Plywood Composite Construction
by Tracy O'Brien
Because terminology seem to change at an even greater rate than the technology it is mew to describe, I will preface this article with the following definition: Plywood composite construction is a method of joining plywood panels by the use of structural fillets and multiple layers of resin bonded fiberglass or synthetic tape.
The method is commonly referred to as "stitch and glue," "tack and tape," or "sewn seam" construction. While all of these terms refer to a similar finished product, they do reflect differences in the procedures used to hold the panels in align-ment during the bonding process. For example, the panels can be held together with twisted wire "stitches," "tacked" together temporarily with staples or nails, or be secured in a jig.
Although the technique has been in use for many years, it has been since the introduction of easy-to-use structural epoxies that it has reached its full potential.
Plywood composite joints enjoy the advantage of nearly 100% duplication of the structural characteristics of the parent material. To understand why this is so, we have to look at the strengths and weaknesses of each of the materials that make up the joint, as well as the physical properties of load-carrying structures in general (see below).
Wood is a low density material made up of unidirectional hollow-celled fibers. The cylindrical shape of these fibers makes them inherently stiff, both because the curve of the cell wall resists buckling, and because the load-bearing surfaces are held apart, maximizing the ability to resist both compression and tension.
Because of these characteristics, wood is an excellent building material. Its stiffness in relation to its weight and its resistance to short load and structural fatigue put it well out in front of other more exotic structures, especially when ex-pense and case of construction are considered. While wood is quite strong in the direction that its fibers run, it is weak, in both tension and compression when subjected to loads across the gain. Plywood overcomes this problem by mechanically re-orienting the wood fibers during lamination. Plywood thus maintains most of the stiffness of solid wood, but tends to be stiffer in the direction of the grain of the face veneers. (This is less noticable as the number of plies is increased for a given thickness.)
At the heart of the composite joint itself are two other materials, each of which plays an important part in the integri-ty of the structure. Multiple layers of fiberglass tape are applied to both sides of the Joint. This is for the purpose of carrying tension loads from the face of the plywood on one side of the joint, to the face of the plywood on the other side. The physical separation of the inner and outer tape laminations provided by the plywood and the structural fillet must be sufficient to prevent the tape from flexing, or the fiberglass will fatigue and fail.
The structural fillet is usually made up of epoxy resin to which one or more thickeners or extenders have been added, yielding a dough-like material that can be applied to the inside radius of the joint with a shaped paddle. The purpose of the fillet is to maintain or even enhance the separation between the load bearing inner and outer surfaces of the plywood. The fillet material also provides the compressive strength that the fiberglass tape lacks.
Much of the splitting seen at the seams of fiberglass-sheathed wooden hulls is caused by repeated swelling and shrinking of solid framing in response to changes in humidity. Because epoxy -coated plywood is extremely stable, and the joint itself contains little more mass than the plywood it joins, plywood composite hulls do not suffer from this problem. The elimination of a significant amount of framing material (and fasteners) allows the desiger to increase the skin thickness of the hull without incurring a weight penalty, thus producing a stiffer, more durable hull. Another important advantage of plywood compos-ite construction is that it is possible to produce a structure with almost no susceptibility to rot, since every surface of every component can be sealed with epoxy during assembly, and the lack of framing eliminates hard-to-clean and trouble-prone areas.
While plywood composite construction methods can be used successfully in the construction of many hard-chine and multi-chine hulls, certain limitations do exist. If any panel in a given design is not the product of conical or cylindrical development (a section of a cone or cylinder), the panel may have to be cold-molded as a separate unit. Ibis would also include designs that require a panel to be forced into a spherical shape at the forefoot. (Some torturing of the panels is possible, but not to the extent that it can be done over a frame or a jig.)
Generally, plywood composite hulls are assembled from carefully lofted pre-shaped panels. Panel shapes are either taken from models or full-size molds, or developed using complex mechanical or mathematical procedures. For this reason, the adaptation of drawings of a conventionally constructed boat may prove to be too tedious and time consuming for a one-off effort.
In the design of plywood com-posite hulls, my practice has been to enhance the inherent stiffness of the plywood by building curvature into the panels and by orienting the face grain of the panels in such a way as to take full advantage of any differences in stiffness.
Open boats are susceptable to twist. In studying numerous derelict wooden boats, I find time and again that their hulls failed along the sheer rail structure. This is particularly true when plywood sheer decks were employed, or when an open rail structure is capped-over forward with a full-width plywood deck. To prevent this type of structural failure, I have adopted the practice of fitting my larger designs with edge-glued, wood grip sheer decks. These decks am supported by a series of epoxy-bonded plywood knees on their undersides, and are held rigidly to the transom by the flotation cover panel, which acts as a huge gusset.
Because plywood composite hulls are assembled from pre-shaped panels, scarphing is an important part of their construction (see Boatbuilder #21, September/October 1986). In addition to scarphing the side and the bottom panels, the rubrails, interior trim and sole battens am generally made up full length prior to installation. In my own designs, the strips that make up ft sheer deck are scarphed in place, as is the second 1/2" plywood layer on boats with I" bottoms.
Assembly procedures very from design to design; but generally consist of temporary fastening. installation of temporary bracing. an alignment sequence, interior bonding, exterior bonding, and finish joinery.
The bonding process usually begins with the interior. A fillet of epoxy and wood flour or epoxy and microballoons is applied to achieve a specified radius at the joint. This is done using paddles cut to die required radius. Any excess is lifted off immediately with a putty knife. Irregularities can be removed with an elliptical shavehook as the fillet cures. The fiberglass tape is precut for each seam and saturated with epoxy using a roller. Multiple layers of tape are usually applied in descending widths (widest applied first), which speeds the feather sanding later on. The pre-wet tape is best handled by rolling it onto a short stick. It is smoothed with a gloved hand as it is rolled out.
On smaller hulls it is often expedient to complete the interior woodwork before inverting the hull and completing the bottom sheathing and seam taping. Transom and stem exterior taping should be applied and sanded prior to the installation of the rubrails.
Exterior bonding begins with the removal of any temporary fasteners (wire stitches, etc.) and the radiusing of the joints. When bottom sheathing is called for, it is applied prior to the application of seam taping. If the bull is to be painted, feather sanding can be reduced consider- ably by the use of a fairing compound made up of epoxy and microballoons. Keel battens, lift strakes and the like should be installed after sheathing and taping the bull.
The interiors of larger hulls are fined out after the "shell" is fabricated, in much the same way as you would a fiberglass hull. This is accomplished by bonding all bulkheads and other interior panels with structural fillets and fiberglass tape, as specified by the designer.
One of the major objections that is raised regarding plywood boats is that it is difficult to maintain the finish on them. This has been true for two reasons: First, without an adequate vapor barrier on the surface of the plywood, sufficient vapor pressure can develop in the plywood to cause the paint to slough off. Secondly, the hard and soft wood exposed in the rotary-cut faces of fir plywood is so different in density that it will cause any paint applied to it to crack from thermal expansion and contraction. To correct the problem of the movement of water vapor you need only apply 2 or 3 coats of epoxy to each face of the plywood. The problem of thermal expansion can only be eliminated by sheathing the fir plywood with fiberglass or synthetic fabric layed in epoxy; or by using marine plywood with a more stable face veneer, such as one of the species of mahogany. In my shop I have found it most practical to use fiberglass-sheathed fir plywood for bottom panels, and mahogany plywood for topside panels and other exposed surfaces
Today's plywood composite designs are surprisingly easy to build, yet are lighter and more durable than the most sophisticated designs of a few years ago. We can expect even greater gains in the near future, as designers continue to integrate plywood composite construction with other wood-epoxy technologies.
Reprinted from Boatbuilder Magazine.