III. Construction of the Hull—A Suggested Design.
By R. R. POOLE, B.Sc.
The construction of speed boat hulls of the 1½-litre class is generally rather beyond the capabilities of the amateur, unless special facilities are available, but the smaller hulls, such as are required for outboard engines and inboards of the nominal 7 or 8 h.p. class, offer no special difficulties, nor do they necessarily involve excessive expense. It may therefore be useful to readers to discuss some of the general features of the construction of a hull suitable for either of the last mentioned motors.
The type of hull giving by far the best performance, for a seaworthy boat, is the single step V-bottomed vessel having an overall length of about 12 to 14 feet and a beam of at least 4 or even 5 feet. The after section is quite flat bottomed as a rule, though a very slight concavity may be introduced by arching the transverse bottom ribs to a rise of about an inch at the centre. In the forward section a sharp entry is carried by a smooth flare to the flat, square edge of the step. The height of the step is not important in itself, but a rise of 6 inches will give a good angle of incidence of the after body, assuming the latter to be about 6 feet long and the boat to be planing level.
” Chassis ” Construction.
The shape of this hull does not lend itself to construction on classical lines, building up from a central keel, so a modified design (Fig. 1) is based on the use of two longitudinal spars or longerons, braced apart by transverse ribs and made rigid by the planking of the after bottom. This rigid structure forms the backbone of the boat, and it can carry nearly all the bending and shearing faces at the step, so that the side planking is not required to stand much of the load, and may therefore be lightened considerably. It will be noticed from the outline sketch in Fig. 1, that the line of the after bottom is continued straight forward, meeting the bow just at the top of the cutwater. The longerons can be carried to a point here and rigidly joined up with quarter-inch 5-ply gusset plates (see Fig. 1C.). The longerons themselves may be made of selected spruce about 3 x 1¼in. with the greater dimension vertical, and 3 or 4 main transverse ribs of similar wood, say ¾in. thick, attached to the longerons by triangular plywood gussets, and possibly reinforced in the corners with lengths of aluminium angle, screwed on. Brass screws of about No. 8 size should be used throughout, and no dovetailing or mortising should be done to any part of the chassis. The ribs may be spindled down a little, for the sake of lightness but the saving of a few ounces is scarcely worth the trouble. A more useful saving would be the tapering of the longerons to say 1½ x 1in. at the ends. The whole chassis can be sprung into shape without steaming as the curves are not too sharp for the thickness of the wood. Light side ribs of mahogany or ash can be attached to the longerons by means of small clips made of about 18 gauge aluminium sheet cut and bent as in Fig. 2. The sides may be about a foot deep behind the step, and can be made from a single piece of quarter inch mahogany planking. Struts of ½in. 20 gauge steel tubing with the ends hammered flat and bent to 45° may be set up from the main bottom ribs to support the gunwale and stiffen the sides. The gunwale need only be quite a light strip, say 1 x 1in., also of spruce or mahogany. Its side elevation will be practically a straight line. If the sides are suitably strutted the decking and upper fairing may be made extremely light. A skin of 1/16in. aircraft plywood on a slender mahogany skeleton will be found easy to build, and the flat profile of the gunwale will avoid difficulty with the intersections of the fairing surfaces. If the deck is made slightly convex about the axis of the boat, a raised ” blister” fairing running down the middle, to cowl both the driver and the engine, can be fitted easily, making the joints, aircraft fashion, with glued-on strips of linen tape, well varnished over.
The part of the boat presenting the greatest difficulty is the curved bottom of the forward section. Bearing in mind the desiderata mentioned in the first article on design, namely a sharp vertical edge of entry or cutwater with a small dihedral angle, combined with a small angle of incidence of the immersed planes, we must arrange the transition from the former to the latter with as gradual a change of curvature as possible. This problem, of having a vertical edge at the bow, swept back to a horizontal edge at the step, presents certain mechanical difficulties. A simple experiment in folding a sheet of paper, will show that it is impossible to obtain a normal ” ship-form ” or convex paraboloidal curve using only a single direction of curvature, such as could be obtained by twisting or bending a flat sheet. It is possible however to obtain a good shape, involving concave cycloidal curves in the plan, by using single sheets for each side, and avoiding the difficult business of planking the curve with narrow fitted strips. The concave form is, in the opinion of the writer, a more efficient plan than the more normal style, as the acceleration of the streamlines is almost constant, instead of reaching a high maximum about a foot from the point of entry.
The shape of the panels can best be found by making a paper or cardboard model, somewhat on the lines of Fig. 3, which is more or less self explanatory. In order to obtain the necessary straight and horizontal keel line in a design which incorporates no actual keel member, the model should be set up on a vertical median plane on which the profile of the keel is sketched. The shape of the transverse sections is best found from the model, as a graphical construction is very involved, and the actual frame sections should be made carefully from about ½in. mahogany. They should be cut to fit between the longerons to serve as ribs, and might be reinforced along the top edge with a strip of wood screwed and glued to one side to take the compression of the longerons. The plywood gussets might be enlarged, possibly even to the extent of covering over the first few feet inside the bows completely with plywood, forming a sort of false bottom.
The bottom panels may be made of 3-ply of high quality about 3/16 inch thick, which, if thoroughly waterproofed inside and out, with many coats of varnish, is fairly serviceable. Aluminium panels are perhaps the most easily adapted to this type of construction, since a better shape can be obtained by making the flare of the bow quite sharp, and this also brings the angles of incidence lower. It is impossible to make a jointless bottom of one sheet as the flare necessitates a slightly concave edge to each half along the centre line for covered joining up. In either case, the joint should be capped with a strip of aluminium about 2 inches wide folded so as to fit along the keel line, where it is rivetted, after doping the join with red lead. The strip should be bevelled down along its edge and the rivets countersunk and filed flush, to reduce resistance.
An inboard engine needs very strong bearers, and if mounted in the forward compartment, these can be attached to the longerons via extra cross members. Lengths of an old car chassis make very good bearer members and can easily be attached lengthwise to the cross members and the motor bolted between them.
An old Morris or Ford frame can be bought for a few shillings from a car breaker. The propeller shaft can be brought out well up in the step, so that the gland is high and dry when the boat is planing. Anyone who has tried to ” stuff ” an immersed gland will appreciate the advantage of not even wanting a watertight fit. If an outboard engine is used, an important requirement is a stout transom designed to take not only the forward thrust of the drive, but to withstand the forces tending to wrench the engine off when the rudder is applied. Some such system of struts as that shown in Fig. 4, would serve to strengthen the transom and in addition give a triangular bracing to the stern of the boat. Three quarter inch 16-gauge steel tube with the flattened ends screwed to the chassis should provide adequate reinforcement.
The final finish of the hull is very important, as friction can be materially reduced by good varnishing. Thin coats of clear copal varnish should be applied until no unevenness is visible. Then the whole of the bottom should be rubbed down with fine glass paper and a little linseed oil until a fine surface is obtained, and two or three finishing coats applied. Shellac varnishes are useless as they absorb water rapidly and become rough and milky. Polishing with furniture polish renders the surface quite waterproof, and is often recommended as a means of improving the slip.