Should Plywood be Fastened to the Frames when Building a Plywood boat?

Building boats planked with sheet plywood started around the World War II era. Before that, the adhesives used to make plywood were, in short, worthless for marine application; the plies would delaminate in a heavy fog. After the adhesives were improved, it took a long time before professional boatbuilders would use plywood because of its previous problems.

Builders of the time planked boats with solid wood with many seams. When plywood started to be used, builders logically built the same framework and attached the plywood in the same way as they had done with planks. Rarely did that work out. Boats planked with sheet plywood wouldn't conform to the shape of a planked boat. A surface developed from a segment of a cone or cylinder was required to enable the sheet material to bend to natural curves. The method is known as "sheet plywood development".

Most builders retained the closely spaced frames with many longitudinals and a zillion fasteners. Then someone reasoned that all that framework wasn't required. Further down the road, many production boats were monocoque; the boats were built over a form with stem, transom, chine and sheer the only framework members. After the hull was removed from the form and righted, they were reinforced with internal framework, primarily longitudinals. This eventually led to current Stitch and Glue boatbuilding.

But before that progression, the sheet plywood boat framework resembled that of its predecessor, the planked boat. At about the same time, more powerful motors such as the automotive V-8's were converted and installed in the new plywood boats. This was before the advent of superior epoxy adhesives and epoxy based putties for fastener holes. At higher speeds, the putty over the countersunk fasteners would come out, water would catch the outer plywood lamination and eventually rip out the outer plywood lamination, starting at the screw hole.

This condition was primarily observed across the plywood at frames that contacted the bottom, longitudinal fasteners caused minimal problems. The logical solution was to eliminate cross fastenings in the bottom frames, and it worked. Some builders even went so far as to relieve the frame so it had no contact to the bottom planking. This also worked well and today is quite common in faster sheet plywood boats, although in slower boats the frame contacting the planking is still prevalent. However, fasteners are preferably NOT used across the planking in any sheet plywood boat.

Fairing a sheet plywood boat framework so the planking will perfectly contact the frames is wonderful in theory, but virtually impossible in practice. The frame will either contact the planking causing a hard spot or the reverse. In many sheet plywood boats, side battens were eliminated or minimized and this accentuated any out of sync frame. Fasteners driven into an out of sync frame causes a visible bump or dish that will mirror through the finished planking.

This becomes more visible on dark painted sheet plywood planked sides. Eliminating fasteners and relieving the frames so they did not contact the plywood solved the problem. Many die hard builders and older texts still retain the notion that sheet planking should contact frames. The latest practice allows the sheet plywood to take a natural bend; it isn't being forced against framework to form an unnatural compound curvature.

Many have questioned why we do not advocate fastening into the frames on sheet plywood boats. The foregoing is our reasoning and is proven in practice. A boat built using the described procedures will not have bumps or hollow spots or framework mirrored to the outside. It'll be a smooth uniform surface and even black painted hulls are practical. Plus, it's proven to be durable. Why build any other way?