Ships and Ship Models: Issue 75 – March 1954

ewan ee. ALAN VIELIERS writes on eo A PERCIVAL MARSHALL PUBLICATION © : | The “A” Class Sharpie x and how to build her In playing chess, the master has his plan of campaign and thinks a number of moves ahead, whereas the beginner blunders from move to move. When building a boat, it likewise is advisable to think several moves ahead. Otherwise, one is liable to find one has created difficulties for oneself, where none should exist. Therefore, it may be advisable for the beginner to study exactly what lies ahead before proceeding further. By way of a start, let us examine the construction plan showing model on building board, which concludes last month’s instalment of this series of articles. This diagram should have been called Fig. 5 and shows the yacht in frame. The expression ““in frame’? means that the hull members are completely set up and the vessel ready to receive her planking. At the bottom of the drawing is the building board with the square cross-pieces A mounted on it. To these are secured the moulds (templates on which the planking is shaped). When we were fixing these to the building board, it was stressed that the cross-pieces in the forward part must have their forward faces on the station lines and those in the after part their after faces on the lines. This is to enable the moulds in the fore part of the boat to have their after faces on the section lines, and the moulds in the after part to have their forward faces on the section station lines. The reason for this is obvious. The hull tapers off from the middle towards the ends of the boat. The moulds are cut to the shape of the section at the station line. Let us take as an example the mould for Section 3. If, instead of mounting it with its after face on the line, we had its forward face there, then when we come to bend the planking round it, the planking would take only on the forward edge. But the planking has to take right across the mould, and by mounting it with its after face on the line, its edges can be bevelled away on the forward corner until the planking takes uniformly right across the mould. The backbone (or keel assembly), also the inwale (or shelf), and upper and lower chines, are positioned in slots in the moulds, and these slots have likewise to be bevelled off so that these frame members also take right across the moulds. This is apparent if the tops of the moulds are examined in the drawing. The keel assembly (or backbone) of the yacht is in three parts. Of these, the centre member is formed by the upper part of the bread-and-butter fin. From (Continued from February issue page 61) 102 BY H. B. TUCKER the drawing of the boat in frame (Fig. 5), it will be seen that there are two } in. thick layers on top of W.L.8. The reason for these thin layers is to provide a sufficient holding for the garboard strake and its fastenings. The upper one of these } in. layers is only necessary forward and stops } in. forward of Section 6, and butts against the frame there. Intending builders are strongly advised to obtain a set of the full-size plans now available from the offices of this magazine, as these shapes can then be taken off easily. The forward and after members of the backbone have a heel which overlaps the centre member and serves to secure them to it. The heel of the forward member butts against the frame on No. 5 station, and the heel of the after one against No. 7 frame. The skeg is put in place before the keel is assembled. Below the keel assembly in our drawing are the lower and upper chines. Although both are of the same scantlings (= measurements), owing to the angle at which it is set, the lower chine looks the narrower. Above these is the inwale which serves the double purpose of strengthening the sheer (upper edge of topsides), and supporting the deck. This is often called the “‘ shelf.” To explain the construction forward, it is necessary to study Fig. 6. The top drawing shows how the stem is attached to the keel. There are two broken lines down the stem. The forward one is the “ rabbet ” line, and the after one the ‘‘ bearding”’ line. The planking is housed in a rebate down the stem. At its forward end the rebate (corrupted to ‘‘ rabbet *’) is the full depth of the planking, but as it comes aft becomes shallower, and finally comes to the surface at the bearding line. Hence the “‘ rabbet line ” and “bearding line.” The third of the three figures shows in plan the bow at the level of the inwale and at the level of the upper chine. These show how the inwales and chines are chamfered off and butt against the stem with their fore ends on the bearding line, so as to back the planking. SHIPS AND SHIP MODELS MARCH, 1954 Fig. 7. Construction at stern It will also be noted from the second drawing that owing to the narrowing of the bilge strake, the chines run together, with the result that they are chamfered off to suit. Before passing to the stern, it may be observed that I have shown the top of the stem cut off as it will eventually be after the hull is planked, though at this stage, of course, it will still have the ear at the top to attach it to the building board. Turning to the stern (Fig. 7), the keel is let into the inner transom for its full depth as shown in the upper diagram. The lower diagram shows how the chines and inwales run together and have to be chamfered off. It will be noted that the inwale runs out altogether just ahead of the inner transom. The outer transom is also shown, and houses (or covers in) the ends of the frame members as well as the ends of the planking. Here again I have shown the inner transom without the extension at the top which serves to secure it to the cross-piece on the building board, On these drawings for the sake of clarity, I have omitted the rabbet line along the keel, and also shown only the outlines of the chines. No mould is required at Section “‘O” as we are on the stem at this point. In building hard-chine model hulls, three-ply wood is frequently used with the edges left exposed. Now the weakness of three-ply lies in its edges, as the lamina can be split apart. It is, therefore better practice to house all edges of plywood. This is done in ““KN” in the following way. The lower edge of the bottom lies in the keel rabbet, and its top edge is under the bilge strake. The lower edge of the topsides is likewise under the bilge strake, and its forward edge housed in the stem rabbet. The outer transom covers the after ends of bottom and topsides. The bilge strake is solid wood, not three-ply, so its edges do not need protection. Our deck is also of Fig. 6 Construction at bow three-ply, but drops inside the upper edge of the topsides along the sheer. The topsides thus protect the deck edges, and their own top edges will in turn be covered by the mahogany rail (covering board, or deck bead) put round the deck. One very simple method of framing a hard-chine hull is to cut the frames from three-ply of suitable thickness and build the boat directly on to these cut frames. This, however, is not remarkably good practice, since it entails putting fastenings into the edges of the plywood, which tends to force the lamina apart. Moreover, this does not make fer lightness in construction. Although an A-class is a large, heavy model, and the hull subject to considerable stresses including the wring of a weighty lead keel, the method recommended will ensure a strong, light hull, well up to its work. We can now proceed to get our moulds and continue the setting up. (To be continued ) 103