Building a hull with frames and plywood generally goes like this:
The first thing is to build the frames, bulkheads, transom and stem-piece as shown on the plans and mark them by frame number. Typically the frames are of 4 to 8 mm plywood with framing of clear pine or cedar. Typically, they are slotted for chine longitudinals (chinelogs), the deck or gunwale stringer and sometimes for a keelson. Actual erection then starts upside down on a rectangular base frame that is about the same length as the boat but typically about 50-60% of the max. beam in width. While a small boat up to say 13' might be built on frame members of standard 2 × 4s, the use of 2 × 6s will be needed for boats up to about 20' and 2 × 8" for boats up to 25', as you don't want the frame to deform as you proceed with the building. Although it's easier to start with pieces that are pretty straight, it's not 100% essential as the frames will be positioned vertically on the frame based on a tight datum line (preferably a wire) that will be at the boat's centerline. It's very important that the frames are positioned accurately, noting which face of the frame is to be located at the given position, and that they also be vertical, square to the centerline and at the right height. Getting this all correct and accurate requires lots of careful measuring as if not done with care, it's very likely your hull will not turn out fair and symmetrical as intended.
Generally, the frames and bulkheads are further held in position by a keelson and two inwales or inner gunwales and these lie in the frame slots so that they are flush with the exterior of the frames. The so-called chine logs will then be added that will make the connection between the bottom and side ply. Then starts the frame fairing process.
The keelson (inner keel), chine logs and the frames themselves, will need to be beveled so that their final surfaces lay in the same plane as the plywood and to check that, you'll need a straight edge. Where there is little or no twist to the plywood, one can use a straight hardwood batten about 30" × 3 × ½" with one edge beveled. Where there is curvature, it's helpful to have a strip of sufficiently flexible plywood about 4' × 8" × 5 mm, that can be bent over the frames to see what bevel will be needed to seat the plywood correctly. One way to fair across the longitudinal members is to use a power plane with a guide bar resting on the adjacent chine log. (Here's a photo of a builder using this approach, taken with appreciation from another web article.)
Keep in mind that at the bow, it's possible that the plywood will be rolled (as around the sides of a cone), and that means that the forward frames will not be cut flat but will have some convex curvature. Careful use of a spokeshave may be helpful here although some final strokes with a long plane will help to keep things fair. Only in a case where the line of plywood from frame to frame stays parallel, will there be no twist. In such a case, the sections can indeed stay flat and an example of this, is the underwater deep-vee of the Buccaneer amas—shown below.
Plywood sheeting will need to be joined with either scarfs or with a butt strap on the back. The latter is easier while the former is neater and lighter. Scarfs need to be a minimum of 6 times the ply thickness (even 8) and a butt strap generally about 3 times as much (ie: 18-24t range). Plywood is usually held in place while being bonded with clamps or with screws and often the latter are left in place so they must be of solid brass and not just brass coated. There is often debate on whether to epoxy-coat plywood.
I've even heard it said "there is no debate; it should always be done!" Well to be honest, I think I've seen more ply boats fail due to poor epoxy inside coatings than from lack of epoxy, so I am hesitant to recommend it without reserve. The Buccaneer shown here was only treated with wood preservative inside and yet with good ventilation, is quite sound after nearly 30 years. By comparison, the amas of another boat of the very same design that were 'triple coated' in epoxy, lasted only ½ that time and were already waterlogged and heavy for much of that time. The reason is that with all the complicated parts on the inside of a wood-framed boat, it's almost impossible to avoid pin holes (or worse) in the somewhat viscous epoxy coating and so water and moisture gets in behind the coating and then cannot EVER dry out. (See Next Photo…)
This ply boat had received THREE coats of WEST system epoxy and even when water got in behind the barrier, the barrier itself stayed intact though the ply delaminated and rotted. Neater, higher quality construction and pre-filling of all crevices and corners could have saved it from the chain saw.
Personally, I am a great fan of using aluminum paint on the inside surfaces of wood above the waterline. It penetrates well, is lightweight, inexpensive and lasts well. The only negative I found is that you have to scrape pretty deep and then pre-clean with acetone prior to successfully bonding any new part to the surface with epoxy. The lesson here is to do all the epoxy bonding BEFORE you paint—and the same would be true if using any other wood preservative. For the lower interior bilge areas, the use of a coal-tar epoxy is a good choice, but first make SURE that the surfaces to be coated are nicely rounded, all cracks filled and that there are no pin holes.
For general tips about working with plywood, there are some good notes on this website: woodworking.about.com — plywood.
The PROS of working with 'plywood on frame', are that both the tools and materials are generally familiar and since the availability of great bonding resins, it's almost impossible to make a mistake that cannot be corrected—even if that means removing parts and re-doing them. Good marine plywood is tough, resistant and fairly light in relation to its stiffness and strength though, as for all wood products, good workmanship will pay off with longer life. It's also readily available and still reasonably cost-effective. The main thing is to provide both good protection against contact with moisture and sun exposure AND also provide excellent ventilation throughout its life—something that is too often ignored.
The CONS are that it can deteriorate fast in the presence of moisture and without ventilation and the shape of a boat's hull of plywood is limited to flat, cylindrical and/or conical forms. In areas that require framing and bulkheads, the system is also somewhat heavier and takes more manhours than more recent solutions that reduce the need for framing. This same framing also takes up more interior space compared to some of the newer systems. There is also the very clear fact that a frame and plywood boat is ALWAYS considered amateur built, so regardless of how well it's done, it will always have a lower resale value than a comparable round bilge or glass boat. But by using plywood in simpler areas above the waterline, one can take advantage of its good stiffness-to-weight ratio and then effectively combine it with the use of another more suitable system to achieve a round bilge below the waterline (see review of alternative hybrid methods).
For the choice of plywood itself, go to: Marine Plywood for some guidance on what to use and why.
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