution to the safe design and operation of large sailing vessels.

1.

INTRODUCTION

Concern with predicting the stability of large sailing vessels has undoubtedly been a matter of interest from far back in history. Earlier books on naval architecture, e.g., [1] 3, have dealt with the subject, and criteria have been further developed in the past, e.g., [2], despite the dwindling number of vessels built. The U.S. Coast Guard has in the past sought to periodically evaluate the stability of its Cutter Eagle (WlX-327), a 295 ft loa, 1730 long ton (LT), threemasted training barque built in 1936 as the Horst Wessel by Blohm & Voss Shipyard in Germany, taking into account more recent stability criteria. It was concluded by investigators [3] in 1980 that the trim and stability of large sailing vessels was far from an engineering science at that time. Continued research in the area of ship motion and sail interaction was thought to be needed

before all possible conditions of stability could be known. More recently there has been an increase in sailing vessels for pleasure and competitive sailing of yachts, restoration of old sailing vessels, and newbuildings of large passenger sailing ships. At the same time, there have been significant sailing ship accidents which have had stability of the vessel as one of the important issues [41. With this background the U.S. Coast Guard sought to investigate the possible improvement of existing trim and stability criteria for the Eagle as a result of the increasing international activity. The first step was the conduction of a worldwide literature search and survey to identify new or developing stability criteria for large sailing ships emphasizing developments since 1985. Once criteria were identified, they were compared against each other, then applied to the Eagle. The

Table 1: EAGLE Principal Characteristics Length Between Perpendiculars

230'- 5"

Beam, Extreme

39'- 5"

Max. Displacement, S.W.

1816 LTs

Draft, Maximum

17'- 0"

Design Draft

14'- 6" Fwd, 15'- 9" Aft

1. U.S. Coast Guard ~' M. Rosenblatt & Son, Inc., New York

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JULY 1996

Presented at the 11 July 1996 meeting of the NY Metropolitan Section of The Society of Naval Architects and Marine Engineers. 3. Numbers in brackets designate References at the end of the paper.

0025-3316/96/3303-0218500.55/0

MARINE TECHNOLOGY

sections of this paper which follow describe the results of this effort [5,6].

H×A z

S n

2.

LITERATURE SEARCH

Approach The literature search was based on an international survey of a broad spectrum of the marine industry which could be expected to represent those segments possibly having carried out investigations on sailing vessel stability since 1985. Sources contacted included the following: • Marine research organizations - 99 in number; • Marine classification societies - 15 in number; • Lloyd's Maritime Information Services, who identified 179 vessels built through the 20th Century; • Builders of 26 sailing vessels since 1980 as identified in Lloyd's Register of Shipping - 11 in number. In addition, a search of written literature was conducted through the following sources: • Society of Naval Architects and Marine Engineers; • Engineering Societies Library - New York; • British Maritime Technology (BMT) Search;

Information Received There were 36 responses which identified various applicable literature. Of these, 11 organizations provided copies of information of interest which has been reviewed and summarized. The review and summary generally have been restricted to material applicable to intact stability of vessels similar to the Eagle.

3.

I N T A C T S T A B I L I T Y M E T H O D S AND CRITERIA

The following subsections outline the principal features of the intact stability criteria under sail which were identified in the search. Table 2 provides a comparison of the various methods/criteria.

United States Coast Guard Stability under sail is based on the requirements of 46 CFR Section 171.055 [7], for all operating conditions, in exposed waters: • positive righting arms to 90 ° heel or greater; • a calculated stability pressure numeral, Sn, which must be greater or equal to stated values for three criteria:

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× 103

A×h

Sl s~ s~ =

A A

= =

=

1.5 LT / ft2 for static balance at deck edge immersion; 1.7 LT / ft2 for dynamic balance to heel angle of downflooding, but not greater than 60 °; 1.9 LT / ft2 for dynamic balance throughout range of stability, but not greater than 120 ° heel; wind heeling arm at zero degrees for each criterion, based on a cos20 relationship to heel (ft); displacement, LT; projected lateral area of sails trimmed flat, plus the spars, rigging and the portion of the vessel above the waterline (ft2); and vertical distance (ft) between the geometric center of projected sail area A and center of underwater lateral area of ship.

Lloyd's Register of Shipping / Wolfson Unit for Marine Technology and Industrial Aerodynamics, University of Southampton, (Lloyd's/Wolfson), England When asked to approve or upgrade stability information for sailing vessels of any length, Lloyd's uses for guidance documents published by the U.K. Department of Transport [8,9]. They also note that the requirements of the International Load Line Convention (ILLC) and Safety of Life at Sea (SOLAS) for passenger or cargo ships would be applied where appropriate. The standard of stability to be achieved is based on vessel length and the requirements are intended for sail training ships between 7 and 24 m in length. It is noted, however, that Wolfson has indicated [10] that their testing has included a 3-masted staysail schooner 56 m in length. In 1987 Wolfson carried out some background research for the U.K. Department of Transport to study the state of the art of sailing vessel stability assessment with regard to the application of stability criteria to U.K. registered vessels [11]. In 1988 they were commissioned by the same department to conduct a more specific program of incorporating model test results and full scale measurements to study stability under sail, particularly wind heel and the effects of gusts. On completion of this study [10,12] they recommended

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Table 2: C O M P A R I S O N OF M E T H O D S / C R I T E R I A

SOURCE CRITERION

USCG

Lloyd's/Wolf son

!

Condition of Sails

All Sails Set

No Requirement

Righting arms

Positive to 90 °

Positive to 90 °

Wind force

-Minimum pressure given -Steady

-No pressure requirement -1.4 gust factor implicit

Sail force

Projected area x minimum pressure

Defined based on downflooding

COS 2 0

COS j3 0

Downflooding angle

Angle at which downflooding occurs or 60 °, whichever less

Aggregate immersed opening area = A/1500 or 60 °

Deck Edge Immersion

-Min. pressure = 1.5 LT/ft.: -Intersection of steady wind heel and righting arm curves

No requirement. Max. allowable steady heel angle based on downflooding and gust factor

Downflooding

-Min. pressure = 1.7 L T / f t 2 -Area under steady wind heel and righting arm curves equal (to lesser of downflooding or 60 °)

-Steady heel angle for which a gust factor of 1.4 would bring vessel to downflooding angle

Knockdown

-Min. pressure = 1.9 LT/ft. 2 -Area under righting and heeling arms equal up to 120 °

-Positive righting arm to 90 °

Sails Furled

U.S. Navy Design Data Sheet for Surface Ships

None

Damaged Stability

U.S. Navy Design Data Sheet for Surface Ships 2-Compartment Damage. Sails Set and Furled. USCG Subdivision

None

Wind heeling arm variation

Other Non-Sail Specific Requirements - Passenger Vessels

None

stability criteria and methods of assessment which have been adopted by the Department of Transport in the two documents cited above for Lloyd's Register of Shipping and which are being considered by the Canadian Coast Guard. The experimental work resulted in the following important findings which have guided Wolfson in developing new standards [10]: All these observations were a result of their aerodynamic testing. • Wind heeling moments cannot be predicted

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SOLAS ILLC

accurately from only a sailplan. The wind heeling moment varies as cosl30 where O is the heel angle. • When struck by a gust, a sailing vessel will heel to the corresponding steady heel angle at the gust wind speed for the duration of the gust. The new standards identify vessel capability without recourse to wind heeling calculations and have provided a method for informing the master of a vessel of the ship's level of safety when sailing. The standards place •

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Table 2: C O M P A R I S O N OF M E T H O D S / C R I T E R I A (continued)

SOURCE CRITERION

Dr. Ing. M. Alimento

ACH

Condition of Sails

Sails Set per Wind Force

All Sails Set

Righting arms

-initial GM .3m min. -max )30 ° -0.2m min. for heel )30 °

-positive to 90° -initial GM ).6m -0.3m min. at 35 ° heel

Wind~rce

-15 m/s mainsails only -10 m/s all sails -20 m/s gust

-13 m/s normal -20 m/s gust

Sail force

-Projected area of sails on centerline in wind speeds identified -Heel no greater than 20 ° in wind speeds identified

-Projected area of sails on centerline in wind speeds identified

COS 2 0

No requirement

Angle at which downflooding occurs or 40 °

No requirement

20% greater than static heel

-250-30° heel -20% greater than static heel

Wind heel arm variation

Downflooding angle

Deck Edge Immersion

Downflooding

-Area under righting arm between static heel and downflooding 0.090m rad minimum -Area between 30° and downflooding 0.030m rad minimum No requirement

Knockdown

None

Sails furled

SOLAS

Damaged Stability

Other

-SOLAS -Sails furled

SOLAS -SOLAS -Sails set per above -GM .60m minimum

None

no restriction on sail area carried, enabling the master to use his or her judgment to set the sails appropriate to the prevailing conditions. The standards require the following calculations at full load and 10% arrival conditions:

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-Heel under gust 45 ° max. -Area under righting arm 1.5 of that under heeling arm

Notes damaged stability requirement would more realistically consider sails furled

The range of positive stability must be 90 ° or greater depending on vessel size. The steady angle of heel (Oe) obtained from the intersection of a "derived wind heeling lever" (DWHL) curve with the righting arm (GZ) curve

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Table 2: C O M P A R I S O N OF M E T H O D S / C R I T E R I A (continued)

SOURCE CRITERION

BV

GL

Condition of Sails

All Sails Set

All Sails Set

Righting arms

-initial GM .3m min. -max }30° -0.2m min. for heel )30 °

-Positive to 60 ° cargo ships -Positive to 90 ° passenger ships -Initial GM 0.6m min. -.3m min.

Wind force

15 m/s average

Not specified

Sail force

-Sails hauled taut -45 ° to longitudinal for square sails -No rigging included

-12°-15 ° heel max. steady wind -Sails for and aft including square rigged -Model tests with sails can be substituted

Wind heel arm variation COS 2 0 Downflooding angle

!

Angle at which downflooding occurs or 40 °

Not stated Not specified

I

Deck Edge Immersion

20% greater than static heel

Downflooding

-Area under righting arm between static heel and downflooding 0.090m rad minimum -Area between 30 ° and downflooding 0.030m rad minimum

Knockdown

None

Sails furled

SOLAS

Damaged stability

None

must be greater than 15 ° , where: DWHL = 0.5 × WLo x Cosi30

WLo=

GZf Cos 1'30s

WL0 is the magnitude of the actual wind heeling

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No immersion At downflooding area under righting arm curve 1.4 that under heeling arm curve

Not addressed None

-SOLAS (sails furled) if failure-proof automatic furl -Otherwise above wind with SOLAS damage should not immerse margin line

Other

222

!

Passenger ships per SOLAS

None

lever at 0 ° heel which would cause the ship to heel to the "downflooding angle" (Oi) or 60 °, whichever is least; and GZI is the lever of the ship's GZ curve at the "down- flooding angle" (OI) or 60 °, whichever is least. The standard [9] also outlines the method for developing curves of maximum recommended steady angle of heel for the prevention of downflooding in the event of squalls, based on the above equations.

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G e r m a n i s c h e r L l o y d (GL), Germany G L ' s requirements [13] for the intact stability of sailing vessels have not changed from those reported to the USCG in 1984. • Initial stability corrected for free surfaces, i.e., gmcorr, must not be less than 0.60 meters. • The range of positive stability must not be less than 60 °. Furthermore, vessels with a large number o f persons on board, e.g., training vessels, the range of positive stability must not be less than 90 ° . • The maximum righting arm, i.e., GZmax, must not be less than 0.30 m. • The steady heel angle due to the wind must not exceed 12 ° to 15 ° , and the deck edge must not be immersed. • The area under the righting arm curve up to the point of downflooding must not be less than 1.4 times the area under the heeling arm curve up to that point. The sail area for this calculation includes the sails trimmed flat, plus the portion of the vessel above the waterline, but not the spars and rigging. It is assumed for this study that the wind heeling moment varies as COS20, since it is not stated by GL.

CH =

• •

• •





Bureau Veritas (BV), France Bureau Veritas, the French classification society, has offered its proposal for tentative criteria for sailing passenger vessels which appears to be dated 15 November 1985 [14]. They note that it has been submitted to the French authorities, and it has been applied to French sailing vessels. For all loading conditions, the following criteria must be satisfied: • Initial stability corrected for free surfaces, i.e., gmcorr, must not be less than 0.30 m. • The wind effect criteria will be calculated assuming full sails and a wind speed of 15 meters per second (m/s). Square sails are assumed trimmed to 45 ° from the centerline and all other sails trimmed flat. Only sails and rigging are used in this calculation. The wind force may be calculated from the following formula or measured in wind tunnel tests:

F~

1/z x

where: F =

G=

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Cs x

CH X p X V2 X A

wind force; shape coefficient, a function of the form of each item exposed to the wind, (a table is given, but sails are taken as 1.0)

• •



height coefficient, a function of the height of each item above the water surface, (a table is given); p = air density; V = wind speed; and A = projected area of item surfaces. The wind heeling moment varies as COS20 where O is the heel angle. For the following criteria, the symbols are defined as: ~R = residual righting lever (~R = gz - ha); Ow = static heel angle due to wind (~.~ = 0); O~ = 40 ° or Of, (heel angle that downflooding may take place), whichever is less; Od = heel angle of dynamic stability; Os = heel angle for which the waterline reaches the margin line. Initial stability corrected for free surfaces, i.e., gm . . . . . must be greater than 0.30 m. The area under the residual righting lever curve between Ow and 30 ° must be not less than 0.055 m/rad. The area under the residual righting lever curve between Ow and OL must be not less than 0.090 m/rad. The area under the residual righting lever curve between 30 ° and Ot must be not less than 0.030 m/rad. The maximum residual righting lever must occur at an angle of heel greater than or equal to 30 ° . The residual righting lever ~R must be at least 0.20 meters for an angle of heel greater than or equal to 30 ° . The static heel angle Ow due to wind effect must be not greater than 75 % of Od or 80 % of O~, and must be limited to a m a x i m u m value of 20 °.

Ateliers & Chantiers du Havre (ACH), France This shipyard recently delivered the Sail Cruise Liner Club Med 2 of 187 meters in length. A C H has made available the criteria [15] which they have used since 1985 for the determination of and approval o f the stability o f sailing ships. They note that other than those certain criteria directly related to the sail equipment, those required by SOLAS 74 and amendments for passenger ships are applied as well. A C H ' s criteria appears to be an adaption of that of Bureau Veritas presented in Section 3.4, or vice versa. The formulation and criteria are identical except for the following: * A C H makes no distinction for sail types and all sails are assumed trimmed to the centerline.

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223

• The wind force is to be calculated for a gust wind speed of 20 meters per second, not the 15 m/s steady wind speed used by BV.

University of Genoa / Dr. Ing. Mario Aiimento (Italy) The rules identified [16] were apparently presented to the Italian shipbuilding community during a national meeting in November of 1990. The requirements are for monohull vessels of length 25 to 100 meters, provided with large sail area, square and/or forward and aft sailplan, fully decked, having relatively low superstructures and not intended for competition. The vessels have also been assumed to be provided with auxiliary engines and are, therefore, subject to SOLAS and IMO (International Marine Organization). Dr. Alimento's criteria are: • The wind force is to be calculated for a steady wind speed of 13 m/s under full sail, and 20 m/s steady wind speed under storm sails. The wind is assumed acting on the beam, with sails trimmed to the centerline. • Initial stability corrected for free surfaces, i.e., grr~ .... must be greater than 0.60 m. • The range of positive stability must be 90 ° or greater. • The heel angle of deck immersion, Ot, should be between 25 ° and 30 ° . • The angle of heel due to wind must be less than or equal to both 80 % Ot and 20 °. • The maximum righting arm, gz, at 35 ° heel must be greater than or equal to 0.30 m. • The heel angle of dynamic stability must be less

than or equal to 45 °, the heel angle at which downflooding occurs, Of, or the second angle of static stability, O~. • The ratio of the area under the righting arm curve, a S, to the area under the heeling arm curve, a~, from 0 ° to Oc or 90 °, whichever is less, must be greater than or equal to 1.5. It is assumed for this study that the wind heeling moment varies as cos20, since it is not stated by Dr. Alimento. 4.

A P P L I C A T I O N T O EAGLE

The stability of Eagle was last evaluated in the 1980's. At that time the following guidelines for intact stability were adopted [3]: • With sails set, intact stability should be in accordance with USCG requirements for sailing vessels given in the Code of Federal Regulations (CFR), Title 46. • Furthermore, the Germanischer Lloyd requirements utilized in the design of the Gorch Foch, built by Blohm & Voss in 1956, based on plans similar to the Eagle, should be met as well. The remainder of this section presents the application of the criteria identified in Section 3. to the

Eagle. Typical Data for Eagle Sail areas and stability data typical to all criteria are presented in Tables 3 and 4.

Table 3: Projected Sailplan Summary [3]

224

Projected Area, feet 2

Center of Area, feet ABL

Full Sailplan, all sails trimmed to 1,

25,355

85.17

Full Sailplan, top 2 square sails set 45 ° to beam wind, lower 3 square sails set 30 ° to beam wind, all other sails trimmed to

22,649

82.97

Full Sailplan, all square sails set 45 ° to beam wind, all other sails trimmed to ~

21,011

83.75

Storm Sails, all trimmed to ~

7274

79.00

Spars

1341

92.85

Rigging

707

73.68

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Table 4: Typical Stability Data with Sails Set

Loading Condition Full Load

Minimum Operating

10% Loads

Displacement, LT S.W.

1731.44

1578.30

1487.91

Draft, feet

15.90

15.06

14.56

KGv, feet A B L

16.32

16.95

17.56

G M ..... feet

3.96

3.45

2.92

Wind A r e a o f Hull, feet 2

4512

4708

4818

Center o f Hull Area, feet ABL

24.86

24.44

24.20

Heel Angle to Deck Immersion

26.3 °

28.5 °

29.9 °

Heel Angle to Downflooding

67 °

~690

~70 o

Range o f Positive Stability

126 °

115 °

106 °

Figure 1 shows the Righting A r m (GZ) curves of the the three loading conditions outlined in Table 4.

Eagle for

L l o y d ' s / W o l fson A summary of the results of these criteria is presented in Table 5 and Figs. 2 through 5.

T a b l e 5: L i o y d ' s / W o l f s o n C r i t e r i a 3

~ F~I Lodd'Condl ~ 7 -r"

/

/

i~linimum~ )pOrating i , Condition ,

Loading Conditions

i

: 20

40

~0

8o

\

10o' ~ -120

Angle of He~l, dcgr©ci

Figure 1: Curves of Intact Stability

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Minimum Operating

10 % loads

Range of Positive Stability, (2 90 °)

126 °

115 °

106 °

Maximum Recommended Steady Heel Angle, ( > 15 °)

40 °

39 °

37 °

;Jiti ,

00

Full Load

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225

5

7 6 ~-~-~

"

--~"

Heeling Arm Curve in Gust to Cause Downflooding

"~ ~ . \

"~

He©ling ~ Curve In Gust to Cause Downflooding

,, 4

~ -r

4

.

Mean Wind Heeling .. '\ Arm Curve \\

~ ;= -r

Downflooding Angle

.

3

, Mean Wind Heeling \ \, Arm Curve ~

',,,

,

Downflooding Angle

2 •. k~

I

/: "2"M a x ~ m ~ ..// Recommended, / Study Heel Angle !

0

2'0

""~ .

\ "\

~ \ \

40

6'0

\,,\\ //

/ : Maximum i /" Recommended, : / SteadyHeel Angle i

"

"\" ~.

80

100

0

120

~. ; .

40

Angle of Heel, degrees

. 60

.

HeelingAngle

"N

. 80

.

I00

120

degrees

A n g l e of Heel,

Figure 2: Lloyds/Wolfson Derived Wind Full Load Condition

\

.

~

' 20

\ ~

Figure 3: Lloyds/Woffson Derived Wind Heeling Minimum Operating Condition

Angle -

i

5O Heeling Arm Curve in Gust to Cause Downflooding

gum in thil region .o

40

< ,

'

~

Mean Wind H e e l i n g " . \ Arm Curve

~k

,,

,. ,,


1.4)

3.43

3.25

Beam wind that will produce 15 ° steady heel angle under full sail

24.4 knots

21.6 knots

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MARINE TECHNOLOGY

---

3.Or

2.5

RAArea/HAArea, 0* tog0" =3.43

,5[

/

• / [~

1

!7-- "

0

/

/

[

r \

Heeling Arm Curve for Steady 15 Degree Heel

/

i\ I :

RA Area / HA Area, 0 to 90 degrees = 3.25

1.5 ',\

i

~

\

\ \\\

\ 0

20

40 60 80 Angle of Heel, degrees

1 oo

120

Figure 7: Germaniseher Lloyd Criteria Full Load Condition

• 0

20

40 6o 80 Angle of Heel, degrees

100

~

120 '

Figure 6: Germanischer Lloyd Criteria Minimum Operating Condition

Bureau Veritas

A summary of the results of these criteria is presented in Table 7 and Figs. 8 and 9.

Table 7: Bureau Veritas criteria

Loading Condition Full Load

Minimum Operating

GM . . . . . feet, ( > 0.98)

3.96

3.45

Area under residual righting lever curve between Ow and 30 °, deg-ft (> 10.34)

5.23

2.02

Area under residual righting lever curve between Ow and Of, deg-ft (2 16.92)

16.78

8.98

Area under residual righting lever curve between 30 ° and Ol, deg-ft (> 5.64)

11.55

6.96

Heel angle of the maximum residual righting lever (> 30 °)

75 °

75*

Maximum residual righting lever (~.R), feet (2 0.66)

2.5

1.9

Static heel angle due to wind, Ow (< 20 ° < 0.75Od ~ 0.8Os)

18.5"

22.5*

Angle of dynamic stability, Od

37*

=50 °

Angle to submerge margin line, Os

25.6*

27.8*

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227

3.0

2.5

2.5

2.0

/

2.0

1.5 i

1.O

\

//

/ :

~-"-4,

\\ \\,\

Heeling Arm Curve for

\ 29.14Knot Beam Wind

, ",

0.5 0.0

/i

~

1.0

~

0.5

20

40

60

80

100

120

/! //

0.0

\ ,9.,dK.otB.oWi.d\

/

~2

ow i 20

0

Angle of Heel, degrees

\

,o. :

--,, ; o~ 4J0

\

~.. 610

'

80

100

120

Angle of Heel, degrees

Figure 8: Bureau Veritas Criteria Full Load Condition

Figure 9: Bureau Veritas Criteria Minimum Operating Condition

Ateliers et Chantiers du Havre (ACH) A summary of the results of these criteria is presented in Table 8 and Figs. 10 and 11.

Table 8: A C H criteria Loading Condition Full Load

Minimum Operating

3.96

3.45

Area under residual righting lever curve between Ow and 30 °, deg-ft (2 10.34)

-0.8

-7.8

Area under residual righting lever curve between Ow and O~, deg-ft (2 16.92)

0.7

-0.6

Area under residual righting lever curve between 30 ° and O~, deg-ft (2 5.64)

-0.1

-7.2

Heel angle of the maximum residual righting lever (2 30 °)

79 °

80 °

Maximum residual righting lever (~'R), feet (2 0.66)

2.3

1.8

Static heel angle due to wind, Ow (~ 20 ° g 0.75Od g 0.8Os)

35 o

44 °

Angle of dynamic stability, Od

~75 °

= 100 °

Angle to submerge margin line, Os

25.6 °

27.8 °

GM . . . . .

228

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feet ( > 0.98)

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-\

3

\ HealingArm Curvefor ,\\38.9 Knot BeamWind

1 \ //

\ HeelingArm Curvef o r ~ ! ~ 3 8 . 9 Knot BeamWind~

~ 3o°i ! i 20

\\

'\ ,.

40 60 80 Angleof Heel.degrees

\

\\

// \,

100

~2

, 3o*;

//

lz0

2'0

Figure 10: ACH Criteria Full Load Condition

I I ~l Io~

\

"\\ \

\

40 60 80 Angleof Heel,degrees

100

120

Figure i1: ACH Criteria Minimum Operating Condition

University of Genoa / Dr. Ing. Mario Alimento (Italy) A summary of the results of these criteria is presented in Table 9 and Figs. 12 and 13.

Table 9: Italian criteria Loading Condition Full Load

Minimum Operating

GM .... feet (2 1.97)

3.96

3.45

Range of Positive Stability (2 90 °)

126 °

115 °

Angle of deck edge immersion, Ot (25 ° to 30 °)

26.3 °

28.5 °

Static angle of heel due to wind, O0 (~ 0.80, and 20 °)

18.5 °

21 o

Righting Arm, GZ, at 35 ° heel, feet (2 0.98 feet)

2. t

1.7

Angle of dynamic stability, Od (~ Oc, ~ Of and ~ 45 °)

34 °

43 °

Heel angle of downflooding, Of

67 o

69 o

Second angle of static equilibrium, Oc

126 °

115 °

Ratio of areas under righting arm curve to heeling arm curve, to Oc or 90 ° whichever is tess (2 1.5)

3.0

2.3

The calculation for storm sails at 20 m/s (38.9 knots) is not necessary by inspection, since the ratio of

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hz for this condition to hz for the foregoing full sail condition is 0.6 and therefore the stability is increased.

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229

3.0

2.5 Heeling Arm Curve for

Heeling Arm Curve for 24.9 Knot Beam Wind

2.0

/

/ ~

1.5

///

, i

i

! ,0

~2

0.5

// /

0.0 0

0t,:

00 ,

, r

20

i ~

1

-

;

'

\.

re

,

6a,

1.0

, : i

\\

,

r

\

0.5 \

,

,

I

40 60 80 Angle of Heel, degrees

100

120

Full Load Condition

DISCUSSION OF C R I T E R I A

The intact stability criteria for sailing ships presented earlier show differences in the wind velocities to be used, amount of sail area, steady heel angle allowed, and other parameters to be satisfied. However, what is common to all, with the exception of the Lloyds/Wolfson criteria, is emphasis on initial stability as a measure of the ability to carry sail and the use of dynamic roll stability based on the traditional approach for conventional motor ships. Evaluation of the ability to carry sail based on initial stability, as measured by gm unduly penalizes sailing ships which operate at relatively large heel angles and have positive righting arms up to and beyond 90 ° , and therefore is not strictly suitable for traditional sailing vessels. Cases in point are the criteria proposed by BV and ACH. In both cases the various criteria proposed must be satisfied up to 40 ° heel, without consideration being given to the stability characteristics beyond this point. These criteria appear to have been developed for modern passenger ships that are sail assisted, and not for fully rigged sailing vessels. The angles of heel permitted are more related to passenger comfort and to motor vessel rules than to the practices of classical sailing ships. The criteria proposed by BV and ACH are more appropriate to large, shallow, beamy ships and catamarans that have significant initial (metacentric) stability, but poor stability at large heel angles. In the criteria proposed by BV and ACH, the residual righting arm area between the steady heel angle and 30 ° must be greater than 10.34 deg-ft. In order to satisfy this requirement for BV, the kg of the Eagle must be lowered by two feet in the minimum operating condition, or the wind speed must be reduced from 15 m/s (29.1 knots) to 10 m/s (19.4 knots). In the case of the ACH criteria, a "gust" of 20 m/s (38.9 knots) is used, resulting in this and several other requirements not being satisfied by the Eagle. Both BV and ACH make

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t /

,

Figure 12: Dr. Alimento/Italian Criteria

5.

.2

0.0

eo= 20

",,

,i

\' 40 60 80 Angle of Heel, degrees

100

120

Figure 13: Dr. Alimento/Italian Criteria Full Load Condition

use of the wind velocity gradient, which amounts to an increase in the heeling moment of approximately 41/z% for the Eagle. BV assumes square sails are set at 45 ° to the ship's centerline, whereas ACH assumes all sails are trimmed fore and aft. The criteria proposed by GL are governed by the requirement to limit steady heel to between 12 ° and 15 ° , provided the deck edge is not immersed, and to give positive righting arms to 90*. For the Eagle the deck immerses at 26.3 ° in the full load condition at 28.5 ° in the minimum operating condition. When the steady heel angle is limited to 15 ° , the maximum allowed under this criterion, the corresponding allowable wind velocities are 24.4 knots in the full load and 21.6 knots in the minimum operating conditions. Because the righting arm to heeling arm ratio at these speeds is very high, the requirement that the steady heel angle not exceed 15" appears too restrictive and out of balance with the stability available. Therefore, an increase in steady heel angle towards deck edge immersion angle would result in a more balanced criterion. If the steady heel is allowed to increase to the point where the deck edge immerses, the allowable beam wind velocities become 34.4 and 32.0 knots, correspondingly. At these wind speeds the steady heel angle is still the governing criterion, followed closely by the dynamic stability requirement. For the 10% loads condition, an 18.3 knot beam wind will heel the ship to 15 °, and a wind of approximately 27 knots will heel the ship to the deck edge, which also just satisfies the requirement for dynamic stability at this point. The criteria proposed by Dr. Alimento (Italian) have some similarities to those proposed by BV and GL. The criteria must be met at a steady wind speed of 13 m/s (25.2 knots). Only the sails are included in the wind area used for calculating heeling moments (not the rig or hull above water and superstructure). However, due to a slightly higher wind pressure, the heeling moment is approximately the same as in the other criteria, which

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use the full wind area for the same wind speeds. These criteria include a requirement that the deck edge immersion angle be between 25 ° and 30 °, and that the static angle of heel due to wind not exceed 20 ° or 80% of the deck edge angle. There is also the requirement that the angle for dynamic stability should not exceed 45 ° . For the Eagle the dynamic stability requirement is satisfied for beam winds up to 29.3 and 25.8 knots at the full load and the minimum operating conditions, respectively. The Lloyd's/Wolfson criteria are derived from analytical studies, wind tunnel tests and full scale measurements as reported in [10 and 12]. The most important innovations introduced to the stability of sailing ships are: (1) the variation of heeling arm as the 1.3 power of the cosine of the heel angle which is more conservative than the square power normally used and more consistent with a lifting surface (sail) than a flat plate (sailing performance analysis uses cos O, (2) definition of what constitutes a downflooding point, and (3) allowing the ship to operate at steady heel angles above the deck edge immersion angle but at or below the derived heel angle. This angle corresponds to one half the heeling arm through the downflooding angle. The criteria to be satisfied are that the derived angle must be greater than 15 o and the range of positive stability must be at least 90 ° . Additionally, the maximum downflooding angle is taken to be 60 °. There is no need to know the amount of sail area. The master is guided by a set of curves of maximum steady heel angle versus wind speed to prevent downflooding in squalls. Using the mean apparent wind speed, the master can reduce sail and hence the steady heel angle, in order to overcome anticipated squalls of any intensity. (It is worth noting that if the upright heeling arms are applied to the full sail area, one can estimate the beam wind velocity as defined by the other stability criteria. These are 39, 33 and 28 knots for the full load, minimum operating and 10% conditions, respectively, for the

Eagle.) Unlike all the other criteria, Lloyd's/Wolfson do not include a factor for dynamic stability. To the contrary, their research [10] indicates that sailing vessels do not roll very much beyond the static angle under the influence of wind gusts, due to the high value of aerodynamic damping exhibited by the sails and rig.

6.

CONCLUSIONS

Various international criteria for the stability of large sailing vessels have been identified. In general, they have elements which differ from each other and with the criteria that have been heretofore applied to the

Eagle.

JULY 1996

The effect of the criteria on the resulting vessel stability cannot be ascertained by inspection of the criteria itself. An application to a vessel in question is required. The criteria developed by Lloyd' s/Wolfson appear to have been based on recent and extensive testing. They also depart from the more traditional approaches based on specified allowable wind speeds and/or sail pressure forces and wind heeling calculations. They appear to afford the master greater latitude in operating the vessel. From the results of the calculations of intact stability under sail for the Eagle, the vessel meets the following criteria: • Lloyd's/Wolfson • Germanischer Lloyd • Dr. Alimento/Italian, although the static angle of heel due to wind, O0, is slightly over the maximum criteria in the minimum operating condition. The requirements of Bureau Veritas criteria and ACH criteria are not met by the Eagle, especially for ACH which uses a 50% higher wind speed.

7.

RECOMMENDATIONS

Based on the discussion herein, it is recommended that the Lloyd's/Wolfson criteria be considered for the Eagle, acknowledging that they have not been applied to a vessel of this size to date, and with the following suggestions and considerations. The Lloyds/Wolfson criteria allow steady heel angles of up to 37* to 40 ° on the Eagle, which are considerably higher than the deck immersion angles. Reference [10] states that "there was nothing in their research that prevented operating a sailing ship with its deck edge immersed." However, immersion of more than the deck edge with relatively large angles of heel, as with the Eagle, may give rise to psychological as well as real operational problems and safety concerns for the personnel onboard, due to the presence of running water on the deck. Furthermore, at heel angles greater than 28 ° to 30 ° the vessel's speed begins to suffer and most masters start shortening sail at this point. On many types of sailing craft, deck edge immersion is used as a practical indicator of the need to shorten sail. We therefore propose this be also included as a practical operational stability criterion. By using the Lloyd's/Wolfson criteria, the downflooding angle in the case of the Eagle is limited to 60 ° , which is below the actual value derived from the downflooding points. Nevertheless, further research is needed to validate the aggregate opening areas that constitute the definition of the downflooding angle,

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231

according to Lloyd's/Wolfson. Positive stability to 90 ° of heel or more is also an important criterion for a vessel to recover from a knockdown. This can happen even under conditions of extremely shortened sail, as in the case of the foundering of the Pride of Baltimore [4], which was sailing under storm sails. Only the Lloyd's/Wolfson and the Italian criteria include this limit, with GL requiring this for vessels with large numbers of passengers (training vessels).

.

REFERENCES

1. White, W. H., A Manual of Naval Architecture, John Murray, London, 1894. 2. Beebe-Center, J. G., Jr. and Brooks, R. B., "On the Stability of Sailing Vessels, SNAME, Chesapeake Section, March 1966. 3. Tsai, N.T. and Haciski, E.C., "Stability of Large Sailing Vessels: A Case Study," MARINE TECHNOLOGY, Jan. 1986. 4. Chatterton, H. A., Jr. and Maxham, J. C., "Sailing Vessel Stability - with Particular Reference to the Pride of Baltimore Casualty," MARINE TECHNOLOGY, April 1989. 5. "Sailing Ship Stability Criteria," Damage Control Book Part II(a) for CGC Eagle (WIX-327), M. Rosenblatt & Son, Inc. Report No. 1002-019-1, Feb. 8, 1993. 6. "Comparison of Intact Stability Criteria" for USCGC Eagle (WIX-327), M. Rosenblatt & Son, Inc. Report No. 1003-008-3, April 12, 1994

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7.

8.

9.

10. 11. 12.

13.

14.

15.

16.

"Subdivision and Stability Regulations; Final Rules," Code of Federal Regulations, Vol. 46, published in Federal Register, Vol. 48, No. 215, Nov. 1983. Code of Practice for the construction, machinerv. equipment, stability and survev of sail training shins between 7 meters and 24 meters in length, The Department of Transport, Marine Directorate, ISBN 0 11 550955 0, England, 1990. The Safety of Sail Training Shins - Stability Information Booklet, The Department of Transport, Marine Directorate, ISBN 0 11 550956 0, England, 1990. Deakin, B., "The Development of Stability Standards for U.K. Sailing Vessels," RINA, 1990. "Sail Training Vessel Stability," Wolfson Unit Report No. 798, U.K., Feb. 1987. Deakin, B., "Model Test Techniques Developed to Investigate the Wind Heeling Characteristics of Sailing Vessels and Their Response to Gusts," Chesapeake Sailing Yacht Symposium, SNAME, 1991. Letter from Germanischer Lloyd to U. S. Coast Guard, GL Reference No. 46462-84-Wag/We/hb, 8 Oct. 1984. "Proposal of Tentative Stability Criteria for Passenger Vessels", Bureau Veritas Regulation, 15 Nov. 1985. "Intact Stability Criteria Applicable to the Cruise Sail Ships Built at Ateliers et Chantiers du Havre, France (Hulls No. 269 and 270)." Letter form Dr. Ing. Mario Alimento, Naval Architect, Genoa, Italy to Dr. John C. Daidola, M. Rosenbtatt & Son, Inc., 24 Nov. 1992.

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