Torturing Plywood in Boatbuilding
Plywood is a great engineering material. Man has taken natures wonder engineering material, wood, and improved on the structural characteristics for use in large sheets. In it's natural form you can't cut wood into 1/4" (6mm) thickness and expect it to remain stable and strong in sizes more than a few inches in width when measured across the grain. It will split, it will twist and it will warp out of the nice flat form that you originally cut. It will be strong along the grain but relatively weak across the grain, which must be taken into account in any structural application.
When that same wood species is cut into thin veneers and they are glued together with the grain direction of adjacent layers oriented at 90 degrees to each other, each layer strengthens those on each side of it in it's weakest direction. The resulting material loses it's linear strength characteristic and strength becomes relatively similar in all directions. The material also becomes much more stable, much less likely to twist, warp and crack. This brings great benefits for designers using this material but it also introduces limitations that the designer or builder must work around. In short, the benefit of plywood is that it is flat and the drawback of plywood is also that it is flat.
Of course the wood species, the quality and number of veneers and the quality of the glue and manufacture all affect the strength and stiffness of the sheet. As a simple rule of thumb, the more veneers, the stiffer and more stable the sheet will be, when comparing sheets of the same wood species. When comparing sheets of different species, the more dense (heavier) specie will likely be the stronger one. Here is an excellent article on plywood quality.
1) Wood can be stretched, within limits. When we bend it, the outer surface of the bend stretches and the inner surface of the bend is compressed. The fibres in the middle are on the neutral axis and are neither stretched nor compressed. The fact that it can be stretched means that it can be deformed in ways that are not necessarily conical or cylindrical in nature, the principles of developable surfaces normally used when designing curved surfaces for sheet materials. It can be somewhat tortured into shapes that mathematics says can't be done. But, push it too far and it will fracture, proving that the mathematics may have been right after all.
2) Wood can be softened and made more pliable if it is wet, especially if it is also heated at the same time. This is the principle used when making steam-bent frames in traditional boatbuilding, softening the wood strip in a steam box. A stiff piece of wood becomes quite soft and malleable when hot and wet, so it can be bent to shape and will hold that shape when it dries and cools. This can also be done with plywood but how do you do it with a big piece that won't fit into a steam box? I have made plywood sheets more flexible by laying them on wet grass, spraying water over the sheets then covering with black plastic and leaving in the sun for a few hours. Others have had similar success using a steam wallpaper stripper on the outside surface while gradually pulling the sheet in with clamps and other mechanical tools. If you have a sheet clamped in place and need to coax more bend into it but don't have a wallpaper stripper, spray water onto both sides of the sheet then cover with black plastic to absorb heat. Even without heat, you can wet the sheet, keep it wet overnight with hessian bags or similar, then find that it is easier to bend in the morning.
|Steam wallpaper stripper can be used to make plywood more pliable|
4) It is easier to deform short sheets a small amount than long sheets a large amount. If you have a panel that is 20ft long and has considerable twist to it, it will be difficult to pull the twist into that long panel if it is all in one piece. It will be easier to get that twist if the panel is fitted in three 8ft lengths that are glued to each other on the hull framing. It will be even easier if fitted in six 4ft lengths but that is only required if the twist is extreme. As example, for the Cape Cutter 19 or Cape Henry 21, with lots of twist in the bow and not much twist aft, the panel would be best fitted as 4ft lengths in the bow, changing to 8ft lengths further aft.
5) You can't easily deform a sheet so that one edge has convex curvature (curved outward) and the opposite edge has concave curvature (curved inward). The stresses in the sheet will likely damage it. The builder of the first Didi Mini to be skinned emailed me to say that he had found it impossible to skin the side panel forward of the mast; the sheet simply could not be forced into the required shape. It has convex curvature at the deck to add reserve buoyancy and concave curvature at the bottom in the form of a hollow bow waterline, to improve wave penetration and increase speed. I told him to cut the sheet vertically so that it was two sheets 4ft long instead of one sheet 8ft long. He emailed back that the problem disappeared and he could easily skin that area.
6) Narrow panels are easier to twist than wide ones. If you have a long and narrow panel that is twisted and another that is the same length and twist but 5x the width, the narrow one can possibly be fitted in one length but the wide one will need to be broken down into shorter lengths to make it easier to fit. My radius chine designs generally have considerable twist in the bottom panel toward the bow. There is no problem twisting this panel because it is narrow in that area.
7) Plywood will take on a set if it is distorted and held in the distorted shape for a few hours. If it won't pull all the way into the shape that you need, don't force it too far, you will break it. Pull it in with clamps, levers and Spanish windlass as far as it will go, then leave it. If you were to unclamp it a day later you would find that it doesn't want to lie flat because it will have taken on some of the bend due to stretching of the wood. Instead, go back 2 or 3 times in 24 hours and pull it in some more. Eventually you should be able to pull it all the way to where you want it.
|Ian Allen, building a Cape Cutter 19, used clamps and levers with|
Spanish windlass to pull in the bottom panels in the bow. He used
an 8ft length and would have found it easier in two 4ft lengths.
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