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Viodel »SUNE~ =s MODEL BOATS BReeeLerE® Toboggan from should a fill planing a 36 in. bulb-keeler, with the fact that they are so much easier to build. By using =4- inch or 1.5 mm ply they are much nearer the true monocoque, stressed skin type of structure and can be made lighter and stronger than anything except a hull of fibre-glass. With the increasing accent on downhill performance and marginal planing ability, it is pleasing to note that this is one sphere where the chine boat is acknowledged to be markedly superior and so Toboggan will have really outstanding sprint ability in dusty weather. The design technique behind the configuration of the appendages has now reached a considerable degree of sophistication, probably near the ultimate in fact. Most trends and innovations develop little by little on a trial and error basis, but in this concept there is no half way stage. Given that a bulb-keel provides the maximum stability, we have to consider what style of fin to adopt. With the deep bulb a fin of normal width is not acceptable. Wetted area would be greatly increased and as the side area is lowered also, the law of diminishing returns would absorb much of any gain in stability. Reducing the width of the fin, however, increases the proportional side area of the hull and skeg-rudder and in the design shown the centre of area of the ‘flat’ appendage surfaces alone, is quite close to that of a conventional design. To cut away need for an easily built Competition “M”’ with a first class performance. ke Chine boats, it must be accepted, have certain disadvantages and advantages compared with a rounded hull. The light weather performance tends to suffer a certain amount, due to crossflow over the chines, and the displacement is usually less for a given profile, resulting, as a rule, in reduced power. In addition the flat surfaces pound somewhat in rough water. These factors may seem pretty formidable at first sight and would be to some extent insurmountable when compared with a conventional hull, but by adopting the well tried and proven Vega style bulb keel configuration, the power is so improved that both displacement and sections can be reduced. The result is that the light weather performance should be rather better than the average size conventional “M’’, while if the edges of the chines are rounded, the eddy resistance also will be much reduced. This will also cut down any pounding, although as it happens, in an “M’’ it is not of great importance, as in this class the forward overhang is so small that the critical first section is very steep compared with the first L.W.L. section on a 10R Having dealt with the disadvantages, we can now take the aft edge of a normal fin a little causes no ill effect, but if area is progressively removed the centre of the re- a look’at the advantages inherent in a chine hull, starting M) DESIGNED BRIDGE ! ; 8 S, Witty COPYRIGHT MODEL MAKER (3-35 For ward sub Frame @” avy, TOBOGGAN ST. HEMEL oF PLANS HEMPSTEAD. SERVICE HERTS DECK “ INDLAN (hi PAINTED CREAM ~ SEAMS INGPLANK PRINTED OM MAHOGANY REO ANO VARNISHED OMENSIONS FOR SHALLOW DRAUGHT +CRSoR SALPLAN SIMILAR FOR BOTH TYRES JUNE maining area will move torward of the optimum position and thus spoil the dynamic balance. By fitting a long based skeg this effect is cancelled out and the C. of L.R. is restored to its proper position. As before, the reduction in fin area increases the proportional size of the skeg. For instance, if the fin area is reduced to negligible proportions, the centre of appendage area will move right back to the skeg-rudder. The skeg size must not be too large, however, or the yacht will be slow to respond to a shift in the wind, but as the width of the fin is only 5 in. or so, it has very little directional stability of its own so the long skeg prevents the yacht being too sensitive. This is perhaps overstating the case a little and the design would sail well with a conventional skeg-rudder but would, as stated, be over-sensitive and less easy to trim. Note that the writer has designed a number of successful conventional Marblehead yachts and only adopted the bulbkeel configuration when its advantages became too apparent to neglect. Like many interested in model yachts I had been indoctrinated over the years with so much pontification on the subject of yacht design that I never doubted it could be all that wrong. While there is a good deal of truth in the old adage that there is nothing new in yachting, in the present day and age this is hardly a progressive attitude. Having explained at some length the thinking behind 1966 TOBOGGAN AN INTERESTING HARD CHINE MARBLEHEAD WITH THE REASONING BEHIND IT EXPLAINED BY S. WITTY the fin design, a few words about the canoe-body may be of interest. To take maximum advantage of the planing potential inherent in chine boats, the hull has a flat floored midsection of shallow draught combined with very easy rocker. This kind of planing hull is rather better than the wedge-form in certain respects. The balance, static, hydrodynamic and aerodynamic (for the above water hull)—is good{due to the compact form, while the high prismatic coefficient permits a long wave length. This also gives more hull volume for the size of midsection, minimising the frontal area. She may appear rather flat underneath but is so designed that her area curve shows no sign of this. A canoe stern was considered at one stage as it has certain advantages married to a chine boat, as the aft L.W.L. shape is dictated by the choice of midsection in a transom hull (unless the lines are forced). But as we only have fifty inches of boat we might as well use it all, and anyway a transom is only a canoe-stern with the least useful bit lopped off! The classic type of planing hull is deep chested and very flat in the run, with twisted plane sections and wedge shaped deck. As the hull when planing does not sit at an angle greater than that of the run at rest, it would seem that most of the lift is generated at the relatively steep and narrow for’d sections. With the Toboggan type hull, all the wide, flat-floored mid-sections become efficient planing surfaces when inclined a few degrees. This form is neither original or untried, but as far as the writer knows, has not hitherto been used in a chine boat. Please note that I am not knocking wedge hulls in general, or the Flying Fifteens in particular, a thing which seems to be a popular pastime for some writers! This form of hull undoubtedly has its own merits. The sections in the fin are fairly sharp nosed and thin and rightly so. Some writers often compare the action of fin sections to that of low speed aerofoil sections, whereas the action is really much more akin to airflow at supersonic speeds! The shock wave and pressure systems are very similar, in fact water channels were used to evaluate supersonic aircraft shapes at one time because of this. The writer recalls how, at one time, every book on model aircraft used to contain a small diagram showing the airflow pattern around a wing aerofoil section and by way of comparison, a similar diagram of a bird wing section, explaining just how much more efficient the former was. Years later, it was discovered that due to scale effect, the small chord sections were more effective when made as thin as practicable (plate). So the birds knew best after all! Very much the same relationship applies in hydrodynamics and the smaller the fin sections, the less the thickness ratio and the sharper they should be. This may well be one reason why the fins of the smaller fishes are (continued on page 236) 233 JUNE 1966 ‘THIS is an inexpensive and very simple model which will surprise you with its performance and allow scope for experiment to increase that performance. As shown, the sail area is just enough to cause the model to fly the windward hull in a gust—rather alarming at first sight, but as the heel increases, the sail power decreases, so a state of equilibrium is reached and the chances of a capsize do not increase. Unless, of course, the wind is fresh enough to blow on the raised hull hard enough to tip it over, but this is one reason for carrying the twin fins in the position shown. With ballast on the fins—perhaps only an ounce or so on each—the boat would be a lot stiffer and faster in a blow, and it would be possible to increase the height of the sail plan and use half as much again sail area on the same length booms. Obviously, somewhere there is a compromise between ballast and sail area which will produce the best all-round results, but this will depend to some extent on the size of the water and the customary wind strength and other conditions under which the individual model is normally sailed. Built as the drawing, and unballasted, most people will factory. find the performance highly satis- A simple-to-build sailing catamaran KITAMARAN Another field for experiment is in steering. Cata- marans tend to sail fairly straight—in fact, turning them quickly is one of the problems facing full-size designers—and ours needs no rudder for normal sailing. However, our 36 in. design Sea Mew (plan MM 533, 9s. Od. inc. post) appears to make good use of a vane gear, so that some sort of steering, perhaps even a version of Braine gear for running only, may well be worth adding to this little model. Constructionally, there are no problems. The prototype was made of } in. sheet balsa, and three sheets 3 in. wide are needed. Pick softish, bendy wood for the hull sides, or there may be trouble bending the bows in; an alternative would be to use 3’s in. or even vs in. for these parts. All four sides are cut from one sheet, and a diagram showing how to mark them out is given. Cut the sides and mark the positions of the bulkheads on the insides—just double measurements taken off the half-size side view. Trace the bulkheads and fins (all shown full-size) and cement the bulkheads to the sides to make a pair of basic hulls. Cement the fins in place, butting against B2, then sheet the top and bottom of each hull between B1 and B3 with x in. sheet, with the grain running across the hulls. (Continued on page 262) 241 Le %” powEL TIES. BIND IN _7 MARKING OUT HULL SIDES ON | SHEET / CENTRE FOR JIB TACK EYE. i ve” x 3″x 36″ BALSA Im STREAMLINE. CENTRE BRIDGE 3 LAMINATIONS Ye” BALSA OR israel TO ; nw SAND MAST ——+) | 2 LAMS. BALSA AND Vie’ PLY CORE. aN OTT ‘N ee 4 — — — BS iain s)he alt B4 BULKHEADS 2 EACH /e” 5! INSIDE DISTANCE HULLS PARALLEL, It SCREW OFF EE 60000 X |ae B2 rT r- Ee © ee \ TIE IN EYE AND TO LEAVE i PEC I8in. sailing model for the not-so-serious \ enthusiast. Side and plan views half full-size. FIN DRAWN FULL ? SHROUD ‘, CUT SAILS POLYTHENE v …SIZE. FROM THIN SHEET BOWSIES FROM PLY OR CELLULOID =I 8%4 w ia) WEYES. KITAMARAN ; BS B4/
