The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

224 Hull Construction Methods
Sheer
W.L.6
W.L.6
<strong>The</strong> stem for this
male mold must be
lofted and built to
support the inner
face of the boat’s
stem. All dimensions
must be consistent
with the lofted lines
of the hull
Inner face of stem
Keel profile
Baseline/centerline
Ribbands
are faired over
the molds and
mold stem, then
fastened securely
Mold
ribbands
Mold
stem
Keel runner
Mold
frames
Profile of stem
Profile of stem
cap or cutwater
Planking
Cap
Stem
Stem
Cap
Planking
Profile of the boat’s hull
Mold stem
Figure 20-4 Lofting and set-up of a mold/stem assembly.
Sheer
the outer measurement of the mold stem assembly. (See
Figure 20-4.)
Even for molds where you will not be inserting a
permanent stem, you will still need to recess the actual
mold stem back further than the permanent stem
would be, and you can easily deduct these measurements
from the lofting board. You’re going to need
some room for stem assembly, whether it remains
attached to the mold or becomes a permanent part
of the laminated hull. For example, if you’re using 3 ⁄4"
(18mm) square ribbands and they meet at the bow
on a mold stem that is shaped to a perfect point, the
ribbands alone will still project a frontal flat area of 11 ⁄2"
(36mm). Thus, you will have to add some material
anyway, regardless of whether it will become a permanent
part of the mold or the hull. This is a practical
consideration to be thinking about when looking at the
lofting board and determining your positioning of the
mold stem piece. You should also keep in mind that a
permanent stem should be large enough to offer sufficient
surface area for a properly bonded joint. Thus
the size or depth of the permanent stem will affect the
location of the mold stem.
Planking the Mold
<strong>The</strong> planking on both solid and stringered molds must
bend in even, fair curves as it is laid over the mold
frames. Knots and severe grain run-out will cause wood
to bend unfairly or even to break, so use high quality
lumber for stringers. When you consider the waste
which results from irregularities in lower grade stock,
high quality wood is really not significantly more
expensive. We usually use clear redwood, cedar, or
white pine for planking molds, but many other species
work well.
As soon as you have finished your lofting and before
you set up your mold frames, decide the thickness of
your ribbands. Subtract this measurement from the
frames just as you subtracted planking thickness. Most
commercially available lumber is already planed to 3 ⁄4"
thick, so we usually use 3 ⁄4" square ribbands. If they will
not bend easily over all curves, we laminate stringers
from thinner pieces or shift them around until they lie
well. If long boards are not available, we scarf the
ribbands before fitting them. Ribbands may be joined
with butt blocks, but we prefer scarf joints because they
take only a little more time and do not result in hard
spots.
We begin planking the mold by identifying its most
difficult curves. Some IOR racers, for example, have
unusually curved shapes to fit a particular rule. Since
these may be unnegotiable if approached from the
wrong angle, we save time and trouble by running our
first stringer so that it best negotiates all of them. This
is the master ribband, and from it we align all of the
other ribbands along the hull.
As you progress up and down the hull, ribbands
aligned with the master ribband may have to be reoriented
to conform to other shapes. If your planking runs
primarily fore and aft, this problem is magnified by the
difference between the girth of the hull’s fat middle and
its thinner ends. Much more planking is needed in the
middle of the hull than at its ends.
A simple solution to this is to leave larger gaps between
ribbands at the center than at the ends of the hull.
Slight variations in the distances between stringers will
very slowly change the direction in which they run.
This permits a gradual directional transition and allows
the planking to bend more easily around more difficult
curves. Be careful not to overdo this, however, because