Designer's Notebook: The case for laminations


In the following we'll discuss some work that has been done with laminations, some unusual and out of the ordinary.

A production method

Several years ago, some southern California professional builders were producing sheet plywood boats with all plywood laminations for keel, stem, chine, and sheer. The method may also have been common to other parts of the country. The boats were built on a form or jig and the laminations made over the form. These boats were in the 20' range with keels laminated from mostly 3/8" plywood, usually about 6" wide aft and extending forward and bending in the forward section to form the stem with the width reduced to about 2". The aft portion of the keel was about six laminations or so thick with the stem section more than twice that. The stem laminations ended stair step atop the keel (into a notched form) to spread the stress. The joining laminations were scarfed or butt joined, staggering the joints so that no two ended at the same point. Numerous clamps or clamping fixtures were used to hold the laminations while curing. I did see one innovative builder clamping a rubber strip from a truck inner-tube to the keel and tightly wrapping it diagonally with the final end secured by another clamp, thus reducing the number of clamps required.

The chines were made similarly from plywood lamination, much wider and thicker than in conventional practice because so much was eventually faired away. The forms were cut off diagonally at the chine point and the chines were set in diagonally across the bottom side junction. Bending was simple, not always easy in conventional chine construction.

Obviously fairing these lams was quite a chore, particularly the chine. The builders used electric planes and routers on special fixtures to accurately fair the members, in some cases disc sanders with very coarse paper were used to initially remove the surplus.

The inner sheers were also plywood, bent in place after applying the bottom and sides and righting the boat. The laminations were applied directly against the side plywood and again they were rather oversize. Those familiar with bending wood know that wide thin uniform strips can't practically be bent around the typical plywood boat with forward flare. Such a member cannot be forced up or down without "kinking" so the laminations were sawn to shape. Templates were made and subsequent laminations duplicated with router or collar shaping. No fairing was required as the member sides to the planking, however, beveling for the deck was necessary. When the hulls were removed from the form they had no frames, though sometimes frames were added.

Note that the boats built using laminated longitudinals were production boats and the expense of the fixtures usually makes it impractical for building a single boat. However, the method may suggest some possibilities to the innovative builder.

In Stitch and Glue

We've used bottom batten and keel laminations in stitch and glue boats for quite some time. However, these are applied after the boat shell has been formed. All plywood laminations or combinations of plywood initial laminations capped with a solid wood-reinforcing member have worked out well. Wider battens spread the support area, provides additional gluing surface, are easier to install, and thinner laminations make the bends more easily. Battens laminated from plywood have never become common on conventionally built plywood craft. In a vee bottom boat, it is difficult to bend the thin wide strips toward the stem in a nice fair curve. Reducing the width as the bow is approached helps, but getting the laminations to bend to a shape that will match the plywood planking is a problem. Insurmountable? No, but again a lot of thought, work and possibly some shaped fixtures may be required.

Common practices

Plywood laminations on the inner surface of longitudinals have long been used to prevent splitting. The centerline keel seam is critical and when a solid member has two halves of sheet plywood fastened to it a local stress occurs. A lamination of plywood applied over a solid wood keel need not be thick or continuous, 1/4" plywood on smaller boats, fitting between frames on the inside, is usually sufficient. If using epoxy adhesives, only minimal fasteners are required; epoxy adhesives, properly used, have proven to be as strong as the wood itself.

Laminated beams for deck or cabin tops are a natural for laminated construction and have been used extensively for years using either plywood or lumber. Laminated beams are stronger and use considerably less material than sawn members but do require a jig or fixture for forming. Three laminations are the minimum to use. Some builders make the laminations wide and rip it into several beams. Always leave a little extra for cleanup, as possible misalignment of the llaminations and cleanup is usually required. A problem with laminated beams is that they will collapse slightly when taken off the gluing jig.

Replacing sawn frames on a vee bottom boat with frames made from laminations is impractical, but advantageous on bent or round bilge boat frames. However, in high-speed vee bottomed boats with solid wood bottom, frames are often laminated with thin plywood to prevent splitting; longitudinal motor stringers laminated with plywood can be significantly stiffened. Note, when I discuss plywood laminations for frames, they are used so that the veneers are parallel to the planking. The longitudinals are fastened to the frames and fasteners do not hold well in edge-grain.

Heavy skegs or keel members made from laminations have the advantage of being more stable and easier to handle. On inboard installations where a shaft hole goes through a heavy skeg, the hole can be progressively cut or drilled in the laminations, eliminating the typical long boring tool required for solid members and avoiding the ever present possibility that the long hole is not drilled in correct alignment.

Although it would be possible to laminate long longitudinals from shorter pieces, it requires a lot of work and glue, and a long scarf joint is a more practical solution.

We've used multiple laminations of solid wood or plywood for transoms, corner knees, stems, keels, battens, planking, cabin tops, decks, hull planking and undoubtedly other places that haven't come to mind. So during your building, think laminations, or at least consider them. P.S. solid wood laminations also look nice.