Ships and Ship Models: Issue 73 – January 1954

TWO. SHILLINGS JANUARY — loside… “rs€ instalment 1954 The -A” Class Sharpie and how to build her “KK poe classes of model yachts are recognised by the Model Yachting Association, each to a different rule. In consequence, the yachts vary greatly in size and type, ranging from the 36-in. Restricted Class (36 in. length overall and weighing 12 lb.) to the A-class (L.O.A. between 70 and 84 in., and weighing between 45 and 62 Ib.). The two smallest classes are the 36-in. and the M-class, and both are very popular. This is largely accounted for by their comparative cheapness, their small size and portability, their suitability for use on small lakes, and also for their simple and easily comprehensible rating rules. The last named reason is hard to understand since the same skill is needed to design a yacht under a simple rating By H. B. Tucker formula as under a more complicated one. Further any additional difficulty, due to a more complex rating rule, is the designer’s headache, not the builder’s or skipper’s. On the other hand, although a model yacht of any of the recognised classes will give good sport, it can confidently be stated that the bigger the class, the: better the sailing. Yet many amateurs are afraid to tackle the task of building a model as large as an A-class, because of the work and expense involved. Now a sharpie is not only far easier and quicker to build than a roundbottomed yacht of similar size, but also much cheaper, so the possibility of building an A-class sharpie is well worth consideration. Hitherto, no designer has made a serious attempt to design a hard-chine craft to the class. Yet provided she is well designed, well built, and well sailed, there is no reason why a sharpie cannot hold her own with more conventional round-bottomed yachts. Moreover, a sharpie need not be an ugly, ungraceful craft. Because a sharpie is easy to construct, and her lines look simple, it should not be imagined that she is easier to design than an ordinary yacht. Both have to embody the same principles of naval architecture and hull balance, and because of her simple lines, it is a more exacting task and calls for greater skill to design a sharpie than a normal boat. When the sharpie embodies the ‘‘ constant chine-angle ”’ principle, it is even more difficult, if a properly balanced hull is to be produced. In an ordinary yacht the sides curve round the bilge into the bottom (see Fig. 1), and when comparing such a craft with a barge or sharpie, she will be spoken of as a “ round-bottomed ”’ vessel. ‘The barge and the sharpie are 27 JANUARY, 1954 \ \ ‘ of rise in the bottom constant through- oS For many years I have been an exponent of the ‘constant chineangle ’’ principle. By keeping the angle Fig. 1. 2. Barge type section as Fig. 3. The present design is on this principle, and I should add that in every other way also the construction has been arranged to make the building as easy as possible. The ordinary sharpie (see Fig. 3) is single chine, but there is another form of sharpie which is comparatively rare, yet possesses many advantages. If we cut off the corner (see Fig. 4), we get two chine angles per side, and this type is known as the “© double-chine,”’? and instead of being in two parts, each side is in three sections, the “ topsides,”’ the “bilge strake,’? and the “ bottom.” The doublechine gives the designer more latitude and enables him to employ a less boxy section. It also enables one to use sounder principles in construction, as will be apparent later. For these reasons, and also because it gives a much shorter quarter beam buttock length, I have used it for this design. Although the importance of the latter point will be apparent to any reader who is familiar with the Aclass rating rule, the novice need not trouble his head about this since it will make no difference to the building, and is a matter purely for the designer From general consideration of sharpies, let us pass Her to a consideration of the lines of “KN.” dimensions are: L.O.A. 71.5 in., L.W.L. (load water line) 54.0 in., L.W.L. beam 12.75 in., draught 12.0 in., displacement 51 Ib. 8 oz., S.A. (sail area) 1,519 sq. in. These are quite normal for a modern A-class yacht, and the only point to note is that her beam is slightly less than the average round-bottomed model of the same L.W.L. length, but as a sharpie, 28 Fig. Round bottomed section angle ’’ type. and the club measurers. ene aeé out the length of the boat, and the angle of flare in the top-sides constant in a similar manner,. construction is greatly simplified. The reason is that instead of the planking having to be bent in two directions at the same time, only a simple bend in one direction is entailed. In the case of a model, this enables the entire side or one half of the bottom to be made from a single sheet of plywood, and eliminates any necessity for fitting plank edges. It will, of course, be obvious that if the angle of rise in the bottom and the angle of flare in the topsides both remain constant, the angle at which they meet along the chine line must likewise remain constant. Hence this type of sharpie is called the “ constant chine- | Sharpie type section a nas | = both “‘ hard-chined ”’ craft, which means that their sides and bottom meet at an angle, known as the “ chine.”’ Their difference is that the barge (see Fig. 2) has her bottom horizontal from the keel to the chine, whereas the sharpie (see Fig. 3) has a rise to her floors. oFa SHIPS AND SHIP MODELS = Fig. 4. Double chine sharpie and with the type of section employed, she will be amply stiff and should carry her sail well. On examining the plans, it will be seen that the centreline of the deck is straight from stemhead to transom, and entirely without rocker. This is a further application of the simple, single bend idea. As all deckbeams are cut to arcs of the same radius, the deck is a simple thwartships (across the ship) bend from gunwale to gunwale, so this also can be a single sheet of plywood. The varying width of the boat ensures the sheerline (side elevation of the deckline) being a graceful curve. Actually deck camber in a model yacht serves no useful purpose, and if desired, the deck can be laid flat from gunwale to gunwale, but in either case the curved sheerline must be retained, as the rating rule insists on the sheerline being “‘ a fair, continuous concave curve ’’ from stemhead to transom. Ifthe camber is omitted, the deck will also be a simple, single bend but this time from bow to stern. A “Datum Line” is shown on the plans. The boat is built upside down on a jig erected on a “building board’ (stocks), and this datum line represents the top face of the building board. This is only mentioned in passing as it is explained fully in due course. The fin is built up ‘‘ bread-and-butter ”’ fashion (i.e. layers of wood, 1 in. thick, glued together). The W.L.’s (water lines) represent the faces of the layers, and it will be seen that W.L.’s 1-7 are identical in shape, each W.L. repeating the one below it. This gives a good streamline effect. The leading JANUARY, 1954 edge of the fin is at an angle of 45 degrees. I very strongly advise builders to fit their yachts with removable keels. With her lead keel in place, an A-class yacht is an awkward and clumsy thing to handle when travelling, and further the weight tends to strain the hull. With the keel off, a yacht can be put on the luggage rack of a railway carriage or stowed on the roof of even a small car. It is my personal opinion that radio control will never play any part in serious model yacht racing. Nevertheless, quite a few people are interested in it, and since ““ KN ”’ would be very suitable, some of my readers may wish to build her for this purpose. I have, therefore, shown alternative lead lines—one for ordinary sailing, and the other for radio-control. If the keel is made removable, as suggested, it would actually be possible to have two alternative keels for use with the same hull and sails, thus enabling the boat to be used either way at the owner’s option. It would, of course, mean making the hatch larger than usual, and also having two sets of running rigging (sheets, etc.). In general, however, “ KN” is intended to be used with vane steering gear for serious racing, and her lines should produce a fast and powerful yacht, capable of winning many races in the hands of a competent skipper. It will be noticed that on the profile drawing of “KN” there is a point on the L.W.L. marked “ C.B.” This represents the fore-and-aft position of the yacht’s centre of buoyancy when she is on her correct fore-and-aft trim. The C.B. in this boat, as in many modern designs, falls forward of the midships section (No. 6). It is absolutely essential that the C.G. (centre of gravity) of the complete yacht with ‘all her equipment aboard should fall at the same fore-and-aft position as the C.B. If it does not, she will trim by the head, or stern, as the case may be. More will be said about this later, but the builder must bear it in mind. I have given the yacht a modern sailplan with a large headsail, and this brings the mast further aft than older type rigs which do not have such a large jib. While this should make for efficiency on all points of sailing, it complicates matters slightly for the installation of radio control, since the after side of the mast is not much more than 3} in. ahead of the C.B. The most satisfactory method of installing radioequipment in a model yacht is to have this contained in a watertight drop-in box. A good example of this apparatus is the E.D. Mark IV three-channel set made by Electronic Developments (Surrey) Ltd., of 18, Villiers Road, Kingston-on-Thames, Surrey, and I have to acknowledge my indebtedness to this firm for information about this set and about radiocontrol generally. The exact weight of the radio control equipment is. governed by the size and type of batteries used. If batteries are used which will last for an entire season, then the size and weight will be increased, but generally the weight will be in the region of 4 lb. for the whole outfit. In order to be on the safe side, I have allowed for 44 lb., and in consequence the lead keel for radio control weighs 35} lb. SHIPS AND SHIP MODELS against 39# Ib. for normal sailing with a vane gear. In either case I have provided } lb. inside ballast for trimming purposes. The size of the drop-in box for the radio-control equipment is 12 in. long by 6 in. wide by 5 in. deep. Hence the hatchway will have to be a full 6 in. wide to enable the box to pass through. Although the box is 12 in. long, it should be possible by tipping it endwise to pass it through a hatchway, say 8 in. long. Now the ideal would be to instal this radio-control apparatus with the centre of the box immediately over the C.G. of the boat (or in other words at the position of the C.B.), but owing to the proximity of the mast to the C.B. it will have to go a little further aft. In order to counterbalance this, I have arranged the 35} lb. lead keel with its C.G. correspondingly further forward. Returning to the lines, it will be seen that the boat has a blunt stem and cutwater, $ in. wide. It has been found by laboratory experiments that this has no detrimental effect on speed, but rather the con- trary. From the point of view of the designer of racing craft, it has the added advantage of saving nearly an inch of L.W.L. measurement, and consequently giving the boat more sail on the rating. It will be noticed that on the profile drawing of the stem, as indicated by broken lines down the topsides and bilge strake, I have shown how the stem would run if the exact technique of a sharpie exercised. As we are well away from the was L.W.L. measurement point, I have taken advantage of the $ in. wide stem to add a little extra forward, rounding off the stem for the sake of appearance. Hence the stem in this part will not be the full * 4 in: wide. For readers interested in designing, I have included the curves of areas of the boat’s canoe body, upright and heeled 20 deg. I may explain for the benefit of beginners that these curves show the volume of water displaced, and how this displacement is distributed throughout the length of the boat. They mean absolutely nothing to the builder, but are all important to the designer. So also is the fact that the C.B. of the boat moves after a mere 0.1 in. when she heels to this angle, but it implies that the hull is balanced to a fine degree, and should perform satisfactorily under all conditions. We can now leave our consideration of the design, and proceed with the construction. (To be continued) We are preparing a double-page spread showing the lines of this yacht for publication in our February issue. This is a small scale reproduction of the complete drawing, which is being added to our Maritime Plans List, reference S.Y.16, price 25s. In this drawing the lines are drawn the actual size for the model, so that tracings may be made for the backbone of the hull, and for cutting out the moulds. They may even be pricked through direct from the print on to the wood. Later in this series, detailed instructions, with diagrams, will be given for making a detachable lead keel, and if required, alternative keels may be made to suit both Radio control and Vane steering. Instructions will also be given for making the highly successful ‘* Guy Blogg *” type of Radial Fibboom—(Enrror). 29