Sail Power and Performance

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-Ûyyyyj' 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 \i<i\yyyyy\' conspicuous than otherwise might be ex S\^éyyyy.' pected from an inspection of driving wiyyyy.' •x-^:iyyyy\' forces alone. In general, the differences in \\izVyyy\' •sA^yyyy'j' speed will be larger in light winds, 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, .âil 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 îyyyy\' 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.

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