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PTTL LULU LE LLOLLoL Lo S1N NUTT TTL LULL LUP LULL S. Witty eee writing SAILS & ee Ee on SAIL BALANCE introduces Z>\. ‘ LJUNGSTROM REDWING his 10 ~ ~ RATER ECAUSE the performance of any yacht, however good the hull, is limited primarily by the efficiency of its sail plan, this short article attempts to summarize past and possible future advances in this field. Since the inception of the modern “Bermuda” rig the height of the sails in proportion to their width has gradually been increasing until now this relationship, known as aspect-ratio, has reached the figure of four to One or even more. While the theorists tell us that this is always an advantage, at least to windward, I myself am not so sure, as, apart from the difficulty of controlling such tall sails I have-doubts that the increase in efficiency is actually great enough to compensate for the considerable loss in stability which results. There have been so many experiments with different rigs and sailplans that it is very difficult to think of something original in this field. Some time ago we experimented with a new 10R fitted with a ten-tenths rig and a double luff mainsail (a ten-tenths rig has the foresail hoist carried above or at the top of the mainsail) and was found to be very tricky to manage indeed, also, owing to the double thickness at the luff the sail was heavy and would not fill properly in light weather. Even- tually an ordinary mainsail was fitted and, somewhat to our surprise, it was found that the mast position had to be moved in inch forward, indicating that the sail was exerting much more thrust than before. Streamlined masts have been tried again and again, both of the fixed and swivelling varieties, without any definite advantage being noticed except, surprisingly enough, downwind, when with the latter type the more or less flat side of the mast adds a FORESAIL RIG 1957 THe ee AUGUST, little unmeasured area. Part of the trouble here is that the sail is not quite the same angle all the way up, even utilising the heaviest kicking strap; so that somewhere along the luff the mast is out of slignment. With all types of streamlined mast this misalignment becomes increasingly worse towards the sail hoist so that at this point the mast presents a definite obstruction to the airflow with resultant deterioration in the stability and general perform- ance. Another difficulty in this respect is tLat with a free rotating mast there is naturally no end rigidity and as a result the mast has to be made rather stronger than is necessary with a fixed type so that the frontal area of the mast even when lined up is always greater than that presented by a modern small diameter metal mast. The stayless Ljungstrom rig is reputed to be able to sail very close to the wind due to the fact that the sail is taken around the mast thus allowing an almost perfect airflow on the lee of the sail, but since the mast must revolve through 360° for each tack this is hardly a practical proposition for a model. In a normal sail plan the respective propor- tion of fore to mainsail has always been a somewhat controversial subject. All yachts with the exception of a few old “Braine” type models with the long skeg extension, have a marked tendency to turn into the wind and to move the sailplan forward to negative this ts a poor compromise since if this is overdone the yacht is unable to point up in light airs. With a large foresail this effect can be minimised, resulting in steadier sailing to windward, but it is true to say that if this is over- done a marked loss of efficiency occurs and 377 i) MODEL MAKER the yacht is unable to sail really close to the belly to form in the luff of the mainsail resulting in a poor airflow, particularly on the windward surface, this effect becoming progressively wind. The modern high aspect ratio sailplan is often compared with an aircraft wing fitted with a leading edge slot but since these slots do not usually exceed 10 per cent. of the wing chord it is not really a good comparison, as quite a lot of yachts carry foresails, the base triangle of which is equal to the length of the mainsail foot. While this slot effect is undoubtedly important for windward work the arrangement is more comparable to that of a biplane with closely-spaced and well-staggered wings, a combination which has incidentally been found to be inefficient in its application to aircraft, both in practice and in wind tunnel tests, owing to the working surfaces being so close together as to cause interference losses and a reduction in lift. While the efficiency may be low with this arrangement, the inherent balance is fairly good because while all single sails have, like curved aerofoils, a tendency to “turn over the barrel” (that is to follow the arc formed by the curvature of the sail) with two sails more or less in tandem, this effect 1s broken up. A disadvantage of the standard rig is that the foresail is tightened in hard to achieve the maximum slot effect, this causes _TOLTEC = 6 MODEL MAKER RO. iA oo WATFOR PN ZL i Mi | tas | : sail foot increases. A reason for the good balance of the normal sailplan as compared to other types is that in gusts of increasing velocity the foresail increases its efficiency more quickly than the mainsail since the turbulance from the mast over the working surface increases rapidly with the airspeed, reducing the power of the mainsail more in proportion. Much of this balance is unfortunately thrown away by yachtsmen who cannot be bothered to fit a proper rigid arm and kicking strap under the foresail, often with the excuse that the sail will not fill properly with such an arrangement; I myself find the opposite to be the case. Even when this system is used the boom is often positioned so low that: the strap is too small to be effective because the yachtsman is unwilling to sacrifice fore triangle area which is rated only 85 per cent. of normal in order to obtain a rather smaller but much more effective foresail. The only other alternative is to mount the attachment part of the way along the foresail boom, ‘ S—S— SERVICE we eee c[p LEAD was uss\ WL 3 \ \\ : we. oe cle HULL 28-25 us \ wu A A Oy ee ee i ey a ; NS BSei ba ORF eee pee SRS ee a RUA) er i i aN , wh “ACY x WEATS nse ry 38, CLa® HENDON PLANS 2 sa worse as the proportionate length of the fore- 378 — et = = AUGUST, thus rendering a strap not quite so necessary. Since such an arrangement brings the foresail pivot point nearest the mast the curvature of the sail is more able to follow that of the mainsail, which results in an improvement in airflow but a loss of balance. As often happens in yacht design it is a case of what is gained on the roundabouts is lost on the swings. Some years ago a very interesting book was published. “Yacht Racing and the Aero-Dynamics of Sails’ by Dr. Manfred Curry in which it was explained that the main driving force is derived from suction on the lee of the sail rather than pressure to windward and that this force is derived largely as a result of the action of the foresail. This was misunderstood in some quarters and in America a full-size six-metre yacht Atrocia was built which had a huge foresail and a very narrow mainsail; a combination which proved quite sailable but not competitive. Nevertheless a type of sailplan which might bear looking into can be achieved. by putting the whole of the working area into the foresail as shown in the sketch. This arrangement should be con- siderably more efficient than the Ljungstrom or Bermuda rig in so far as there is no turbu“oo Is vi 6 : 12 93.757 Ww = 5 o _— o 59-5″ <4 lin 3 52:5 = 45:5 7 as ‘2 lence caused by the mast over the working area at all plus the added gain which is obtained by concentrating all the area in one sail allowing larger and more efficient aerofoil sections. Such a sailplan would naturally have its own attendant snags, not the least of which would be structural since the mast must carry a terrific downthrust, particularly when running. I think also that there might be some slight difference of opinion regarding the maximum permissible spinnaker size but if these problems can be solved the rig may prove useful in its application both to model and full-size yachts. The new sail materials now available result in a considerable improvement in performance especially in medium and heavy water. This improvement is often quoted at twenty per cent. but there is at least one case or record where a big “M” class race was won by the only yacht having cotton sails, all the rest of the field using terylene; while this may be the exception which proves the rule, sails made of this material do not seem to pull quite so well in light weather. Also due to its having two dimensional rigidity, any small amount of curvature in the mast can cause the appearance of ripples in the luff. Toltec has been designed for those yachtsmen who do not wish to try any new-fangled notions but who want to build a really good straightforward ten rater. Before drawing the first line I resolved to find out the most popular requirements of the enthusiasts sailing this class. While such information is often difficult to analyse because there are so many conflicting opinions, it would seem that the average dimensions, etc., of a good all-round ten-rater should lie between the following limits: L.W.L. Draught wn brs Disp. < HEELED 25°(TO LEE DECK RAIL) ALSO CURVE IN “i Hep aca 35815 ae Sail A.R. a WEDGE AT 25° a ||Eee,tea =e A fas, 1957 54in. to 55 in. 11in. to 12in. 27 to 29 lbs. 3.5 approx. Also most people seem to regard a beam dimension of 12in. as more or less essential for any sort of performance in heavy weather. Most yachtsmen look on the ability of the hull to plane as a great asset even if only as fa “safety valve” to prevent broaching on the run. All these requirements have been developed into Toltec in a manner likely to prove attractive to the majority and she should prove a very robust yacht with a fine performance whatever the strength and direction of the wind. Ml MODEL MAKER 1957 National MARBLEHEAD Championship Witton Lakes, June 8th, 9th, 10th pitts A LAST minute cancellation by two boats left only fourteen boats to compete in the 1957 Marblehead Championships, held at Witton Lakes, Birmingham, over the Whitsun weekend. Out of the seventeen entries one had scratched the week previous, and as preparations had gone forward with a sixteen boat card, the first round was sailed giving a bye to two boats in each heat. Time permitted a complete two rounds and it was possible to rearrange the race card to avoid byes in the second round. The race started at 2.30 on the Saturday, in a light to moderate south westerly wind, giving a broad reach out and a close reach home. A thunderstorm terminated activities at 7 o’clock by leaving absolute calm in its wake, but by this time nine heats had been sailed, although due to the byes the scores meant very little. More south in the light breeze on Sunday meant that the closer trimmed boats just fetched the leeward mark on their first leg, although later in the day the wind veered further and some boats sailed tack and tack. At the end of the first round the order of boats was as follows:— Gretel ... 5 is Tweedle-dee & Nymph Zoe ee am: os 43 42 40 Flamingo oe ee. Gannett & Hustler... 37 33 Hopalong a a Se Second place boat Grete] starts a board with Puffin in our heading picture. Below, left to right, Hopalong tied with Nymph for third place but was two lengths out in the sail-off; Tweedle-Dee and her skipper; twelfth placer Puffin, less camera-shy than most of the other boats, was one of several Duck (or Duck-influenced) designs sailing 382 AUGUST, Towards evening the wind dropped and the programme slowed down. 1957 Bournville, just managed to hold off his club- When six heats of mate Ronnie Harris with Gretel. Tweedle-dee is, incidentally, a Littlejohn design. The sailoff for third place was a very close race, and A. H. Garbutt, of Bristol, with Nymph, having and Zoe were tying for third with 59 each, and just made the leeward flag two lengths in front of J. G. Meir’s Hopalong. the second round had been sailed a halt was called in proceedings for the night, and at this point Gretel still led with 63 points, Hopalong had moved to second with 61, and Tweedle-dee although Tweedle-dee had a resail in hand. On Monday morning a steady N.N.W. breeze with more power in it obtained, and some lively sailing resulted. Four of the last seven heats had been completed by lunchtime and at this stage there were still only five points between the first five boats. Gretel and Tweedle-dee stood at 73, Hopalong 72, Zoe 71 Nymph 68.- In the first heat after lunch all four leading boats lost their runs, and Nymph taking five off Gretel moved level with her into third place, Tweedle-dee and Zoe having won their windward boards. Gretel came back with five in the next heat, Tweedle-dee took her beat and Nymph her run, and with only one more heat to go their scores were 78, 79 and 75 respectively, and the final result was still very much in doubt. In the final heat Tweedle-dee and Gretel lost their runs, Nymph won hers, but whilst the first two won their beats Nymph lost hers. Hopalong got her five and drew level with Nymph for third place. In a most exciting finish Tweedle-dee, sailed by the young skipper Mick Widdows, of lost the toss, sailed from the leeward’ berth One of the notable features of the race was Spero’s performance in the second round: she took five points in each of the last five heats, and achieved the highest score of this round with a total of 47. Final results were:— |. 2. 3. Tweedle-dee Gretel ... Nymph 4. Hopalong 5. Spero ... 6.2067" 3s 7. Flamingo 8. Gannet Fs EChO. 2... 10. Hustler Il. Cresta ... 12. Puffin ... 13. Witch ... 14. Nimrod... The scratched ... rs ins bse aes 5 ok
