Developable Surfaces for Plywood Boats


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Clarence E. Werback/Understanding stresses and curvatures of plywood in boat building.

by Chas. P. Burgess

Laying out a hull so that the true surface of it can be readily determined.

The rapidly growing popularity of plywood and sheet metal for the construction of small boats and yachts makes the problem of designing developable bottom and sides one of timely interest to both professional and amateur yacht designers.    Strangely enough, it appears that very few designers, even among the professionals, know the solution to the problem, and yet it is quite simple, and can provide the amateur with a lot of fun when he knows how the trick is turned. All curved surfaces may be classified as "developable" or "undevelopable." The side of a cylinder is a familiar example of a developable surface, and a sphere an undevelopable one. A sheet of paper may be rolled into any form of developable surface, but it cannot be formed into a sphere or any other undevelopable surface without crinkling or stretching. It is generally supposed that a V-bottom boat with straight lines in all cross-sections is developable; but, a matter of fact, it is not strictly developable unless th angle of deadrise is constant throughout the length, though it may be near enough to true developability to permit forcing plywood into it. A better shape of bottom, and one which is strictly developable, can be designed with slightly convex sections forward, and with the angle of deadrise increasing towards the bow. A widespread and quite erroneous belief is that a developable surface cannot be curved in two direction at right angles to each other, e.g., longitudinally and transversely in a boat.    The fallacy of this belief may be seen at once by imagining two diagonal sections through a cylinder, intersecting one another at right angles. Both of these sections have curved intercepts with the surface of the cylinder. In fact, with the exception of lines parallel to the axis, all lines on the surface of a cylinder are curved; and yet a cylindrical surfae is indubitably developable.

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