The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 16 – Lofting 177
conform to the buttock, then the buttock will have to
be changed again. What you are looking for is that all
of the lines that you put on the half breadth and the
profile view are fair and, when the distances from the
center or baseline to these lines are transferred to the
body plan at each section, the body plan is also fair.
<strong>The</strong> next line we would pick to fair would be a
diagonal that intersects the hull somewhere between
the load waterline and the keel. This diagonal section
view will also use the half breadth baseline as its centerline.
At this point, some loftsmen like to draw a line
parallel to the baseline and intersecting the station lines,
but on the opposite edge of the lofting floor, using this
line as a centerline for the diagonals to keep them
separate from the waterlines. Because diagonal sections
are sometimes very similar in shape to the waterline
sections, using a common centerline might become
confusing. However, we usually use the baseline as the
centerline for the diagonals because later on it will be
quicker to pick up information necessary for manufacturing
various parts of the boat.
Laying out a section view of the diagonal is similar to
laying out a waterline. <strong>The</strong> diagonal will intersect the
item and transom at certain heights above the baseline.
For the stem, this might be on a waterline, making
things very easy. Just follow that waterline to the point
where it intersects the stem in profile. If it is not on a
waterline, as in Figure 16-9, take the measurement
height y from the baseline to the intersection of the
diagonal and the centerline in the body plan (point r).
Take this distance y to the profile view and mark off the
intersection with the stem (point r). <strong>The</strong>n use the
square to drop a line vertically down to the baseline to
locate the point at which the diagonal starts (r).
For fairing the transom intersection, you need to use
the point where the diagonal intersects the outside edge
of the transom (point s). In our example, this would be
at height x above the baseline. Transfer this distance x
to the profile view and determine where it intersects
with the transom (point s). Project this point down to
the half breadth view (s1 ). Pick up distance z from the
body plan, measured along the diagonal from centerline
to transom line. Mark distance z perpendicular to s1 in
the half breadth plan to get the point s representing the
intersection of the transom’s edge and the diagonal B.
You will fair the diagonal to this point.
Now lay the pick-up stick with its sharp edge just
touching the diagonal on the body plan and one end
touching the intersection of the diagonal to the centerline.
Label the stick according to which diagonal it is
and which end touches the centerline. <strong>The</strong>n, mark the
distance from center of each of the stations on the stick
and label. Do this, of course, on both sides of the body
plan so that all stations are included on the pick up
stick. <strong>The</strong>n transfer these distances to their respective
stations in the half breadth plan, labeling them in light
pencil as you transfer them.
Follow the standard fairing procedure of placing a
batten on the marks and checking for fairness, moving
the batten the minimal amount possible so that it forms
an even, smooth curve. Draw the line in light pencil
and make corrections to the body plan at the stations
where the batten had to be moved from the marks to
lie fair. If several sections of the body plan have to be
grossly changed in order to conform to the diagonal
section, the diagonal may have to be readjusted in that
area in order to fair the hull.
It is now just a matter of going through this fairing
process with the rest of the diagonals, buttocks, and
waterlines that shape the hull. Since everything is still
being drawn in light pencil, take care in labeling all the
lines to keep track of what’s what. As mentioned before,
there will undoubtedly be some necessary changes that
will affect some of the lines that have already been
faired. An example would be in fairing a diagonal that
is close to a buttock that is already faired. In order to
get the diagonal to lie fair, you may have to change a
couple of station sections, which, in order to keep the
station sections fair, might necessitate altering one of
the buttocks lying close to the diagonals. This sounds
like work, and it is. Any adjustment may necessitate refairing
of other lines, but at this point the changes will
be quite small to make everything fair. Care in lofting
will pay off in ease of planking.
In some respects, lofting reminds us of sanding a hull
and getting ready to paint it, in that a decision has to be
made as to when to stop. Most people are quite happy
if they achieve plus-or-minus 1 ⁄16" (1mm) accuracy in the
lofting. In special boats built to a rule, we usually carry it
out finer than this, trying to be as accurate as possible so
the boat will fall within the designed specifications of the
class in which it is racing.