the area of Ihe so-called "fore-triangle"), the overlapping part of headsail does not contribute to the driving force. This im­ plies that it does pay to have o large genoa only if the area of the fore-triangle (or 85 per cent of this area) is taken as the rated sail area. In other words, when compared on the basis of driving force produced per given area (to be paid for), theoverlapping genoas carried by racing yachts are not cost-effective although they are rating- effective in term of measurement rules (Ref. 1). In this respect, the rating rules have a more profound effect on the plan- form of sails thon aerodynamic require­ ments, or the wind in all its moods. As explicitly demonstrated in Fig. 2, no rig is superior over the whole range of heading angles. There are, however, con­ sistently poor performers such ns the La­ teen No. 3 rig, regardless of the course sailed relative to thewind. When reaching, this version of Lateen rig is inferior to the Lateen No. 1 by as rnuch as almost 50 per cent. To the surprise of many readers, perhaps, there are more efficient rigs than the Berntudan such as, for example. La­ teen No. 1 or Guuter, and this includes windward courses, where the Bermudon rig is widely believed to be outstanding. With the above data now available, it's possible to answer the practical question: how fast will a given hull sail on different headings when driven by eoch of these rigs? Results of a preliminary speed predic­ tion programme are given in Fig. 3, A and B. These present comparative speeds for the two distinctly different hull types at the same true wind velocity Vx= 12 luiots. A displacement type of hull with a length/ beam ratio of 5 was chosen as a typical plank-built boat found in many parts ofthe world (Ref. 2). The boat was fitted with a shallow keel and her basic measurements were: Length (L) = 8.8m (28.9 ft) Displacement (A) = 2.5 tonnes Sail Area (SA) = 20 mM215 sq. ft.) Displacement/length ratio A/(0.01L)' = 104 Aslenderligbtweigbtoutriggercanoewas also deliberately selected to provide a con­ trast with the fii-st type of hull. It was SAIL POWER assumed that this canoe will be fitted with some form of stabilisation (float to wind­ ward) which would not increase the liull resistance. To achieve reasonably good close-hauled performance the canoe AND PERFORMANCE would have a dagger board or leeboard. Her basic measurements were: Length (L) = 9.0m (29.5 ft) Displacement (A) =1.5 tonnes TONY MARCHAJ CONCLUDES HIS INVESTIGAHON SaUArea(SA) = 20 (215 sq.ft.) Displacement/length ratio 'Common sense is not so common." Fig. 2 illustrates the magnitude of the A/(0.01L)' = 58.5 Voltaire 11764) driving force comi>onent (Cv) for three The speed performance calculations were representative points of sailing, selected done on two simplifying assumptions: for coherent examination of thespeed per­ First, the effect of waves was not consi­ fWy he central theme of this lost article formance. These heading angles are: dered, so the predictions of speed made I on sail power is au estimate of the close-hauled, 30 degrees; close reaching, good to windward are likely to be -R- effect of various rigs on the speed 60 degrees; and running, 150-180 de­ optimistic OS compared to real conditions. performanceofthesamehull.lnprinciple, grees. It will be seen that even with one Secondly, the added resistance due to heel the greater the propulsive force, other type of rig there are conspicuous differ­ angle was also ignored. It is known, how­ things being equal, the faster the boat will ences in the driving force, depending en­ ever, that this effect is relatively small up travel. tirely on the course soiled relative to the to about 16 degre^ of heel. As distinct apparent wind. For instance, the Bermu- To refresh readers' memories, Fig. 1 — from pleasure boats, such heel angles are dan mainsail with small jib is more effi­ repeated here from Part 3 — gives the seldom exceed by working Ashing croft. cient on reaching and running than the overall potential thrust produced by all As would be expected, the canoe hull rigs tested. Such a presentation, however, same mainsail with larger jib. These re­ with its much lower displacement/length takes no account of differences in sail sults corroborate earlier tests made by the ratio is consistently faster, but otherwise forces at porticular heading angles rela­ author in connection with the 12 metre tive to the apparent wind. And these dif­ rig. Those tests showed that if the total the relative rankings of the rigs are virtu­ ferences can be quite significant. areaof hendsails is taken into account (not ally identical on either of the hulls. This implies that the choice of rig can be made 62 PRACTICAL BOAT OWNER H =BERIv1UDAN m =LATEEN =SPRIT o =GUNTER iii IIII =LUGSAIL o ft: = =CRABCLAVV RIG TYPES (see key) o 11. KEY TO COMPARISON BAR CHARTS '/// :• CJ Bars of same lype should be read in same order as set out below > • V///, 9 Bermudan + small jib a: f ////. I Bermudan + large jib o Bermudan mainsail only Bermudan with modified mainsaif MM Lateen 1 >»////, 0m Lateen 2 IJ! '^m Lateen 3 CLOSE REACHING RUNNING HAULED Sprit 2 Fig. 2: Showing the comparison of driving forces of rigs in close-hauted, reaching and ^ Sprit 3 running attitudes relative to the apparent wind. Sprit with small jib •W; Gunter il Lugsall : Crabclaw 1.5t CANOE J. 1: Comparison of overall potential potA/er SAIL AREA=20m' rigs tested in windtunnel obtained by mea­ WIND SPEED=12f(ls suring areas underthe driving component Cv plotted versus heading angle relative tothe apparent wind. \1 no matter the type of hull, provided that O the stability and the hull's efficiency in z generating sideforce are comparable. A glance at Figs. 2 and 3 will reveal that •x-^Uyyyyj' theorderofmerit given in tertnsofdi iving 111 »*Kvyyyi' force coefficients (Fig. 2) is reflected in a ^z-Kyyyy'.' »H''y/yyyV predicted speeds (Fig. 3). However, the »-ZiiVy/y.' ••i'Sf-yyyy'" speed differences ore quantitatively less \iwinds, when Wèy'yyyZ' the hull operates in the frictioaal-regime, éihyyyyjiM^üyyyy> and less pronounced in stronger winds •x»éyyyy.* when the hull is driven in the wave-malc- SPEED REACHING RUNNING ing regime. The reason is as follows: in MADE GOOD light winds, when the hull resistance against motion primarily depends on the =BERMUDAN bvdrodynamic friction, there's a nearly "LATEEN stent ratio between the boat speed, the COMPARISON OF SAILING SPEED •••».•: 1 driving force and the wind velocity, .^ail aerodynamic forces due to wind ac­ /^J =SPHIT tion, and water resistance forces actingon X; =GUNTER the hull, both vary approximately at the 9-1 IIII =LUGSAIL same rate; i.e. as thesquareof the wind and 2.5t Dispit Hull SAIL AREA=20mi = =CRABCLAW boatspeeds.Thus,aseitherthe windspeed 8, or the rigefficiency is increased, thespeed WIND SPEED=12kts of the boat must increase proportionally until the balance of aero nnd hydrodynam­ 7 ic forces is reached. For instance: if a boat speed Vs is to be doubled, the hull resis­ 6 yyyy.' tance will increase four times, so the driv­ O ing force delivered by sails must increase 5 in the same proportion. Wave-making re­ sistance at low wind speeds is not impor­ 4 <.yyyy\' tant, and the boat speed then depends ut mainly on friction resistance which is di­ CL ^iyyyy\' rectly related to the wetted surface of the W 3 iyyyyV mm hull and its smoothness. In such a condi­ i^yyyy.' K'yyyy\' mm tion, the differences in speed of boats 2-1 yyyy.' iiyyyyj' driven by the rigs in question will be re­ yyyy.' éyyyy'j ^yyyy.'mm y'Vyyyj' %yyyy\' flected quite distinctly. %yyyy\' %yyyy'j 1 ^f'yyyy.' %yyyy\' iiyyyy'j- This is not longer so in strong winds. The tmim Vyyyyy.' basic relationship between the boat speed and the driving power of the rig is more SPEED REACHING RUNNING complex. Whereastheaerodynamicforces MADE GOOD vary, as before, with the square of the wind, the hull resistance against motion risessharply, andmay increase as much as Fig.3: Predictedspeedsofthetwohulls—a lightweight outrigger canoeandamonohull the fourth or even fifth power of the boat —driven by different ngs to show differences in performance. No 264 DECEMBER 1988 53 speed. This is because of rapidly glowing tt) The u])pcr yard of the snil wo.s Fu nily re^sistonce due lo wnvc-malcing. Thus, if a attached lo the mast and the tack was boat speed Vg Is to be doubled, the driving rigidly controlled from the bow. power should increose as much as sixteen b) Both the tnck and halyard (upper or even thirty-two fold! At the snmc (ime, yard) were eased so that thesail ossumed a the possibilities of achieving such a sail position some distance lo leewai-d of the power increase are limited either by the mast and the bow. stability of the boat or by the strength of Aerodynamic forces were measured for the hull nnd/or rigging. Higher winds heading angles ranging from 20 to 55 de­ malce this situation worse —soils must be grees. The relevant polar diagi'ams of lift reefed or their trim altered to reduce aero­ nnd drag coefficients, CL and Cu, are pre­ dynamic forces. This radically changes sented in Fig. 6. It will be seen that at a the relation between sail power and boat heading angle of 30.4 degrees the sail speed. As a result, differences in boat firmly controlled (combination a) devel­ speed driven by rigs of differentiated po­ ops about 16 per cent more driving force tential power shown in Fig. 2 may be very than the sail set loosely (combination b). small or even negligible. The difference in sail performance, ini­ It should be added that the comparison tially negligible at small heading angles, ofsailing speed in Fig. 3 is relevant to the increases when the boat bears away. true wind speed Vj- = 12 luiots (Force 4 The deterioration in performance when Beaufort) in which wave-maldngbegins to tack and upper yard are not rigidly at­ contribute to the total resistance of the tached to the hull and most is due to the boat. The foUowingapproximate relation­ difficulty in controlling the camber and ship between the increase in the driving Fiq.6: Effect of two different rigging meth¬ twist of the sail. Theoiy and experiments ods of Crab Claw sail on lift and drag. power ofa rigand tbechange in boat speed both agree that the basic conditions for obtaining high efficiency from the Crab % change in driving power. % change in boat speed Vs Clow are: no camber, no twist. Figure 7 illustrates the meaning of the 0.4 10.0 10.0 term camber as related to this particular 0.8 10.0 2.0 sail. The preferable (and achievable) cur­ vature between the upper and the lower 1.4 10.0 1.1 yard, through sections A-A nnd B-B, is marked by number 3. The soil should be as (expressed in terms of speed length ratio) on lift and drag. Some details of these flot as possible. The shape marked 1, with applies reasonably well to most monohuU settings together with the result of mea­ bulges close to the yards is undesirable if boats. surements are shown in Fig. 5. As the sail With the above in mind, it should scarce­ pos ition is altered from high to low, thot is 1 2 3 ly liesurprising that theCrab Claw rigdoes by increasing the sweepback angle, the not stand out in Fig. 3 as superior to the maximum lift coefficient rises substan­ r f others, as the wind tunnel resuhs (Fig. 1) tially from 1.5 to 1.9, i.e. by about 25 per might imply. It will be seen in Fig. 3 that in cent. This advantageous shift towards close hauled condition this rig is marginal­ higher lift is, however, associated with ly better than other single sail rigs except disadvantageous reduction in lift/drag Sections through Lateen No. I, but gains impressive super i- ratio which controls close hauled perfor­ A-A and B-B ority over all rigs in broadreaching.Tbis is mance. Thus, to achieve best speed to shown in Fig. 4 which compares speed windward the sail should be set in the performances of sailplans tested on the medium position, but for reaching the best Fig. 7: This Indicates the cross sections best course for the Crab Claw rig. Restdts efficiency is obtained when thesailissetin (camber) ofthe Crab Claw sail. are relevant to the canoe hullsailingatlSO low position. High position offers no ad­ degi'ees to 12 Icnots true wind. vantage in either respect; it produces maximum lift is to be obtained. The pri­ Tests indicated that the efficiency of the neither large lift nor high lift/drag ratio. mary function of the leading edge of a Crab Claw sail is sensitive to the way it's The way the Crab Clawsail is rigged and slender foil — and the Crab Claw type of set relative to the mast (sweepback angle). its shape controlled also greatly affects its sail belongs to this category — is to fix the Three different positions of sail were in­ efficiency. Two different systems were in­ flow separation line from which strong, vestigated in order to establish this effect vestigated, namely: conical vortices roll-up. These generate lift. Straight, rigid edges ensure intense growth of these vortices. On the other Fig. 4: Speed prediction of all sailplans Fig.SrLiftdragofCrabClawtestedinthree hand, blunt, round edges behind the yards tested, when broad reaching on a canoe ditferent positions relative to mast (differ- — bulges as shown in Fig. 7, section 1 — hull. Best heading for Crab Claw rig. ent sweepback angles). preclude the generation of strong and ef­ fective vortices. In heavier winds it may, however, be­ come necessary to put a limit on the force produced by the Crab Claw sail. This can be achieved by allowing the lower yard to move up and thus reduce the distance between yards. As a result, the sail camber will shift toward that shown in Fig. 7, section 1. This may cause sufficient reduc­ tion of the aerodynamic force to suit pre­ vailing wind strength and the stability of the boat. Such n deliberate modification of the sail shape is similar in its effect to reefing. The infiuence of the leading edge on the performance of the Crab Claw can be seen in Fig. 8. It reveals variation of the driving force coefficient Cx with the apparent wind angle for onesailmodified in shape in two different ways as follows: In the first series of tests, the driving power of the Crab Claw sail with straight yards was established. The upper curve in Fig. 8 pre­ sents the results. Such a sail, set in the low position, closely resembles in its planform

54 PRACTICAL BOAT OWNER Tho astonishing crab claw in action cannot doubi llmtthecaudal(tail) finpluys nn important part ingeneratingthethrusi that fishes exhibit.

REFERENCES TO NOTES IN ARTICLE /. C. A Marchaj Sailmg Theory and Practice Adiard Coles. UK I9S2. 2. Analysis ol Wind Tunnel Data on Representative Anisanal Fishing Boat Rigs. Rep 3446/01 1985. GiHondandPanners.Computeranatysiscarriednutai Ihe request olMacAlisterElliot Fanners Ltd. 3 efficiency Characteristics of Crescent-Shaped Wings and Caudal Fins C. P. Van Dam —Nature. 29Januar/ 1987 4. Minimum Induced Drag of Wings with Curved Planform d.^kenbergandO. Weis lofAiwmft. January 5. Animal Locomou'on Sir James Gray — Publ Weidenfeld and Nicotson London 1968.

The Crab Claw type of rig, although of lower aspect ratio lhan that of caudal fins shown in Fig. 9, belongs to the same cate­ gory of foils. It should perhaps be added that the winglets attached to the keel ofthe 12 Metre Star and Stripes — victorious Americau Chollenger in the 1987 Ameri­ ca's Cup contest — have planform of that the notoriously poor Lateen sail No. 3 (see ference, bearing in mind that at first sight type. B. 1 and 2) with high degree of sweep- all other factors may appear to be the Such shapes were invented and practi­ .clt. Presence of the lower yard makes same. cally applied some hundreds years ago by ine only difference. One more peculiar lift-producing plan- the Polynesian people, who must have de­ Subsequently, the lower yard was re- form deserves mention. Due to the action veloped them by trial and error, probably movedso the canvas took a typical shape of of selective evolution operating in Nature, inspired by clever observation of efficient the Lateen sail — no longer rigidly sup- mony aquatic anim.als that cruise fast and pored along its foot and hence much more sometimes for long distances, such as dol­ flexible with large camber and twist. The phins, tunnyfish, swordfish, mackerel- measurements were then repeated for the shark, whale (Fig. 9) have developed cau­ same range of apparent wind angles. The dal fins (foils) of the crescent-moon shape. results are depicted by the lower curve in Also, wings of certain efficient soaring Fig. 8. It's evident that the lack of support birds, such as albatross, display chaiac- by the lower spar has a shatteringeffee t on teristicbackwardcui-vatureoftheleading sail power. At the heading angle 30 de­ and trailing edges. grees (i.e. in close-hauled attitude, see One of the claims of classic, low speed points A and A' marked on the curves) the aerodynamicsb thattheminimum diagof Tlio mackerel-shark Lamna Crab Claw develops about 45 per cent a wing, or any lift generating device for more thrust than the same sail supported that matter, is obtained on an untwisted by the upper yard only, i.e. the so-called elliptical planform. It's believed that the Lateen configuration. In close reaching well knovra SpitRre aeroplane enjoyed conditions, at the heading angle 50 de­ some of its wartimesuccess from itsellipti- grees (see points B and B') the Crab Claw cal wing form. rig develops about twice as much thrust as Onemayaskwhy, aftermillionsofyears the Lateen type of soil! An enormous dif- of evolution. Nature should produce pecu­ liar, moon-like shaped foils (Fig. 9) when it's generally known (since M. Munk Tho tunrtytish Thunnus ) 1 proved it mathemaUcally — see NACA > Rept. 121, published in 1921) that the elliptical planform is the most efficient 1 lifting surface? i Theanswerisratherstraightforward— apparently Nature knows the subject bet­ B , ter than the most able mathematicians. More recently, some scientists (Ref. 3 and 4) have shown that crescent shaped foils, with backward curvature of the leading The swordfishX/pA/i/s o / Crab Claw Sail edge, are more efficient: they produce more lift for given drag than the elliptical plonforms considered best in classical wing theory. Any study of fish locomotion must con­ sider how a fish can product the thrust f / A. * • needed to overcome the water resistance / ateen Sail and maintain the speed observed. It has 1, been found that a tunnyfish about the size A' of o man can swim ten times as fast as the Fig. 9: Nature appears to favour the cres­ / cent-shaped fins (with backward curva­ II Olympic champion! And certain fishes ture of the leading edges) such as tail fins may produce an acceleration ot4gin their and bird wings. lethal lungingattack (Ref. 5). Fishes have 0- 10' 20- 30' 40- 50- 60' been evolving for hundreds of millions of forms produced by Nature. This ties up years, and we know that in this process of Apparent wind angle with the remark expressed by Sir d'Arcy 8 evolution any quality, such as speed, that Thompson (1860-1948) in his book i ncreoses the chance of su rvi val—a sort of "Growth and Farm": "There is never a Fig. 8: Driving force developed by the same "survival value" — is most likely to get discovery made in the llieory of aerody­ sail butin two different configurations: asa moreand moreincoi-poratedinto thechar- namics but we find it adopted already by Crab Claw and a Lateen rig. acteristics of succeedinggenerations. One Nature." « No 264 DECEMBER 1988 55