Sail power and performance

SAIL POWER

AND PERFORMANCE

TONY MARCHAJ CONCLUDES HIS INVESTIGAHON

'Common sense is not so common." Voltaire 11764)

fWy he central theme of this lost article I on sail power is au estimate of the -R- effect of various rigs on the speed performanceofthesamehull.lnprinciple, the greater the propulsive force, other things being equal, the faster the boat will travel.

To refresh readers' memories, Fig. 1 — repeated here from Part 3 — gives the overall potential thrust produced by all rigs tested. Such a presentation, however, takes no account of differences in sail forces at porticular heading angles rela-tive to the apparent wind. And these dif-ferences can be quite significant.

Fig. 2 illustrates the magnitude of the driving force comi>onent (Cv) for three representative points of sailing, selected for coherent examination of thespeed per-formance. These heading angles are: close-hauled, 30 degrees; close reaching, 60 degrees; and running, 1 5 0 - 1 8 0 de-grees. It will be seen that even with one type of r i g there are conspicuous differ-ences in the driving force, depending en-tirely on the course soiled relative to the apparent wind. For instance, the Bermu-dan mainsail with small jib is more effi-cient on reaching and running than the same mainsail with larger jib. These re-sults corroborate earlier tests made by the author in connection with the 12 metre rig. Those tests showed that if the total areaof hendsails is taken into account (not

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, consistently poor performers such ns the L a -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. L a -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 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 would have a dagger board or leeboard. Her basic measurements were:

Length (L) = 9.0m (29.5 ft) Displacement (A) =1.5 tonnes SaUArea(SA) = 20 (215 sq.ft.) Displacement/length ratio

A/(0.01L)' = 58.5 The speed performance calculations were done on two simplifying assumptions: First, the effect of waves was not consi-dered, so the predictions of speed made good to windward are likely to be optimistic OS compared to real conditions. Secondly, the added resistance due to heel angle was also ignored. I t is known, how-ever, that this effect is relatively small up to about 16 degre^ of heel. A s distinct from pleasure boats, such heel angles are seldom exceed by working Ashing croft.

As would be expected, the canoe hull with its much lower displacement/length ratio is consistently faster, but otherwise the relative rankings of the rigs are virtu-ally identical on either of the hulls. This implies that the choice of rig can be made

RIG TYPES (see key) KEY T O COMPARISON BAR CHARTS Bars of same lype should be read in same order as set out below

9 Bermudan + small jib I Bermudan + large jib

Bermudan mainsail only Bermudan with modified mainsaif

MM Lateen 1

0m Lateen 2

'^m Lateen 3

Sprit 2 ^ Sprit 3

Sprit with small jib • W ; Gunter

i l Lugsall : Crabclaw

J. 1: Comparison of overall potential potA/er

rigs tested in windtunnel obtained by mea-suring areas underthe driving component Cv

plotted versus heading angle relative tothe apparent wind.

no matter the type of hull, provided that the stability and the hull's efficiency in generating sideforce are comparable.

A glance at Figs. 2 and 3 will reveal that theorderofmerit given in tertnsofdi iving force coefficients (Fig. 2) is reflected in predicted speeds (Fig. 3). However, the speed differences ore quantitatively less conspicuous than otherwise might be ex-pected from an inspection of driving forces alone. I n general, the differences in speed will be larger in light winds, when the hull operates in the frictioaal-regime, and less pronounced in stronger winds when the hull is driven in the

wave-malc-ing regime. The reason is as follows: in

light winds, when the hull resistance against motion primarily depends on the bvdrodynamic friction, there's a nearly stent ratio between the boat speed, the 1 driving force and the wind velocity, .^ail aerodynamic forces due to wind ac-tion, and water resistance forces actingon the hull, both vary approximately at the same rate; i.e. as thesquareof the wind and boatspeeds.Thus,aseitherthe windspeed or the rigefficiency is increased, thespeed of the boat must increase proportionally until the balance of aero nnd hydrodynam-ic forces is reached. F o r instance: if a boat speed V s is to be doubled, the hull resis-tance will increase four times, so the driv-ing force delivered by sails must increase in the same proportion. Wave-making re-sistance at low wind speeds is not impor-tant, and the boat speed then depends mainly on friction resistance which is di-rectly related to the wetted surface of the hull and its smoothness. I n such a condi-tion, the differences i n speed of boats driven by the rigs in question will be re-flected quite distinctly.

This is not longer so in strong winds. The basic relationship between the boat speed and the driving power of the r i g is more complex. Whereastheaerodynamicforces vary, as before, with the square of the wind, the hull resistance against motion risessharply, andmay increase as much as the fourth or even fifth power of the boat No 264 D E C E M B E R 1988 o iii o ft: o 11. CJ

>

IIII

IJ!

Fig. 2: Showing the comparison of driving forces of rigs in close-hauted, reaching and running attitudes relative to the apparent wind.

\1

MADE GOOD REACHING

C O M P A R I S O N O F S A I L I N G S P E E D 9-1 8, 7 6 5 4 3 2-1 1 2.5t Dispit Hull SAIL AREA=20mi WIND SPEED=12kts RUNNING =BERMUDAN " L A T E E N •••».•: / ^ J =SPHIT X ; =GUNTER

IIII

tmim

mm

mm

mm

MADE GOOD REACHING RUNNING

Fig.3: Predictedspeedsofthetwohulls—a lightweight outrigger canoeandamonohull —driven by different ngs to show differences in performance.

speed. This is because of rapidly glowing re^sistonce due lo wnvc-malcing. Thus, if a boat speed Vg Is to be doubled, the driving power should increose as much as sixteen or even thirty-two fold! At the snmc (ime, the possibilities of achieving such a sail power increase are limited either by the stability of the boat or by the strength of the hull nnd/or rigging. Higher winds malce this situation worse —soils must be reefed or their trim altered to reduce aero-dynamic forces. This radically changes the relation between sail power and boat speed. As a result, differences in boat speed driven by rigs of differentiated po-tential power shown in Fig. 2 may be very small or even negligible.

It should be added that the comparison ofsailing speed in Fig. 3 is relevant to the true wind speed V j - = 12 luiots (Force 4

Beaufort) in which wave-maldngbegins to contribute to the total resistance of the boat. The foUowingapproximate relation-ship between the increase in the driving power ofa rigand tbechange in boat speed

0 . 4 1 0 . 0 0 . 8 1 0 . 0 1 . 4 1 0 . 0

(expressed in terms of speed length ratio) applies reasonably well to most monohuU boats.

With the above in mind, it should scarce-ly liesurprising that theCrab Claw rigdoes not stand out in Fig. 3 as superior to the others, as the wind tunnel resuhs (Fig. 1) might imply. It will be seen in Fig. 3 that in close hauled condition this rig is marginal-ly better than other single sail rigs except Lateen No. I , but gains impressive super i-ority over all rigs in broadreaching.Tbis is shown in Fig. 4 which compares speed performances of sailplans tested on the best course for the Crab Claw rig. Restdts are relevant to the canoe hullsailingatlSO degi'ees to 12 Icnots true wind.

Tests indicated that the efficiency of the Crab Claw sail is sensitive to the way it's set relative to the mast (sweepback angle). Three different positions of sail were in-vestigated in order to establish this effect

Fiq.6: Effect of two different rigging meth¬ ods of Crab Claw sail on lift and drag.

% change in boat speed V s 1 0 . 0

2 . 0 1.1

on lift and drag. Some details of these settings together with the result of mea-surements are shown in Fig. 5. As the sail pos ition is altered from high to low, thot is by increasing the sweepback angle, the maximum lift coefficient rises substan-tially from 1.5 to 1.9, i.e. by about 25 per cent. This advantageous shift towards higher lift is, however, associated with disadvantageous reduction in lift/drag ratio which controls close hauled perfor-mance. Thus, to achieve best speed to windward the sail should be set in the medium position, but for reaching the best efficiency is obtained when thesailissetin low position. High position offers no ad-vantage in either respect; it produces neither large lift nor high lift/drag ratio.

The way the Crab Clawsail is rigged and its shape controlled also greatly affects its efficiency. Two different systems were in-vestigated, namely:

% change in driving power.

Fig. 4: Speed prediction of all sailplans Fig.SrLiftdragofCrabClawtestedinthree tested, when broad reaching on a canoe ditferent positions relative to mast (differ-hull. Best heading for Crab Claw rig. ent sweepback angles).

tt) The u])pcr yard of the snil wo.s Fu nily attached lo the mast and the tack was rigidly controlled from the bow.

b) Both the tnck and halyard (upper yard) were eased so that thesail ossumed a position some distance lo leewai-d of the mast and the bow.

Aerodynamic forces were measured for heading angles ranging from 2 0 to 5 5 de-grees. The relevant polar diagi'ams of lift nnd drag coefficients, C L and C u , are pre-sented in Fig. 6. It will be seen that at a heading angle of 3 0 . 4 degrees the sail firmly controlled (combination a) devel-ops about 16 per cent more driving force than the sail set loosely (combination b). The difference in sail performance, ini-tially negligible at small heading angles, increases when the boat bears away.

The deterioration in performance when tack and upper yard are not rigidly at-tached to the hull and most is due to the difficulty in controlling the camber and twist of the sail. Theoiy and experiments both agree that the basic conditions for obtaining high efficiency from the Crab Clow are: no camber, no twist.

Figure 7 illustrates the meaning of the term camber as related to this particular sail. The preferable (and achievable) cur-vature between the upper and the lower yard, through sections A - A nnd B - B , is marked by number 3. The soil should be as flot as possible. The shape marked 1, with bulges close to the yards is undesirable if

1 2 3

r

f

A - A and B - B

Fig. 7: This Indicates the cross sections (camber) ofthe Crab Claw sail.

maximum lift is to be obtained. The pri-mary function of the leading edge of a slender foil — and the Crab Claw type of sail belongs to this category — is to fix the flow separation line from which strong, conical vortices roll-up. These generate lift. Straight, rigid edges ensure intense growth of these vortices. On the other hand, blunt, round edges behind the yards — bulges as shown in Fig. 7, section 1 — 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 C x with the apparent wind angle for onesailmodified in shape in two different ways as follows: I n 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

Tho astonishing crab claw in action

the notoriously poor Lateen sail No. 3 (see B. 1 and 2) with high degree of sweep-.clt. Presence of the lower yard makes ine only difference.

Subsequently, the lower yard was re-movedso the canvas took a typical shape of the Lateen sail — no longer rigidly sup-pored along its foot and hence much more flexible with large camber and twist. The measurements were then repeated for the same range of apparent wind angles. The results are depicted by the lower curve in Fig. 8. It's evident that the lack of support by the lower spar has a shatteringef feet on sail power. At the heading angle 30 de-grees (i.e. in close-hauled attitude, see points A and A' marked on the curves) the Crab Claw develops about 45 per cent more thrust than the same sail supported by the upper yard only, i.e. the so-called Lateen configuration. I n close reaching conditions, at the heading angle 50 de-grees (see points B and B') the Crab Claw rig develops about twice as much thrust as the Lateen type of soil! A n enormous

dif-o )

1

/ Crab Claw Sail

A. f / / *

•

1,

/

No 264 D E C E M B E R 1988

ference, bearing in mind that at first sight all other factors may appear to be the same.

One more peculiar lift-producing plan-form deserves mention. Due to the action of selective evolution operating in Nature, mony aquatic anim.als that cruise fast and sometimes for long distances, such as dol-phins, tunnyfish, swordfish, mackerel-shark, whale (Fig. 9) have developed cau-dal fins (foils) of the crescent-moon shape. Also, wings of certain efficient soaring birds, such as albatross, display chaiac-teristicbackwardcui-vatureoftheleading and trailing edges.

One of the claims of classic, low speed aerodynamicsb thattheminimum diagof a wing, or any lift generating device for that matter, is obtained on an untwisted elliptical planform. It's believed that the well knovra SpitRre aeroplane enjoyed some of its wartimesuccess from itsellipti-cal wing form.

Onemayaskwhy, aftermillionsofyears of evolution. Nature should produce pecu-liar, moon-like shaped foils (Fig. 9) when it's generally known (since M. Munk proved it mathemaUcally — see NACA Rept. 121, published in 1921) that the elliptical planform is the most efficient lifting surface?

Theanswerisratherstraightforward— apparently Nature knows the subject bet-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 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 needed to overcome the water resistance and maintain the speed observed. It has been found that a tunnyfish about the size of o man can swim ten times as fast as the Olympic champion! And certain fishes may produce an acceleration ot4gin their lethal lungingattack (Ref. 5). Fishes have been evolving for hundreds of millions of years, and we know that in this process of evolution any quality, such as speed, that i ncreoses the chance of su rvi val—a sort of "survival value" — is most likely to get moreand moreincoi-poratedinto thechar-acteristics of succeedinggenerations. One

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 type.

Such shapes were invented and practi-cally applied some hundreds years ago by the Polynesian people, who must have de-veloped them by trial and error, probably inspired by clever observation of efficient

Tlio mackerel-shark Lamna

Tho tunrtytish Thunnus

The swordfishX/pA/i/s

Fig. 9: Nature appears to favour the cres-cent-shaped fins (with backward curva-ture of the leading edges) such as tail fins and bird wings.

forms produced by Nature. This ties up with the remark expressed by S i r d'Arcy Thompson ( 1 8 6 0 - 1 9 4 8 ) in his book "Growth and Farm": "There is never a discovery made in the llieory of aerody-namics but we find it adopted already by

Nature." «