The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

310 Hull Construction Methods
hull fullness, and the amount of compounding within
the structure. It will usually require some adjustment to
make up for curvature at the hull’s midsection. Take all
of these factors into account when determining the
ultimate sheer line. Remember that the sheer line must
curve evenly over its length, with no hollows or flat
spots.
For basic clues on the sheer profile and height, look at
the profile view and a reasonably drawn version of the
body plan. <strong>The</strong> aft and midsections of a hull usually
demand more curve from the plywood, and the sheer is
usually therefore higher at the bow, where the sections
are much straighter. To develop a straight sheer line,
add material in the midsection as shown in Figure 25-31
to make up for the curve there. Measure along the
anticipated hull profile at each station to determine
how much extra plywood is needed and add this to the
mid-sheer section of the layout. <strong>The</strong>n cut the panels for
the first model.
Building the Model
<strong>The</strong> first model is usually only a rough attempt to
achieve the hull you desire and will serve as a guideline
for refinements that can be made with the next model.
This first model will also give you a much better understanding
of the compounded plywood hull development
process. We have always built our models on a scale of
1" to the foot because we have learned by experience
which thicknesses of plywood we can use for the model
that will scale up with reasonable accuracy to full size.
We have used the following thicknesses of aircraft birch
plywood purchased from aircraft supply houses. <strong>The</strong>se
thicknesses seem to be universally available.
Aircraft birch plywood
Scales to these full size panels
(okoume plywood)
.8mm
5
⁄32" 3-ply (4mm)
1.0 mm
3
⁄16" 3-ply (5mm)
1.2 mm
1 ⁄4" 3-ply (6mm)
1.5 mm
1 ⁄4" 3-ply (6mm)
While aircraft birch plywood is expensive, you can
make a lot of models from one standard 4' � 4'
(1200mm � 1200mm) sheet. Certainly the models are
much cheaper than making full-size mistakes that are
costly not only in materials, but also in time. Scaling
between these two materials has been quite accurate.
Several other factors must also be scaled properly,
however, such as the manufactured keel and the sheer
clamps. Remember that errors made in the models are
projected to full scale by a factor of twelve, so everything
you can do to maintain accuracy when building
the models is essential.
Build the model as you would a full-size boat. First
prepare two identical panels. Temporarily fasten them
together and make and cut the keel, bow, and sheer
lines to the dimensions established in your drawings.
True these up and make a master template, a third
panel identical to the first two. This will serve as a
record of the panels in this model and can be used as a
pattern against which to make small changes in future
models. Make a master template for every model so
that you can clearly see the changes each new panel
dimension brings.
<strong>The</strong> first step in assembling the panels is to install sheer
clamps. <strong>The</strong>se will, by necessity, be larger than scale. A
1" (25.4mm) square sheer clamp reduces to 1 ⁄12" (2.1mm)
square, which is too flimsy to adequately represent the
real sheer clamp, so in this case we would increase the
model sheer clamp to 3 ⁄16" (4.7mm) square. Apply epoxy
to the sheer clamps and hold them in place with spring
clamps or clothespins until the epoxy cures.
Bevel the exterior radius of the plywood, as you would
on a full-size hull if you were using nylon webbing.
Join the two panels along the keel line and at the bow
with duct tape or masking tape. Be careful to line the
edges up perfectly. Surmarks help guarantee alignment
fore and aft.
<strong>The</strong> next stage of the design is to decide how far apart
and at what points to spread the panels to achieve the
hull shape that you desire. If you have drawn a rough
body plan view of the shape that you hope to get out of
the compounded plywood, you can use this as a beginning
point by simply measuring from the body plan the
angles of the hull panels as they emerge from the keel
line on both sides.
<strong>The</strong> panels join at specific keel angles. Divide the hull
into four equal parts by drawing lines through it at the
hull midpoint, a point between the bow and the
midpoint, and a point between the midpoint and the
stern. <strong>The</strong>se three lines become measuring points for
determining the keel angle. If the hull has a transom,