Most have never heard of Sam S. Rabl, a naval architect, author and innovator of small boat building methods. Sam was at the height of his career back in the mid-forties, when he wrote several books and numerous boatbuilding articles. Most of his works are now long gone, so why should you care about this man?
Sam developed a method of fairing a boat framework to accommodate sheet plywood that is simple and accurate; it takes the guesswork out of fairing. We call it the “Rabl Method”.
Sheet plywood boat planking is bent to the shape of a segment of a cylinder or a cone, a design process known as “conendric development”. Sam Rabl used this design method as the basis for his novel fairing method.
Most sheet plywood boat hulls are easy to fair in the aft section (incidentally, the word “fairing” means shaping a boat framework so the plywood will lay flat or mate to all members). Using the forward bottom of a typical sheet plywood hull as an example (see Plate 18-C) a series of equally spaced points are drawn along the curvature of the stem/keel and chine. The spaces are usually about a foot apart, although more points can be added if desired.
Starting at the stem/chine junction the points are labeled progressively "A”, “B”, “C”, etc., along both the stem/keel and chine. Use a straight-edge extending between points “A” on the stem and “A” on the chine to determine the amount of bevel required; file a notch in the respective members so the straight edge lies flat. Continue doing the same at each point, going aft until the bottom is flatter in section; the bevel at each frame is then filed in the keel and chine.
Use a plane sweeping between the points in a fair line and you have an accurate faired surface that the plywood planking will contact. The same method can be used to fair the surface for the side planking, but in practice using the frames as a guide and notching the chine or sheer will suffice.
As you sit back and admire your accurately faired hull give a thought to Sam Rabl, the guy who made it so easy.
Editor’s Note: A clever adaptation of the Rabl Method can be viewed in the Fairing Frames without Trouble video posted by Klaus Silbernagl immediately preceding this post in the Boatbuilders' Blog.
You can find dozens more "Designer's Notebook" articles by Glen L Witt in the arhives of the Glen-L WebLetters, the forerunner of the Boatbuilders' Blog.