The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

286 Hull Construction Methods
athwartship frame edges be even slightly higher than
any of the longitudinal members. This is especially
important if you are using thinner plywood for the
exterior skin because slightly high frames tend to
produce a slight athwartship distortion, which may be
noticeable in the finished hull. Edges that are too low
may also distort the hull shape.
Use a fairing batten and a piece of plywood measuring
approximately 1' (300mm) wide and 4' (1200mm) long
to determine overall fairness in both directions. <strong>The</strong>
plywood gives an indication as to how the panel will
actually lie when it is installed on the frame surface.
Because you will be fairing flat surfaces, longer hand
planes, such as jack planes, are very handy for final
fairing.
Some boat plans may call for compounding or bending
of the panels in two directions at the same time. You can
accomplish this by curving the frames slightly between
plywood joint positions, thus making the plywood
bend in both directions. While this compounding does
add to the stiffness of a plywood panel, difficulty can
quickly arise if caution is not used on hard chine
frame-supported plywood structures. Usually, the hard
chine method of construction is chosen because of its
ease and simplicity, and when you try to compound or
bend plywood in two directions on a frame-supported
surface, unwarranted complications can develop that can
easily nullify any benefits gained. <strong>The</strong>re are excellent and
simple methods of compounding plywood which are
explained in Chapter 25, but this compounding is done
using a unique concept, where the compounded plywood
surface does not have to be made to fit a preformed
framework. (Also see discussion of compounding sheet
plywood on decks and cabin tops in Chapter 27.)
Fitting Panels
You must first decide how large a plywood panel you can
apply easily at one time. Generally, this depends on the
size of the boat you are building. With smaller dinghies,
it is common to apply a complete prescarfed panel over
one section of the hull in one bonding operation. With
larger boats, it will be easier to install smaller panels
and perform the scarf joint assembly right in place on
the boat hull, as discussed in Chapter 12.
Other benefits in using shorter panels include ease of
handling and more efficient use of material because
shorter panels create less scrap and are easier to mark
and fit prior to final application. In any case, a 16'
(4.8m) long panel is about the maximum length that
you will want to install on any hull at one time. Our
experience has been that it takes a very organized and
coordinated effort by a number of people to apply large
panels and to do a good job both in installation and
cleanup. <strong>The</strong> main problem is time. You need time to
apply adhesive to all of the stringers, frames, chines,
keel, stem, etc. More time is required to position the
panel and apply proper clamping pressure with staples.
Even after the panel is in final position, a little more
time will be needed to clean up the excess adhesive that
squeezes out between the ply and the framework before
it cures.
When scarfing in place on a hull, it’s usually best to
locate joints over bulkheads, frames, and other solid
components. If you are using plywood that is at least 3 ⁄8"
(9mm) thick, however, and have ample longitudinal
stringers for support, it’s possible to scarf anywhere
without the totally rigid support of a frame or
bulkhead. With thinner plywood, you can scarf over
temporary, plastic-wrapped butt blocks if a frame or
bulkhead isn’t handy.
<strong>The</strong>re are a number of ways to mark panels for rough
sawing and accurate scarf alignment. <strong>The</strong> easiest
method is to clamp a sheet of plywood over the area it
will cover and adjust it for minimal waste. <strong>The</strong>n draw
pencil lines on the underside of the panel, using the
chine and keel or sheer and chine as guides. Cut the
sheet and check again for fit, and mark stringer and
scarf locations. To speed things up later, tack the panel
in place for this fitting and leave the nails in the
plywood to help align it. Surmark the panel to facilitate
installation.
Scarfs should be evenly centered on frames. To pin down
their locations, first determine the length of the scarf
bevels. This should be eight times the plywood thickness,
so if you were applying 1 ⁄4" (6mm) plywood the
joint would be 2" (50mm) long. To mark a 2" scarf on
the panel, lay it out so that it centers on a 3 ⁄4" (18mm)
thick frame and its edges extend 5 ⁄8" (15mm) on either
side. (See Figure 24-6.) When the two panels are