The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

294 Hull Construction Methods
design parameters we use in developing hull shapes and
instructions on designing and building models. If you
have purchased plans, begin with the specific techniques
for building described in the first portion of the chapter.
If you are developing your own compounded plywood
hull, however, read over the sections on design before
you start. In either case, the first step of any project
should be to make up samples for testing. Experiment
with keel joints and be sure that the plywood you have
chosen will be able to bend around the hull’s hardest
curves.
Compounding Plywood
To compound a sheet of plywood, you bend it in two
directions at the same time. All plywood can be
compounded, but as we will discuss later, the amount
of compounding possible for a given sheet of plywood
has not been firmly established. In general, the
“compoundability” of a panel is determined by its
thickness, by the number of plies used to make it, and
by the species of wood used. Plywood thickness is a
limiting factor in the size of the hulls that can be built
with compounded plywood construction. We have
used up to 1 ⁄4" (6mm) 5-ply panels and usually choose
the thickest plywood that can be bent to shape.
Different woods of the same thickness bend with varying
difficulty. Although different species of woods may be
used for compounded plywood construction, we almost
always choose the very best Okoume marine grade or
birch aircraft plywood. Each species has its pluses and
minuses; the important thing to consider with this
boatbuilding method is that flaws and voids within a
laminate may cause it to break when it is compounded.
High quality panels should therefore be used.
Construction
To begin building a compounded hull, make two
identical flat plywood panels. Each of these should be
slightly larger than one half of the hull skin, so it’s
usually necessary to scarf two or more 4' � 8'
(1200mm � 2400mm) sheets into a single piece.
Sometimes we have been able to lay out both panels
on a single sheet by alternating them end for end.
Precoating plywood with WEST SYSTEM ® epoxy increases
its stiffness and affects compounding. We precoat only
areas like the bow interior which will be difficult to reach
in the folded hull, and these only if little compounding
is required of them. Several of our compounded
plywood designs called for applying a layer of fiberglass
cloth to the outside of panels before folding them into
shape. As we discuss later in this chapter, fiberglass
sheathing increases stiffness significantly. Should you
decide to use this modification of the compounded
plywood technique, make both a model and a test
panel and check to be sure that it will be able to
negotiate the hull’s most severe curves.
<strong>The</strong> shape of a compounded hull is determined primarily
by its rocker profile, its keel angles, and its sheer
line, all of which are explained in greater detail in the
design section of this chapter. <strong>The</strong> amount of rocker cut
along the bottom of the panels where they join to form
the keel and the keel angles are the main factors in
Figure 25-2 Plywood panels for a Tornado catamaran. <strong>The</strong>se
panels are cut from 5mm okoume plywood.