- Description of contents
WAAL \4 : —— inl MODEL MAKER} ITH the advent of the present trend towards ultra heavyweight ten-raters, many yachtsmen owning lighter hulls of less than 30 Ib. or so tend to be a bit morbid about their chances in the larger open events. Without doubt this is due to a large extent to the success of the current Lewis designs, although not so long ago a boat of 301b. was regarded as a heavyweight. On reflection, though, this is the usual effect when any yacht using a different design approach makes good. I remember the flurry Triplane made when she won the championship. Sum total of popular opinion at that time was (quote) “‘If that’s how a ten-rater ought to look we might as well put an axe through the rest”. I believe quite a few skippers at that time felt that a yacht so unbalanced by accepted standards and so different had no business to perform so well, let alone win. Scaling about 21 Ib. she was a real lightweight even then, and so was Restive at 23°5 Ib., an elongated ‘‘M”’ design which just missed taking the Championship in 1952. The following year again saw a lightweight triumph Ke 10 R&TER OfSION % Gt BLUEFIN NI See in this event. A few more performances like these might very possibly have started a trend towards lightweight ten-raters, but instead the more stereotyped RO, WATFORD, and except The Flying Fifteen concept was not taken up by the majority of yachtsmen, it being possibly a case of “All, or nothing at all”, with attempts at improving the balance, etc., resulting in mediocrity. Probably 35lb. represents the acceptable limit of 10 R displacement, and any further increase will result in an unacceptable downwind performance, however good she might be on the beat. For the design shown, however, the medium displacement of 301b. was chosen, which, in conjunction with her twelve-inch plus beam and draught, will enable her to hold her own with the heavies in hard weather to windward, while still being light enough to take the points coming back. Compared with previous design Toltec, she is approximately 30 per cent. more powerful with little, if any, reduction in either light —— t f ee MODEL MAKER PLANS SERVICE CLARENDON prevailed, are not readily apparent except to the expert. S. Witty. 38. form for the increase in size, changes in the hull form — HEARTS aie > } ——————— fou iy 4 ww (I wwe x is Ld WEEN T iad el, : £ 209 ave See SSS LA — Seaways Senn SES wli2. u eed suit 1ST 2nc 3RO 4TH =a au eens Saics SAI, AREA –~————4, SECTIONS SPACED——– 4° 5 26 — ES ser Ses JANUARY, weather or planing ability. Curiously it is not in itself too difficult to design a hull with good heavy weather characteristics, but this is not enough. Due to the vagaries of our summer weather and the widely differing local conditions of the various ponds, the design must also be capable of winning at the other end of the scale. Apart from obvious sail to wetted hull area considerations, performance in light weather depends to a large extent on the angle of the bow waterlines, as is generally recognised in full scale practice. (Models are different, but not that much.) As the waterline bow angle becomes finer, so do the light weather capabilities tend to improve. Briefly, at low speeds the water flow remains in laminae and passes around the hull, following the shape of the waterlines, hence there is little or no wave formation. As the speed increases the water cannot get around quick enough this way, and is forced to take the shortest path under the hull until on a full wave system there is very little cleavage at all, and the flow lines remain largely in parallel. To facilitate the action of the former I decided that the y —— ——— J iat ! | eS ae ee +e Ny Stee = 1959 AN INTERESTING NEW TEN-RATER DESIGN Bluefin BY S. WITTY waterline itself should form a fair curve from stem to stern. A reasonably shallow hull section takes care of the latter even when heeled to the deck edge. In recent years it has been standard practice to rake the fore edge of the fin approximately 40 deg., but with a fin deeper than 11 in. the angle must be steepened to allow for a low lead position. Providing the sections are sharp and slim this has no ill effect. Previous theory on this was that a very steep angle would cause the yacht to gripe, but it seems that this only occurs when used in conjunction with blunt nosed sections of the subsonic aerofoil variety. This combination eventually causes the yacht to stub a toe when really hard pressed. SOSESAIL. pes s0:s*x 47°3″ 3 44°5°x 41-0″ = 39-0″x 35-25″ U@ 633-5″ x 30-25″ HST. MAINSAIL. In some Yankee designs the fin leading edge HST. is actually swept forward, the effect being similar in ‘that the displacement curve of the fin is lengthened, thus reducing C. of P. travel, but this type of fin is not allowed under our rules. The position of the mast can make or break the performance of a yacht. Unfortunately there [59-0 20-25% 675°. 69-7″ | 73-07 52-5″ 20-25″x $9-0″« 61-5″ 64-57 46-5″ 2025’s 51-0″x 53-8″ 56°” 42-0) 18-0°x 45-0″ x 47°5° sO-Ss° are so many variable factors to be taken into er — ieee ee account that it is virtually impossible to calculate. ses, The C. of P. movement of the hull cannot be calculated and neither can that of the sailplan. In any case their respective positions vary in differing proportions as one might expect, since the velocity of one medium is allied, but not dependent on that of the other. All sorts of things have to be taken into account, for instance, a sharp turn in the hull lines amidships will require the mast to go further forward than | a hull with a smooth turn in the lines. Should (eae, you perhaps enlarge the sections of any design a IN MODEL MAKER} in order to increase her power as is so often done, then the mast must be advanced a little from the original position. Although various text books describe a rough system where by the centre of area of the sailplan can be ent on gusty days, so it is better to make it good and strong. Too often does one see flimsy makeshift efforts going wrong at the most awkward timzs. A good tip here is to allow for the adjustable part to move two or three degrees fixed a percentage of the waterline ahead of the centre laterally, so that it can follow the slight bowing of the foresail hoist and avoid straining the deck fitting. These days all ten-raters are designed to plane downwind, and Bluefin is no exception, her beam and moderate canoe body draught will allow her to go very well. In the right high wind conditions I have noticed one or two designs which are capable of actually planing to wind- of area of the underwater profile, becaus e of the variables described, it is of little value. On the design shown the mast positi on is about 2 in. further aft than is usual on a tenrater, due to the high inherent balanc e of the hull. Previously, designs using the writer’s particular system of balance tended to show the mast a little forward of the best optimum position, but with Bluefin the position as shown should occur within the normal limits of 1 in. each way. As with my other designs I advise builde rs to fit a fore-sail kicking strap. While this is by no ward in short bursts, this being most likely to happen when the gusts are coming from a slightly varying direction, as is so often the case. Obviously this is a feature to be encouraged, as these sprints can help a lot, providing that she does not screw into the wind too much. A main factor here is good dynamic balance, means essential, it does permit better control, not only to windward, but also on the run, although naturally the tension must be eased to allow the sail to fill properly. There remain s some considerable otherwise the boat will very likely lose more than she has gained before filling away properly. On the whole, while she is no heavyweight, her potential is such that she can be placed in the power-plus bracket. She is also the first of my designs to have her lines checked by the use of second degree curve construction, thus eliminating any small inaccuracy. scope for development re- garding these fittings, as they are not quite so simple to design as one might expect. For instance, the fitting must not extend throug h the deck level, otherwise the sailplan cannot be adjusted. It has to take a good deal of punish
