Tortured plywood was originally a description of how Tornado catamarans were built by ‘torturing’ the thin plywood skins into shapes that were previously thought impossible for plywood. It was a very innovative way to build a beautiful round-bilge boat from sheet material. Some argue that there is no such thing as ‘tortured plywood’ because plywood cannot be bent into compound shapes no matter what method is used. That is their theory but science has proven over and over again that theories are there to be broken.
Plywood is a great engineering material. Man has taken nature’s wonder engineering material, wood, and improved on the structural characteristics for use in large sheets. In its 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.
This is a forward bottom panel from the Cape Cutter 19. It has been clamped in place and allowed time to rest and distort to the shape of the hull. You can see how it has retained most of the twist and is ready to be glued to the hull.
The Paper Jet bottom panel has nearly 90° of twist over its length, with most of the twist in the bow. Starting from the bow and working toward the stern, the twist can be pulled in much more easily than starting from the stern.
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° to each other, each layer strengthens those on each side of it in its weakest direction. The resulting material loses its 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 man-made 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 laminated sheet. As a simple rule of thumb, the more veneers that it has, the stiffer and more stable the sheet will be, when comparing sheets of the same wood specie. When comparing sheets of different species, the denser (heavier) specie will likely be the stronger one.
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. I regularly design panels that my surface modelling programs tell me can’t be skinned with sheet materials but knowledge of the materials that will be used allow me to bend the rules a bit.
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. It doesn’t have to be a wood specie that is naturally flexible; it is done with oak and other strong timbers. 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 or use a heat gun to warm it. 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.
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 inner surface of the curve 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 because the soft spot at each kerf will show on the outside of the curve if the kerfs are too far apart. 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. What is happening is that when the plywood is being twisted it tries to keep one diagonal of the sheet as a straight line, to keep to the conical or cylindrical curvature of a developable shape. If the diagonal is 20ft long there are large stresses imposed in overcoming that natural tendency of the sheet. If the diagonal is only 4ft or 5ft long the stresses are considerably reduced so the sheet complies more easily to the shape that you want.
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, even though the sections through the panel are straight lines. 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 a considerable amount of the bend due to stretching/compressing of the wood. Instead, go back two or three 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.
8. If you do have a long panel that you want to skin in one large piece for some reason, don’t start by gluing the easy end first then working toward the twisted end. That will almost guarantee that you will use some choice words along the way and have to use a lot of brute force to pull in that difficult end. Instead, glue the twisted end first then gradually pull in the twist by working along the panel toward the easy end. Also, don’t just dive in and start gluing the panel in place before you have tested it for proper fit. That needs you to clamp it against the framing at the twisted end first and gradually pull it in and adjust the position as needed until it fits properly. Then leave it there for a few days, wetting/heating occasionally so that it can relax into the shape that you need.
9. Plywood that is twisted diagonally will bulge outward, due to the conical/cylindrical forms adopted by sheet materials. This extra hull volume may be desirable for some boats, like planing powerboats. For other boat types that extra volume may not be desirable, for performance reasons. Running a stringer or two through the bulkheads/frames will add strength and rigidity to the panel and will also allow the builder to pull the panel inward against the frames with screws, to glue it to the stringer/s. This makes a finer bow, with smaller bow wave, improved wave penetration and greater speed.
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 its partner on the other side of the boat to the form that you need and they will hold most of the twist when released, then it is time to glue them 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. These processes take time but it is better to take your time than to rush it, then have to deep-six the panel.