The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Scarfing
Scarfing remains an essential boatbuilding skill. This chapter has few changes from the 1985 edition.
It does, however, discuss additional ways to cut scarfs in plywood.
It’s very difficult to buy lumber and plywood in the
right lengths for boat construction. When we do
manage to find wood that is long enough, it is usually
too expensive to consider. As a result, we’ve developed
our own scarfing procedures and use them to make
long boards, panels, plywood, and veneers out of
shorter ones.
Scarfing is a basic and essential boatbuilding skill.
When a well-made scarf is bonded with WEST SYSTEM ®
epoxy, it can be as strong as the material it joins.
Desired lengths of lumber and plywood may be built
up with no increase in the width or thickness of the
material at the joint.
Although there are several different kinds of scarf joints,
we use only one, the simplest and most reliable. We
machine identical bevels at the ends of the pieces to be
joined, fit these matching bevels, and then permanently
bond them with epoxy. In many boatbuilding situations,
we use a ratio of 8-to-1 to determine the size of the
bevel, so that a 1" (25mm) thick board will have a
bevel 8" (200mm) long. When high-density, highstrength
wood is used in a critical area, a mast for
example, 12-to-1 proportions may be required. For
extra strength and safety, we increasingly recommend
12-to-1 scarfs for lumber.
<strong>The</strong> main object of the bevel is to develop enough
surface bonding area to exceed the strength of the
wood itself. If you have any doubts about the amount
of surface area needed with the wood you intend to
use, you can manufacture a sample joint and destruction
test it. <strong>The</strong> sample does not have to be large: 1�2" thick � 1" (12mm � 25mm) wide is sufficient.
Machine 8-to-1 bevels on two pieces and join them with
an adhesive mixture. When the epoxy has cured,
support the ends of the sample (well beyond the scarf
joint) and then apply force in the middle of the scarf
CHAPTER
12
joint until it breaks. If failure occurs within the bond of
the scarf itself, you will have to increase its surface area
by increasing the scarf angle to a higher ratio for the
stock thickness.
<strong>The</strong> direction of the load on the scarf joint itself is of
little significance in comparison to the development of
bonding surface area to withstand that load. To develop
maximum surface bonding area with minimum waste
of material, you should bevel the side of the lumber
with the largest dimension. For example, if you had a
piece of 2" � 4" (38mm � 89mm) stock, no matter
what the load direction, you would make the 8-to-1
beveled joint on the 4" side rather than the 2" side.
Although the principles of scarfing lumber and
plywood are the same, the actual procedures differ.
In this chapter, we first discuss methods of scarfing
boards and then move on to a discussion of plywood
scarfs. We rarely scarf veneers for boats, but when we
do, we follow the basic steps used with plywood.
Scarfing Lumber
Making scarf bevels by hand
Depending on the size and type of lumber, you can
use a variety of hand tools to cut bevels. Other methods
of beveling are faster, so we make bevels by hand only
on large stock or when we have just a few pieces to
work on.
Check to make sure that the end of your board is
sawed square with its length. Plan the bevel dimensions
carefully and mark them on the end of the board.
Measure the length of the bevel in from the squared
edge and, using a square, draw a line parallel to the
edge. This will represent the end of the bevel; it is
usually referred to as the bevel line. Draw diagonal
lines on each side of the board to connect the bevel
line with the squared edge on the opposite side.