The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 20 – Building a Mold or Plug 223
Figure 20-2 A plug for a 32' (9.7m) catamaran is polished
and waxed, ready for a mold to be cast. <strong>The</strong> mold for the
upper surface will be female in the areas of the hulls and
cabin top and male in the cockpit area.
the planking easily. When making the mold frames,
remember that you must not only deduct the planking
thickness of the hull, but also the planking thickness
that you will use on the mold. If the hull is to be
1 3 ⁄2" (12mm) thick and the ribbands are to be ⁄4" (18mm)
thick, you must deduct a total of 11 ⁄4" (30mm) planking
thickness from the lofting in order to arrive at properly
sized mold frames. Because of this unusually large
deduction, you must pay careful attention to the
planking thickness deduction procedure. (See
Chapter 16 for a detailed discussion of lofting.)
Once you have properly set up the mold frames and
completed final fairing, you can begin planking the
mold. Although it is not always necessary, many
molded hulls are built with provisions in the mold
to insert a permanent keel and stem which can be
removed with the hull from the mold. It is relatively
easy to insert a permanent keel into the mold and to
attach the veneers permanently to the keel during the
molding process.
If the keel is to be 4" (100mm) wide, leave a gap along
the centerline of the mold slightly over this width,
allowing approximately 1 ⁄8" (3mm) clearance between
each side of the keel and the mold. We run a permanent
ribband, which we call a keel runner, along each
side of the keel. This then becomes a joining point for
all the rest of the ribbands on the mold that would
Keel
runners
Ribbands
faired and
secured to mold
Mold
Keel
Figure 20-3 A mold with a recessed keel.
Notch so keel may
sit fair with
surface of mold
intersect the keel at a diagonal angle. If the keel is
thicker than the ribbands, recess the mold frames
whatever extra distance is required so that the keel
will fair up smooth with the ribbands on the mold
surface. (See Figure 20-3.)
Installing a permanent stem is somewhat more complicated
because you must construct the mold using an
inner mold stem on which the permanent stem will
repose during the laminating procedure. You need the
inner mold stem as a point on which to attach the
ribbands as they approach the bow stem area. This
interior mold stem must continue on up to the keel
area where the keel runners intersect so that every
ribband running in from the hull can either connect
with the inner mold stem or the keel runner. No
ribbands can be allowed to end at the stem or keel area
without being attached to one of these solid points. A
difficult part of this installation is to determine the exact
location and shape and size of the inner mold stem. To
determine this information, you will have to go to the
lofting board and work back from the outer stem area.
<strong>The</strong> location of the permanent stem itself might be
back a distance due to the effect of the planking and
stem cap that protrude out in front. Thus, from the
outer profile of the boat, you might have to deduct the
stem cap and the permanent stem before determining