NECESSITY IS THE MOTHER OF INVENTION by

P au l

B o n n in

Capitaine de V aisseau (e.r.) de la M arine F ran ç aise

1. To consider carrying out a h y d ro g rap h ic survey by m ak in g use of some of the in stru m e n ts th a t g u n n ers em ploy w hen aim ing th e ir guns is not perhaps to be recom m ended. Everyone know s th a t shells only very ra rely score a d irect hit. However, th is is exactly w h at w as done in 1953 by the F ren ch Survey Vessel B eautem ps-Beaupré (here abbreviated to B.B.) off the coasts of M au ritan ia in F ren ch W est Africa. B ecause of local circum stances, a satisfac to ry survey could not be achieved by m erely em ploying the conventional m ethods available to s u r­ veyors in the 1950s. W e w ere therefore obliged to deviate som ew hat from the classic appro ach by com bining these conventional m ethods w ith a w holly original m ethod w hich we devised bearing in m ind th a t th e B.B. m ounted three guns and had ad equate gun d irection equipm ent. 2. W e w ere re q u ired to survey a n a re a of about 600 k m 2 off a b arren , featureless coast w here a violent sand-laden w ind blew alm ost all the tim e reducing the visibility to a few kilom etres. T he area to be surveyed stretched along about 45 kilom etres of coastline seaw ards to the 10-m etre line w hich w as 15 km out beyond a subm arine ridge th o u g h t to be coralline and likely to prove dangerous. T he coastal m ark s w hich h ad been set u p and tria n g u la te d the previous year w ere n o t visible in the b etter p a rt of the survey zone on account of the atm ospheric im p u rity . F u rth e rm o re the a re a betw een this subm arin e ridge and the coast h ad to be considered dangerous, at least initially, since in 1935 the Sem inole, a h y d ro g rap h ic survey vessel on an exploratory m ission, w ould m ost ce rtain ly have foundered th ere if the presence of rocks subm erged u n d e r only 2 m of w ater h ad n o t been revealed at the very last m in u te by an ab n o rm ally large swell. 3. In view of the B .B .'s tonnage, one could not plan to have the ship h erself ru n the sounding lines. U n fo rtu n ately , th a t m ean t losing all h er advantages, since w ithout too m uch risk it w ould have been possible to p ro fit from th e sh ip ’s ability to m ain ta in a steady course an d provide a stable p latfo rm , navigating in good conditions, w ith fre q u e n t fixes using p red eterm in ed trian g u lated shore positions. T h u s it w as n ecessary to ru n the sounding lines w ith the sh ip ’s survey launches, and th is in spite of the fact th a t they w ould be m oving blind, as they could not see the

trian g u latio n m ark s. M oreover th e ir survey p arties w ere usually unable to m ake accu rate optical m easu rem en ts on account of spray, and finally th e p latfo rm they provided w as u n stab le in the very choppy sea, w ith wave h eig h ts of up to 1.5 m. 4. One solution w ould no doubt have been to w ork only w hen atm ospheric conditions were favourable, th a t is to say, on days of fla t calm , since it w as the trade-w inds th a t produced both the poor visibility an d the choppy sea. However, th is w ould have delayed the survey w o rk enorm ously, and n a tu ra lly the survey w as an u rg e n t m a tte r — as it n ea rly alw ays is ! Its o b ject w as to allow a governm ent service to choose a coastal site from w hich ores could be exported by sea. One of the possible sites (now adays d esignated T an a t) w as the one to be investigated by us. It could, we th o u g h t at first sight, prove of in te re st since the su rf th e re is not too heavy w hich is exceptional on th is coast. T he site w as also to be exam ined because it w as in all likelihood th a t in this neighbourhood som e 200 years ago there had existed a tra d in g post know n as P ortendick, of w h ich no d efinite trace re m a in s b u t w hich p ro sp ered to su ch an extent th a t in tim es gone by it w as tak en by force of a rm s by several nations in tu rn . T his h isto ry led to the belief th a t it w ould be possible to establish p o rt in stalla tio n s in the area. 5. To re tu rn to m y n a rra tiv e — the h y d rographic problem w as to survey th e w hole of Zone C (see figure 1). Zone B h ad been sounded by survey lau n ch es w hich h ad fixed th eir positions according to classical m ethods on previously tria n g u la te d la n d m a rk s in Zone A, an a re a th a t extended a little inlan d. B ut th ere rem ained Zone C, and here th e lack of visibility p rev en ted th e use of classical m ethods. It w as th u s decided th a t for this zone the sounding lau n ch positions sh o u ld be p rim a rily fixed by bearing an d by distance relative to th e B.B., these tw o coordinates being supplied by the ship. In o rd er to do this the B.B. an ch o red in tu rn in positions P, Q, B, S, T, etc., w hich w ere each about 10 km from the shore. At each of these an ch o red statio n s the B.B., th a n k s to h er stability as a p latfo rm , could d eterm in e h er position by reference to the lan d m ark s, tak in g ad vantage of sh o rt periods of favourable visibility. A sandy bottom was chosen so th a t the ship did n o t d rag an ch o r even w hen it w as at long stay. E very day, the w ind slow ly changed direction fro m NNE to NNW , a n d blew steadily w ith the re su lt th a t the B.B. did not yaw. T h u s her a m o u n t of sw ing an d consequently h er position, could be considered u n altered for as m u ch as an h o u r at a tim e. 6. T he m ethod consisted essentially, then, of ru n n in g sounding lines m ade up of a netw ork of s ta r p a tte rn s each of w hich w as cen tred on one of the B.B.’s anch ored positions. To th is end we set up tra in a b le tra n s it m ark s on th e ship by erecting a w ooden fram ew o rk on the b a rre l of one of its guns. T his fram ew ork was 12 m long, an d at each end we fixed two large rcflccto rs fitted w ith electric lam ps of sufficient pow er to pierce the opaque atm osphere. An experienced g u n lay er aim ed the gun — and consequently the alignm ent

F

ig .

1

C o a stlin e . .............................. L a n d w a rd lim it o f Z one A (tria n g u la te d a rea of co ast). S e a w ard lim it o f Z one B (a re a of c la ssic a l sh a llo w so u n d in g ). L im its of Z one C, th e a re a so u n d ed w ith th e m e th o d d esc rib e d in th is a rtic le . ^

L im its o f th e v a rio u s so u n d in g n e tw o rk s c e n tre d on th e B.B.’s p o sitio n s. O

T he B .B .’s a n c h o re d p o s itio n s .

®

T ria n g u la tio n m a rk s u tiliz e d fo r o b ta in in g c a lib ra tin g th e B .B ’s tele m e te r.

re fe re n ce

a z im u th s

and

fo r

of the a tta c h m e n t — in a co n stan t bearing c “=n° + b°, relative to a reference bearing n “, w hich w as obtained by continuously sighting from the B.B. on a tria n g u la tio n m ark . Angle b° w as ad ju ste d d u rin g the course of o p eratio n s, according to the course the launches w ere to follow for th eir so u n d in g lines. F o r each p a rtic u la r case one of the reference signals (E, F, G, H, etc.) w as chosen in term s of its g reater distance from the vessel an d its position in a q u a rte r w here the visibility was the least un fav o u rab le. T he “B lack ” lau n ch (she flew a large black flag), th u s guided, could follow7 a stra ig h t course controlled precisely by th is alignm ent. To expedite the w ork tw o o th e r launches, one “R ed ” and the o th er “G reen” w orked a b reast of th e first, each follow ing a course a t angle of d° from the “B lack ” la u n c h ’s course.

F ig . 2. — D ia g ra m sh o w in g a se c to r w h o se w id th is covered b y th e th re e la u n c h e s a t each change of a lig n m e n t. ......

L im its o f a se c to r co v ered b y th e th re e la u n c h e s. ■—“ T ra c k o f “ B la c k ” la u n c h .

............. - ...........T rac k o f “ R e d ” la u n c h . ______ T ra c k o f “ G re e n ” la u n c h . c

:

b e a rin g o f a lig n m e n t.

d

:

a n g le o f d iv e rg e n c e o f th e la u n c h tra c k s , a n d a ls o th e w id th o f th e sector co v ered by each la u n c h .

T he sector so u n d ed th u s h ad a 3x rf° spread for each change in the alig n m en t (see figure 2). The re tu rn trip s of the th re e launches th u s led to so u n d in gs over a sp read of fixrf" since the ce n tral b earin g w as altered by 3 X d ° w hen th e lau n ch es arriv ed at the far end of th e ir tra c k before tu rn in g back.

7. T h e b ea rin g s of the lau n ch es w ere co n tro lled from the B.B. by observa­ tions in a z im u th using th e targ et in d ic a tio n sight located on the bridge. T his in s tru m e n t w as a u to m atically set on bearin g c", and it w as possible to sh ift it a t will to c “ ± d°. T h e B.B. m ain tain ed c o n sta n t ra d io te le p h o n e co n tact w ith the lau n ch es, an d th is m e a n t th a t they could be g u id ed from the targ e t in d icatio n sig h t’s ob serv atio n s in azim uth. T his guidance w as not in fact essential for th e “B la c k ” lau n ch (the one u sin g the im p ro v ised tra n s it m a rk ). F o r the o th er two lau n ch e s it w as indispensable, even tho u g h th ey q u ick ly becam e able to ju d g e th e distance they should keep fro m the “B la c k ” lau n ch as th e ir d istan ce fro m the B.B. varied. All th a t wras req u ired w as freq u en tly to tell the tw o outside launches w h eth er th e y w ere very little, a little, or too fa r rig h t (or left) of th e ir course. T im e signals w ere radioed every 3 m in u tes : a l m in u te x for the “B la c k ” lau n ch , an d at x ± 1 for each of the o th er two. At each “m a r k ” th e B.B. m easu red the angle betw een th e relev an t lau n ch an d the la n d m a rk being u sed for the reference bearing. T h is th en w as the w ay in w hich w e d eterm in e d one of the co o rd in ates of the la u n c h e s ’ position, i.e., th eir b e a rin g or az im u th from th e B.B. 8. To d eterm in e the o th er coordinate — th e distance betw een the lau n c h an d th e B.B. — we used the sh ip ’s stereoscopic visual ra n g efin d er. T his w as an optical in stru m e n t w ith a 3 -m etre base w hose in stru m e n ta l e rro r w as 20 m a t 5 k m and 80 m a t 10 km (ta k in g into acco u n t the visual ac u ity an d tra in in g of a com petent o p erato r). T he erro r w as in fact less in practice. As the th ree lau n ch es w ere m oving in com pany th e ir d istan ce from th e B.B. w as the m ean of th ree ra n g e fin d e r m easu rem en ts. Also as th e ir speed w as very n ea rly co n stan t it w as possible to d eterm in e fa irly ac cu ra te ly the distance th ey ra n betw een tw o stations. F in a lly the in s tr u ­ m ental e rro rs arising from the fact th a t ra n g efin d ers a re in fluenced by atm o sp h eric conditions w ere alm ost e n tire ly elim inated, since th e in s tru m e n t w as ca lib ra te d before, d u rin g , an d a f te r each series of so undings a g a in st a k n o w n d istan ce of about 10 km betw een the B.B. an d the reference la n d ­ m a rk ash o re, a d istan ce a c cu ra te to w ith in several m etres. T his w as done for each sea statio n occupied by th e B.B. 9. In actu al fact angle d° (the divergence of the so u n d in g lines) w as u su ally fixed as e ith er 9° or 6°, b u t som etim es less (in a re a s w here th e sea bed seem ed uneven), and the so u n d in g lines w ere betw een 4 and 6 k m in length. H ow ever, w h atever e rro rs the co o rd in a tes (bearing an d d istan ce) m ight co n tain , th e accu racy of th e m ethod p roved satisfac to ry in the eyes of the h y d ro g ra p h e r in charge of the technical w o rk of th e su rv ey (a fte r the d a ta o b tain ed a t the in tersectio n s of the lines belonging to th e v ario u s n etw o rk s h ad been cross-checked). As to the le n g th of tim e re q u ire d to com plete th e w o rk , less th a n 2 m o n th s sufficed to survey the w hole of Z ones B an d C. It w as th erefo re possible to d eliv er to th e re q u estin g agency th e 1/50 000 scale sheet th a t they desired.

10. It m ay be h elpful to add to the essen tial in fo rm atio n given above a few re m a rk s concerning som e of the m eth o d s actu a lly used. (a) A nchorage positions w ere ta k e n u p a t P, Q, R, S, an d T in tu rn . In th is w ay the B.B. could move from one statio n to th e next w ith o u t an y h azard , since she w as alw ays nav ig atin g w ith in an a rea th a t h ad ju s t been sounded. (b) R adiotelephone co n tact betw een th e directing vessel — the B.B. — an d th e so u n d in g launches w as essential, as th is p erm itted th e tran sm issio n of a tim e check to fix the exact tim e for each station. Also it m ade it possible fo r the lau n ch es to re p o rt any in fo rm a tio n th e y th o u g h t necessary, an d in p a rtic u la r to in d icate the in sta n t a t w hich they crossed the su b m arin e ridge, w h ich they had to do in o rd e r to re a c h safe ground. F inally, th a n k s to th is lin k the B.B. could give the lau n ch es new in stru c tio n s at the fa r end of each line. (c) T he end of the o u tw ard course w as d eterm in e d by rang efin d in g , and, for th e lines to seaw ard, in addition by th e re p o rts of the launches an n o u n c in g th a t th ey h ad crossed the su b m a rin e ridge. At a given signal all th re e lau n ch e s w ere o rd ered to tak e a 9 0 “ tu rn . T he cen tral bearing was at th e sam e tim e altered by 3 X d°. W h e n this h ad been done the “B la c k ” la u n c h could d eterm in e its re tu rn course on the new alig n m en t w ith o u t fu rth e r help. T he B.B. notified th e o th er two launches ind iv id u ally w hen th ey arriv ed a t ± d° of the re tu rn alig n m en t. All th ree launches then tu rn e d to w ard s the B.B. 11. T he crew s of th e lau n ch es obviously h ad som e very bum py rides, b u t this did n o t p re se n t g re at difficulties since they w ere accustom ed to n av ig atin g in such con d itio n s. F o r th em th e big advantage of the m ethod was th a t th ey w ere only re q u ire d to m ake sim ple a n d fa irly sp arse observa­ tions, a n d th is avoided th e ir hav in g to re m a in too long exposed to the sp ra y o u tsid e the sh elter of the forecastle. T he crew s w ere alw ays suffi­ cien tly fresh an d e n th u sia stic to w ork long h o u rs each day, a n d they had the a d d itio n a l en c o u rag em en t of the w a rm w elcom e given them by the sh ip ’s co m p an y of th e B.B. each tim e th ey n eared the vessel. In fact the spectacle — at least fo r those w ho w ere no t directly involved in the o p eratio n — h a d all the a ttra c tio n of a circus act, so p ictu re sq u e w as the sim u ltan e o u s a rriv al of the trio of launches, th e ir flags sn ap p in g an d sp ray fly in g a ro u n d them as th ey ra n in to w ard s the sh ip — for all the w orld like p o rp o ises speeding in p ast th e bows in a m oonlit and phosp h o rescen t sea.