Wood-epoxy #1 – Plywood Single and Multi-ChinePlywood is an excellent material when epoxy-coated. It is as resistant to decay or structural degradation as any other timber. The simplest form of plywood structure is single chine. Chine construction is the term used when a hull is made up from several flat panels, the chine being the join between the panels. Single chine has just the one join, so there is a topside panel and a bottom panel. The 8' sailing pram - Design No. 035 is a typical single chine pram dinghy. An even simpler form of single chine, such as Design No. 181, has a single bottom panel with a topside panel each side, but this construction can be somewhat limiting in terms of hull shape.
The two methods of putting together a small single chine vessel are “stitch and glue” and “frames and stringers”.
The shapes of the stitch and glue panels are predetermined either by the designer or by the builder. The developed shapes are cut from the ply sheets. A developed shape is a shape that needs to be cut from a flat panel to give the correct shape when that panel is curved and set at its angle in the structure.
The developed panels are laid down on a flat surface and joined together by copper or stainless wire, threaded spirally through holes bored for the purpose at close intervals along the edges. As the panels are sewn together in this way they automatically pull up to the correct boat shape (or reasonably near to it). The assembly can be laid over a simple jig or pulled up onto elements of structure, such as transoms. Once to shape, the stitched joins are epoxy-filled and reinforced inside and out with woven glass tape and epoxy resin.
With modern computer aided design techniques, it is perfectly possible to build the majority of the interior components first, and then drape the stitched hull skin over them. However, because the shapes that can be achieved are limited, “stitch and glue” is usually confined to dinghies and small craft - though it can also with advantage used for multi-hull construction in larger sizes.
Frame and stringer construction consists of relatively lightweight transverse frames, set at well-spaced intervals, plus the transom (or transoms, if a pram dinghy). Around these are bent the longitudinal elements – the backbone, the chine pieces, the shelves or gunwhales, and if necessary, intermediate stringers. Design No. 012 is an example of single chine construction in a larger boat.
The shape and bevels of the transverses can be easily found by the designer and given to the builder in the form of simple data. Longitudinals develop their shape naturally and bevels can be discovered from the transverses.
Once the transverses are set up, and the longitudinals formed around them, the shapes of the ply panels can be derived directly and simply off the hull sub-structure. The panels are bonded to the sub-structure and the whole structure is wood epoxy coated inside and out as usual.
Multi-chine is essentially the same as single chine – there are just more panels and chines. Double chine and triple chine are quite common. The Naja 30' sailboats, which were built by Whisstocks, and also supplied in kit form all over the world, are a good example of a triple chine (four hull panels).
Ply can also be used in place of solid timber in many of the other wood epoxy construction systems described in other technical bulletins.
© George Whisstock. This article is for information only and may not be comercially reproduced in any form or used in any way without permission.