- Description of contents
SEPTEMBER 1970 THREE SHILLINGS U.S.A. & CANADA SEVENTY-FIVE CENTS HOBBY MAGAZINE SEPTEMBER 1970 FIG. 13. DETAILS OF INTEGRAL BACKBONE ASSEMBLY FIG. |4, DETAILS OF SHADOW Inset Transom Bow- 3/8″ Slot for integral backbone assembly —=—— 3/8″ Notches for inwales _— Shadow Mount Building Board RACING MODEL YACHT CONSTRUCTION FIG. IA. DETAILS OF RUDDER POST Part Four — the ‘A2’ method of building a round bilge hull. BY C. GRIFFIN – * AS’ PREVIOUSLY stated, a degree of experiment was done when this method of construction was employed. The chosen design was S. Witty’s Skippy, which was published in the February issue of Model Boats and features a fine bow with a forward flare, an integral bumper and a_ heavy tumblehome at the mid-sections. It was decided to attempt to build as light a hull as possible using mahogany planking. The problem of weight ratio of all-up hull to lead bulb is more evident when the hull is constructed from planks than when a fibreglass moulded hull is used. Most designers do, in fact, show alternative bulb keel weights, usually varying by about 1 lb., as they are aware of the problems of construction, especially in one-off building. To provide a solution to the weight ratio aspect and to utilise the full bulb keel weight, with its greater stability factor, a ‘new’ approach to the building method was adopted. Fortunately, the idea proved successful in that the completed model is only 1 oz. overweight and the lead keel is 1 oz. below maximum designed weight. The full outline of the hull, including station lines, is traced from the full size plans onto a piece of s in. thick marine faced mahogany plywood 6 ft. x 2 ft. As illustrated in Fig. 13 this outline includes the kingplank, backbone, fin and skeg, and a cut away transom. All excess wood is removed using a coping saw; the backbone at A-Al, B-B1l and C-Cl is undercut by 3/32 in. as shown, and the fin and skeg shaped to the designed chords. This latter operation involves thinning the skeg to 5/16 in. thickness and building the fin up to 4 in. thickness by the addition of a layer of 1/16 in. mahogany on either side of the fin. A bow block is fitted over the forward end of the ‘skeleton’ whilst the after end is cut away to facili- 367 tate a % in. deep inset transom by allowing the planks to overlap on to a ‘false’ transom by ¢ in. A built-in handgrip is fitted between sections 6 to 7, being constructed from 4 in. wide mahogany planks, see Fig. 13. The angle of the rudder post is drawn on the skeleton and the immediate area is built up by the addition of two pieces of 4 in. thick obeche or spruce so that a hole may be drilled % in. dia. to receive the rudder tube. The rudder tube is cut away as shown in Fig. 13A and glued into position using epoxy and } in. x No. 2 countersunk headed brass screws. The shadows No. 1—T are traced from the body plan allowing 1/32 in. for the ribs and 3/32 in. for the planks, ie. 4 in. in all. Notches ? in. wide by $ in. deep are drawn for the inwales and 1/16 in. is allowed for the inlaying of the deck. From Fig. 14 it can be seen that the shadows are notched } in. wide to the body depth to allow the integral backbone to be dropped into position. Trace the corrected shadows Nos. 2-—T, eleven in all, on to + in thick plywood and cut to size. The bow block, which will be covered by the planks, is shaped from the tracings of the body plan at the stations 0 and 1 allowing 3/32 in. only for the planks, cut } in. thick, recessed to a depth of in. (see Fig. 15), and faired into position. A new shadow (see Fig. 16) has to be drawn into the body plan, allowing 3/32 in. only for the planks. Shadow T1 is traced on to } in. thick marine faced mahogany plywood and faired into position. When all the shadows have been cut, shaped, and checked for accuracy and symmetry, they are set up on the building board at the designed section spacings. Check for alignment and that they are at 90° to the centre line. The integral backbone assembly is then dropped into the slots and once again checked. MODEL BOATS Note that if the section spacings are drawn onto the hull profile as suggested, the position of the shadows is automatically verified. All shadows from No. 5 forward have their leading face on the station line, whilst the shadows No. 6 and aft have their after face on the station lines. As explained in method Al the inwales are constructed from + in. thick, % in. wide spruce and laminated into position; they are glued into notches in the bow block and the transom T1. Inwales constructed in this manner have the advantage of being both stronger and less prone to subsequent movement. It may be necessary to join two pieces of } in. spruce together to get the necessary length (spruce is easily obtained in 36 in. lengths) but this is rela- tively simple if a half joint is used. A certain amount of reshaping of the inwales may be neces- sary to present a clean, true surface for the sheer strake planks. The ribs, cut from 1/32 in. marine plywood } in. wide, are placed around the shadows, being notched into the inwales and laid in slots 1/32 in. deep in the backbone. Sections 5, 6 and 9 require small rectangular holes cut at the top of the fin and skeg to enable the ribs to fit through the backbone. A rib is run around the shadow T as this forms a base for the ‘overlapping’ planks at the transom. The planking process used differs from that previously described in that, instead of each plank being spiled and fitted, all the planks are cut parallel sided. It was the writer’s opinion that this would be the only way to achieve the very fine bow sections dictated by the design. This method has an indirect fringe benefit in that it does reduce the amount of time required to fit a pair of planks, especially if a FIG. 15. Photo shows at right hull on completion planking, of with deck beams, etc. fitted. Note handgrip and chainplates in place. ower saw which has a tilting table can be used. It is possible, on occasions, to fit three planks on each side of the hull in one day. During cold weather some form of heating is required whilst planking, as Aerolite 306 glue dries best in a temperature of 16-21° C, 60-70° F. Four mahogany planks are cut 5/16 in. wide, + in. thick, and these are used to construct the sheer strakes. One plank is glued into position on either side of the hull with about +4 in. overlap to allow for recessing the deck and trueing up. The plank can be held in position whilst the glue is drying by using either G cramps, clothes pegs or even insulating tape. The second plank on each side of the hull has the edge which contacts the first plank slightly FIG. 16. DETAILS OF TRANSOM 1/8″ Fillet along sides of Planking DETAILS OF BOW BLOCK 3/8″ wide recess 3/8″ deep > True transom Tl – No.Rib 3/4″ Note planks meet at centre line of backbone 4 368 Backbone Assembly SEPTEMBER 1970 FIG. 17, DETAILS OF BOW either side of the fin is glued into position and held by temporary pinning using map pins. When all the garboard planks have been affixed, the garboard radius is checked to comply with the minimum 1 in. Note planks meet at centre line of backbone rule and sanded to form a smooth surface. Sand the hull by using wet and dry carborundum paper graded 120 to 400, working across the grain of the wood, checking frequently with templates that the sections are accurate and symmetrical. Sand the fin and skeg to the final chords. Remove the shadow mounting blocks from the building boards and remove the shadows after placing some temporary deck beams in position. Make the chainplates from 20 gauge stainless steel plate as shown in Fig. 18. Cut suitable recesses in the inwales to receive the chainplates and fix by bolting through the inwales using 6BA countersunk headed bolts and epoxy. Fix shroud strain bearer beams diagonally across the hull as shown in previous photographs chamfered so that it fits snugly against the first plank. Glue this pair of planks into position. A further four planks are cut 3 in. wide, chamfered on the contacting edge and glued onto the bow block, ribs and transoms T and Tl. This width parallel planking is continued until a point is reached approximately ~ in. below the apex of the bow block; from here on the planks are narrowed to } in. wide to enable a greater amount of twist to be put into each plank in order to obtain the fine bow required by the design. It is necessary to shape the backbone to a vee to enable the planks to lie flat on the backbone. Unlike the previous method, the planks are not overlapped but are fitted to meet at the centre line of the backbone (see Fig. 17) at the bow and after sections. Fillets of + in. thick spruce are shaped to fit close to the fin and skeg to form a rebate for the planks that follow, these planks are cut 5/16 in. wide. The process continues in this manner until the hull is complete. The garboards are constructed from + in. thick mahogany planks shaped to size by tracing off the lines of each plank from the design. Each plank on 20 Gauge Stainless Steel Inwale aul tee 2 Shroud Holes 3/8″ Apart to augment FIG. 19. and N 8 9 10 T ee ‘ LL] Reape lll a clamps Brass Bolts eels PuORpestrton i | holding planks. ; 1/2″ No. 6BA DETAILS OF CHAINPLATES AND FIXING IE} lo) 2 ak Sat —oa FIG. 18. DETAILS OF DECK BEAMS Photos at left show stern after removal of false transom, bow block, and the hull half planked. Note use of black tape fe pegs in FIG ISA, NOTCHING DECK BEAMS INTO INWALES Si and notch the deck beams into the inwales at the predetermined positions. See Figs. 19 and 19A. ae for alignment of fin and skeg as indicated in ig. 20. The transom shadow T can now be carefully removed from the hull leaving the true transom T1 in position with the planks protruding approximately = in. Sand the protruding plank ends at the bow so that a flat surface is left upon which is glued (epoxy) the +} in. thick cork bumper. Sand the protruding planks at the transom so that the overall length does not exceed SO} in. Run a fillet of Plastic Padding around the outside of the transom on top of the protruding planks and, whilst still wet, cover with a preshaped layer of mahogany veneer. See photograph. Drill a 3 in. or 4 in. dia. hole in the transom and insert a drain plug, this can either be screwed into a ferrule or just a push-fit cork. Clean and true the surface of the inwales and cut the deck from .8mm marine plywood. Make the deck a reasonably good fit into the recess formed by the plank tops with not more than 15/1000 in. clearance around the periphery. At this stage it is best to stain and line the deck if this finish is desired. The lining can be done reasonably easily by using an ordinary marking gauge which has a ¢ in. hole drilled through the end. A ‘Parker’ Tee-ball black refill fits neatly into the hole and, using care, the deck planking can be drawn on to the stained wood. A kingplank picked out in 369 { a darker stain and jogged into the deck planks enhances the appearance of the deck. Drop the deck FIG, 20. CHECKING ALIGNMENT OF on to the inwales and mark off the position of the FIN AND SKEG FIG. 21, handgrip and the rudder tube. Cut out both and recheck for fit. Drill 1/16 in. holes, 2 in. apart, around the periphery of the deck about 3/16 in. from the edge and also around the edge of the SCRIBING BLOCK handgrip. 68 Pencil Give the inside and the outside of the hull and both sides of the deck two coats of thinned varnish. Translac clear polyurethane or P.U.15 varnish, No.6 Screws exterior quality, both give good results, although the writer prefers the latter. Rub the varnish well into the grain, as these two coats are primarily for filling purposes. When the varnish is dry give the contacting surfaces of the underside of the deck and the inwales and deck beams a coat of turpentine thinned Sealastic. Screw the deck into place using 3 in. x No. 0 or 1 countersunk headed brass screws. The watertight integrity of the boat can be improved Add layers to required height. FIG, 21IA. MARKING THE WATERLINE and, in the writer’s opinion, so can the appearance, by the addition of a + in. wide, + in. thick deck bead or rail. This is sprung into position hard up against the sheer strake. Varnish the rail and the edge of the deck and whilst the varnish is still wet, fix the Pictures below show the build-up of the ‘skeleton’ to give adequate thickness for peaway. the rudder tube. Extension of planking pet transom can be seen. Lower photo shows handgrip fr ‘om beneath. Note § in. sq. rebate strips have not yet been added to sides of fin. rail into place, using small brass sprigs or nails. A similar rail run around the edge of the handgrip adds to the appearance of the finished model. Sand both rails down to 1/16 in. or 3/32 in. thickness and varnish. Give the hull a rub down with 400 grade wet and dry paper and then give about three coats of varnish, allowing each coat to dry thoroughly before rubbing down with 600 grade wet and dry paper. Allow the final coat to harden for at least 72 hours and then polish using ‘T-Cut’ or a similar product. Finish with a coat of good quality wax polish. Give the deck a final coat of varnish and the hull is ready for the fitting of the lead keel and other gear; these aspects will be discussed in a later article. Whilst both planked type hulls have described the use of mahogany for the planks and subsequent varnishing, the author in no way decries the use of other woods such as silver spruce, yellow pine and white cedar. These are all excellent materials for planking, but the main problem will be finding a supply of the required wood. Similarly, there is no reason why the hull need not be painted instead of varnished, this is really a matter of personal preference. Provided a good quality paint is used (nowadays this invariably means a polyurethane resin type paint of either the one or two pot type) and adequate care and attention is used in the finishing process, a painted hull can be very attractive. The use of two colours, i.e. a different colour above and below the waterline or a contrasting boot top which divides the topsides from the bottom both add to the appearance of the model. The waterline is marked by setting up the hull on blocks so that the waterline marks fore and aft are at the same height. A simple scribing block, Fig. 21, incorporating a 6B pencil is used to draw a line around the hull. Masking tape can be used to separate the colours but a better method is to use gummed paper strip which is removed by soaking when the paint is absolutely dry. Apply the paint in three or four thin, smooth layers, rubbing down between coats with 400 grade wet and dry paper. Rub down the last coat with 600 grade paper and apply the finishing coat. Allow the paint to harden for at least 72 hours then polish with burnishing cream or “T-Cut’. 370
