The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

130 Core Boatbuilding Techniques
weakening of the wood fiber and sets up favorable
conditions for dry rot. Any leaking should be detected
early, so that the fitting can be removed and rebedded.
High moisture levels in wood also contribute to other
problems. When wet wood swells, the wood between
the fitting and the flat washer or backing plate will
crush. When the wood dries, it shrinks, so the fitting
will be looser than ever. Wet wood also significantly
reduces holding power. Data from dozens of fastener
extraction tests show wet wood has up to 50% less
holding power than dry wood. In addition, when
moisture seeps beneath paint and varnish, it will
cause a premature failure of the finish. This often
begins at a leaking fitting. Finally, if dry rot develops,
the entire fitting may fail.
To overcome these problems, we have developed a
different approach to attaching hardware, which we
call hardware bonding.
<strong>The</strong> Concept of Hardware Bonding
<strong>The</strong> goal of hardware bonding is to distribute loads
on fasteners and fittings over as large an area of wood
fiber as possible through adhesive bonding with WEST
SYSTEM epoxy. We use WEST SYSTEM epoxy to achieve
this load distribution bond in two distinct ways:
• Fastener Bonding. All fasteners—all screws, bolts,
and threaded rod—are bonded directly to the
surrounding wood fiber. This is particularly
effective where the fastener is stressed in tension.
• Hardware (or Fitting) Bonding. In addition to
bonding the fasteners, the fitting itself is bonded
directly to the wood fiber on which it rests. This
is particularly effective where the fastener is stressed
in shear.
When you install hardware using the hardware bonding
techniques explained in this chapter and install deck
blocking as described in Chapter 27, the hardware can
withstand dramatically more load than hardware which
is simply through-bolted and bedded in position.
WEST SYSTEM epoxy has the capacity to bond dissimilar
materials, which makes hardware bonding practical.
WEST SYSTEM epoxy is an excellent adhesive for wood. It
can also be used to bond metal to wood. For a variety of
reasons, other types of glue typically used on wood are
not appropriate for gluing metal to wood. Additionally,
not all brands of epoxy will work. Many are too flexible;
a few are too brittle. Careful formulation of the resin
and hardener is needed to create a cured epoxy capable
of transferring the loads from metal to relatively soft,
flexible wood. WEST SYSTEM epoxy has this capability.
When a screw is inserted in wood, its threads break
the wood fiber. <strong>The</strong> screw keys into the wood. This
mechanical keying makes the screw effective, but it
does so imperfectly. Voids remain between the wood
and the metal. When the pilot hole for the screw and
the screw itself are wet out with epoxy, however, the
epoxy molds to the shape of the screw threads and
flows into all minute voids. Mechanical keying becomes
far more efficient and the screw is therefore able to bear
a greater load. <strong>The</strong> wood-epoxy matrix immediately
surrounding the fastener is stronger than wood alone,
and it distributes the fastener load more efficiently to
the surrounding wood fiber.
Basic Approaches to Hardware Bonding
Overall, hardware bonding relies on three basic
approaches.
1. Wet out a standard size pilot hole with epoxy.
<strong>The</strong> simplest and most common way to bond small
fasteners is to wet out a standard size pilot hole,
using a pipe cleaner or syringe to work epoxy
completely into the hole and all of the wood fiber
immediately surrounding it. Dab epoxy over the
screw’s shank and threads. <strong>The</strong>n insert the fastener
and allow it to cure. <strong>The</strong> bonded screw will have
much greater load capacity than an identical fastener
inserted in an identical hole without epoxy. Our tests
showed increased fastener load capacity in tension
by up to 100% on low-density woods. We gain
this phenomenal increase with little expense or
additional effort and use it in many moderately
loaded hardware applications.
2. Cast the fastener into an oversize hole filled
with epoxy. To increase the amount of epoxy
surrounding the fastener and, therefore, its holding
power, a fastener may be inserted in an oversize
hole. This creates an annulus, or ring, of epoxy
between the metal and the wood. <strong>The</strong> diameter
of the hole may be considerably larger than the
diameter of the fastener, up to twice the diameter
of a small screw. We more typically recommend an
annulus of between 1 ⁄8" and 1 ⁄4" (3mm to 6mm) per