The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 25 – Compounded Plywood Construction 313
Block
Section line
Marking on
waterline
Flat surface Model secured plumb
and level on blocking
Waterline
Figure 25-33 Marking waterlines and station lines on fold-up
models.
plane as a guide. Make sure that the model is also
parallel in the athwartship direction during marking.
(See Figure 25-33.)
With waterlines permanently inscribed on the model,
you will now be able to take off measurements from the
hull so that you can determine the center of buoyancy
and the total displacement available at given waterline
heights. You will also be able to figure your prismatic
coefficient together with wetted surface area. For
detailed instructions on how to figure all of these
factors, consult Chapter 23 of Skene’s Elements of Yacht
Design.
With this first model, you will undoubtedly find some
parts of the hull shape that are not entirely to your
liking. <strong>The</strong> object then is to determine what changes
you can make in the next model to persuade it to look
more like the hull you envision. Remember, you might
have to compromise because what you would like to
get in a hull shape may be an impossibility with this
method of construction.
If you decide to make a second model, you can
measure the effect of the changes made in the second
model against the first. From this data, you can assess a
cause and effect relationship of any change. <strong>The</strong> more
models you build with changes, the better your understanding
of the compounded plywood process will be.
We would advise anyone designing a hull for this
process to spend at least a couple of days developing a
number of models just to gain experience in the
compounded plywood building method.
Going from the Model to Full Size
When you have developed a model hull that you are
happy with, expand all the measurements of the model
up to full size. <strong>The</strong> most important set of measurements
will come from the master template, that is, the exact
duplicate of the hull panels used in the successful
model. Position the master template on 1 ⁄4" square graph
paper with the anticipated waterlines as parallel as
possible to the horizontal lines of the paper. Draw the
perimeter of the template on the graph paper using fine
lead with a sharp point for the best accuracy. Draw a
longitudinal line through the approximate horizontal
center of the panel. This becomes the reference line for
all future measurements, and you can draw other longitudinal
reference lines parallel to it if necessary. (See
Figure 25-35.) Make measurements of the keel line and
sheer line using 1' to 2' (300mm to 600mm) intervals,
depending on the size and amount of curvature
involved. Closer measurement intervals, although they
involve more work, produce more accuracy. In areas of
rapid change such as might occur in the bow or
transom, you may want to reduce measuring intervals
down to as little as 4" (100mm) to achieve enough
accuracy. Transfer these measurements to a full size
plywood sheet that also uses a drawn centerline as a
main point of reference for all dimensions, with lines
drawn at right angles to the centerline at each predetermined
measurement point. Lay down the centerline
using a tight cable, just as is done in lofting, for the best
accuracy.
Figure 25-34 A model of the outriggers shown in Figure 25-27. This model has been marked with waterlines and section lines.