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.

Plywood structure. Plywood must always have an odd number of veneers, so that the surface veneers on both faces run in the same direction. This is called balanced construction and adds stability to the sheet. If it has an even number of veneers then the surface grains will run at 90 degrees to each other and result in tensions within the sheet that will cause it to warp.

Ever since plywood was first invented, man has been figuring ways to distort it into shapes other than the flat sheets in which it is formed. There are various ways to do this and understanding the nature of the material helps to figure the best way to deform it to suit our needs.

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

3) Wood can be bent by making saw-cuts (kerfs) across the piece on the inside of the curve. This reduces the compression loads on the inside surface and effectively moves the neutral axis locally to the mid-point of the thickness that is left after the kerf is cut. If the kerf is half-way through the wood then the bend characteristics become more like a piece of half the thickness than one of the full thickness. The more kerfs that are cut the more that the piece is softened and the smoother the curve that results. This works with plywood also but must be judiciously done. The kerfs shouldn't be cut deeper than half-way through the plywood, or the structure of the plywood will be destroyed. If you cut them so deep that only the surface veneer remains, you will almost certainly fracture that surface veneer along the kerf line. The kerfs should also not be excessively wide because they will have to be filled with epoxy to regain the lost strength after the panel is glued in place. The width of a circular saw blade or narrow router bit, so about 3mm, is a good kerf width. It gives enough width to allow the sheet to bend without the edges of the cuts closing so much that they can't be filled.

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.

If you are building a boat that has a skin panel that has a large amount of twist that is mostly concentrated in one area then you are almost certain to need one or more of these methods to get the plywood to conform to the hull shape. You can combine various methods from the list, to supercharge the process. Softening with water/heat or steam, combined with kerfs and fitting narrow sheets will allow plywood to take on considerable twist. Once you have managed to pull that panel and it's partner on the other side to the form that you need and they will hold most of the twist when released, then it is time to glue it into place. Don't consider getting one side done and glued in place before starting to twist the other side, you will seriously limit your options for clamping any edges where the two panels meet. You will also load the framework on one side and possibly pull it out of alignment so that the completed boat is asymmetrical. It is best to prepare the one side then set it aside while you prepare the other side, then glue both on in parallel.

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