- The “A” Class Sharpie and how to build her “K.N.” Part 5 BY H. B. TUCKER
fier. —. A —S PERCIVAL é oe _ 8 Seature zs~~ RY. BRITANNIA’ MARSHALL PUBLICATION ‘ | The “A” Class Sharpie x and how to build her we” since the mould is in two parts, these have to be held together during building operations. To effect this, temporary ties are nailed across. On no account screw the ties, or nail them as if they are to be permanent parts of the structure, since ultimately they will be prised off with a screwdriver. When the two parts of the mould are tied together turn over and paste the paper template on the reverse side. Line the template up with the line representing the top of the floor exactly on the junction of the two pieces of wood. This presents no difficulty as the wood will extend beyond the template. The reason for using paste is that when the boat is off the moulds, the paper on the floor can be damped and removed easily. When the paste is dry, run a sharp knife (or a razor blade) along the floor line to separate the two parts of the template, and thus facilitate the removal of the upper part of the mould later on. Cut the wood just outside the line and then finish to the line. All the moulds are similarly made. Two points to note are that an iron-back, finetoothed saw is the easiest tool for cutting out the moulds, and that the nails holding the ties should not be long enough to go right through the wood and project. ‘This will avoid having to turn the points over, or alternatively lacerating your hands on them. Since gluing will shortly be necessary, it may be as well at this juncture to deal with this matter once and for all. Many old-fashioned builders still favour various brands of Casein glue. Casein adhesives have a very low degree of water resistance. When damp, their adhesion strength drops almost to nothing, though they do recover a portion of it when they dry out again. With each successive wetting, their adhesion becomes less, and in a comparatively short time vanishes altogether. Their only successful use in model yacht building is when they are kept well protected with paint or varnish. Modern synthetic resin glues are to all intents and purposes completely water resistant. Under any circumstances their adhesive strength is vastly greater than that of Casein glues. Moreover they set faster than Casein. It will be noticed that I use the expression “‘ waterresistant.”” No glue is waterproof in the sense that a rubber seaboot is waterproof, but a water-resistant adhesive will neither lose strength nor be soluble when exposed to water. Hence my advice to the * Continued from April issue p. 184 138 BY H. B. TUCKER builder is not to use Casein on any account, but employ a resin glue. Resin glues can be had both “ hot-setting ’’ and “cold setting.”’ Since the former require special ovens and apparatus, only a cold-setting variety is suitable for our purpose. An excellent example of a cold-setting synthetic resin glue is Aerolite No. 306, which is in powder form. The disadvantage of made-up resins is that they will only keep about three months from date of manufacture, whereas this powder glue has a life of two years. When required for use, the powder is mixed with cold water, and it then becomes Aerolite Resin No. 300. The latter is an ordinary resin but has the added property of being “‘ gap-filling.” This means that it can be used to fill a gap up to 1/20 in., which (whisper it softly!) might occasionally be found useful by less expert builders, though I am certainly not advocating slovenly fitting. On the other hand, for bread-and-butter constructions, where a thin glue line is important, a thinner glue is advisable. In such case, if 10 per cent. more water is added, our resin becomes Aerolite 301. It should be added that when we have transformed our powder into resin by adding water, it then starts its three months life as resin, so the glue is very economical in use. For the benefit of those unfamiliar with resin glues, it should be explained that these glues consist of two parts—the resin and the catalyst (or hardener). The resin by itself is a grey treacly substance, that does not set or harden until brought into contact with the catalyst. The hardener is liquid being an acid compound, and by varying its composition, the makers have been able to produce a range of catalysts of different speeds, so setting times vary according to the hardener used. Speed of setting also varies with room temperature, but makers supply tables giving all particulars. The point to observe about this is that the “shuffling time” (i.e. time for assembly, lining up, and cramping before the glue starts to set) varies in direct proportion to the setting speed. ’ as the “combined application”? type, and the chemical action of setting starts immediately the acid is added to the resin. Others, including Aerolite No. 300, are of the “separate application ” type. With these, one surface is coated with the resin, and the hardener is applied to the other face of the joint. Nothing happens until the coated surfaces are brought into contact, when the chemical processes begin. This separate application type is much the best for the model builder, as he can take his time in spreading the resin thinly and evenly. When this is accomplished to his satisfaction, the hardener can be brushed on to the other part of the joint, and the joint assembled. Aerolite No. 300 is put up in a 6s. tin, which is a handy size for the model maker. It is used in conjunction with the makers’ GB range of hardeners. There are several of these to choose from, but for our job, one of the slower ones is the most practical, since this gives more time to align parts. Because of their hardness, resin glues are liable to Right: G. Honnest Redlich’s “A” class yacht beating to windward under radio control at Hove Below : Mr. Redlich adujsting his yacht. Col. Bowden is on the extreme left and Mr. Carrington Wood is busy taking a photograph Photos by Gabor Denes * eS Mention was made of the keeping qualities of the resin, and hardeners keep indefinitely. With some resin glues, the hardener is mixed with the resin before application to the surfaces to be joined (or “bonded”’). These glues are known blunt tools fairly quickly. so all surplus glue squeezed out of a joint should be wiped off before it is hard, using a rag dipped in warm water. , It is advisable to cover any cut or abrasion on the hands before using resin glue, and also wash the hands thoroughly afterwards. For spreading the resin, use a spatula cut from a piece of thin wood, and apply thinly and evenly. To apply the hardener, make an applicator by fixing a piece of felt on to a wooden handle. The resin may be kept in a tin, but the hardener must be kept in china or glass, and never in a metal container. One important hint on working with these glues is always to use a sheet of newspaper to prevent the work sticking to anything it is not meant to (e.g. the building jig). _When a thinner protection is needed, cellophane answers excellently. The best idea, however, it to buy some anti-adhesive lacquer, and varnish the jig with it. A suitable lacquer is made by Stanley Smith & Co., Worple Road, Isleworth. In case builders have difficulty in getting Aerolite, I may mention that its makers are Aero Research Ltd., Duxford, Cambridge. (To be continued) 185
