The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

312 Hull Construction Methods
Flat areas can distort or
“oil can” during fold-up
Figure 25-32 Considerations for deck plans on folded hulls.
compounding that is induced into the plywood.
U-shaped hulls with a lot of fullness down low and
narrow decks will induce more compounding stresses
within the plywood than a narrower hull with a wider
deck. Deck width can be measured as a percentage of
the height of the panel at its maximum point. A typical
deck width-to-hull panel ratio is around 75%. For
instance, if our panel height were 24" (600mm) at its
maximum size, a typical deck width would be approximately
18" (460mm). Any narrower ratio than this
might exert extra bending and compounding forces
upon the plywood for which you should make some
allowance in the initial design by reducing some of the
other compounding factors.
<strong>The</strong> degree to which the plywood panels will be folded
toward each other (or deck width) also affects the
rocker profile. Narrow decks produce less rocker and
wider decks promote more rocker. It is easy to witness
this phenomenon by making several deck jigs for your
model at varying widths and measuring the amount of
rocker that is produced at the different widths. <strong>The</strong>
compounded plywood concept of construction
demands overall fair curves for all segments, including
the deck profile. <strong>The</strong> profile of the deck should have
gracefully flowing lines and not have any unusual
fullness in the bow or straight lines that might
contribute to distortions in the panels as they are being
compounded. (See Figure 25-32.)
Make the deck jig for the model by carving one half of
the deck line profile out of thin plywood— 1 ⁄8" (3mm)
thickness works well—and then make a duplicate for
the other side. Join these two pieces of plywood with
cross-braces at the bow and the transom and a third
brace in the middle to complete the deck jig. Make sure
that the plywood is thick enough and big enough so
that no distortion of the deck jig will take place with
Too much fullness in deck line can
cause distortion in fold-up
only one brace in the middle. <strong>The</strong>n fold up the
plywood model and insert it into the deck jig. <strong>The</strong>
cross-braces at the bow and the transom, together with
the one in the middle, act as guides to hold the deck jig
at the general sheer line position on top of the foldedup
model. It is usually easy to fold up the model by
hand and insert it into the deck jig, but occasionally
another person is needed to help with this operation.
So that you can remove the deck jig and more closely
evaluate the hull, complete the model by bonding on a
precut deck to hold the hull in permanent position. For
the deck, use the same thin birch plywood that is used
to fold up the hull. Cut out two deck panel pieces, one
forward and one aft, that will fit between the three
braces on the deck jig. Even though you can’t cover all
of the hull with the deck sections because some of the
deck jig braces are in the way, this partial deck will give
enough support to the sheers so that there should be
no distortion once the deck jig is removed. Attach the
deck by bonding it in place with thickened adhesive
and holding it down with light weights until the
adhesive cures. To keep the adhesive from bonding the
deck to the deck jig and/or the deck jig to the hull, coat
the deck jig carefully beforehand with a mold release
(paraffin works well).
With the deck jig removed and the deck plywood
faired smooth with the sheer, you will have a model
hull that you can now study closely. Establishing waterlines
on the model hull is necessary so that
displacement values can be calculated. Choose the
desired waterline position at both the bow and transom
areas and set the model upside down on a flat plane.
Prop the model up on blocks so that the marked fore
and aft waterline positions are the same distance from
the flat plane surface. Use a block and a pencil to draw
a continuous waterline around the hull using the flat