The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 24 – Hard Chine Plywood Construction 283
Next, install plywood gussets at each joint on one
surface only. <strong>The</strong> size and thickness of the gusset
material are usually in proportion to the size and thickness
of the timber stock used for the frames. Typically,
we use 1 ⁄4" to 3 ⁄8" (6mm to 9mm) thick plywood gussets
on 3 ⁄4" (18mm) thick frame material. At the present time
there are no scantling rules worked out on this relationship,
but if you are in doubt, make a sample frame joint
and try to break it. If a break occurs in the joint, make
the gusset bigger.
Because we use gussets of a few standard radii, we can
round and sand their edges and precoat them before
installation on the frames. It’s not necessary to make
them as circles, but as much standardization of gusset
size and shape as possible saves a great deal of time and
labor. Study the lofting of your frames to determine the
best gusset shape for your boat.
Use a high-density adhesive mixture to bond the gussets
and use either clamps or staples for temporary pressure.
Clamps are simpler to use, so choose them wherever
possible. If you are stapling, use either narrow-crown
alloy staples or wide-crown staples. <strong>The</strong> former can be
left in the hull, but the latter should be removed when
the epoxy has cured. Be sure that any staples left in
place are clear of areas that will be notched or beveled.
After applying the gussets to one side at each joint,
install a spall for additional support in the upper half of
the frame following instructions in Chapter 17. Use
temporary diagonal bracing if you are working with a
particularly large or unwieldy frame. This will help
maintain accuracy when you turn the assembly over to
attach gussets to the other side.
When you have installed gussets on both sides, reposition
the frame on the body plan and check it for accuracy.
Mark the centerline, appropriate waterlines, and sheer
measurements just as you would in the typical frame
assembly procedure discussed in Chapter 17. It’s
convenient to lay out, cut, and finish limber holes in
the frames with epoxy at this point.
Prefinish the frames as much as possible before
installing them in the set-up. You may want to do this
before marking the centerline, waterlines, and sheer for
the set-up. If you have already prefinished frame and
gusset stock, touch it up to make sure that all surfaces
are coated.
Setting Up
<strong>The</strong> process of setting up hard chine frames is identical
to the standard procedure explained in Chapter 18. In
practice, hard chine frames are easier to work with than
regular frames because the chines serve as reference
points for checking for minor errors in frame location.
In some cases, the frames between the chine and keel
will be straight, but in areas where the hull twists or
flares out abruptly, such as the forward sections of a
power boat, frames are designed to curve and accept
the developed shape of the plywood. 1
It’s also considerably easier to fair hard chine frames
than it is to fair frames for hulls with round bilges. In
the initial stages of fairing, only the keel, chine, and
sheer areas are important. Lay a batten halfway between
the chine and keel or the chine and sheer and make
necessary corrections on the frames. Move the batten in
either direction to check the bevel of the frames. Lay
another batten on them from chine to keel and chine to
sheer to check for irregular curves.
When the set-up is complete and the frames are finally
faired, install the stem, keel, and transom as described
in Chapter 19. Sheet plywood construction is similar to
the stringer-frame method in that it uses frames and
stringers to support a thinner skin, but there are two
major differences between the techniques. With sheet
plywood, stringers are not always needed for support
during building. Very little loading develops on any one
stringer when plywood is applied over a hull surface, so
generally there is little cause for concern about deforming
stringers. Stringer size can therefore be based on the
structural needs of the hull skin rather than on mold
requirements.
1 In cases like these, the hull is formed as a series of cones and/or cylinders. <strong>The</strong>
plywood is still straight along certain elements, but these elements may no longer be
parallel to the frames. <strong>The</strong> design of developed skin plywood hulls can be complex.
For further information, we recommend Modern Boatbuilding by Edwin Monk or
Skene’s Elements of Yacht Design by Francis S. Kinney.