The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

132 Core Boatbuilding Techniques
may be higher than the value derived above, but we
recommend using the conservative value for estimation.
For most applications where the shear strength of the
wood is greater than 800 psi, we recommend a hole
sized 1 ⁄4" (6mm) larger in diameter than the major
diameter of the bolt. This gives an epoxy annulus of
1 ⁄8" (3mm) around the bolt. We vary the length of the
bolt and depth of the hole to balance the breaking
strength of the bolt versus the withdrawal resistance
of the epoxy plug. <strong>The</strong> 800 psi epoxy shear strength
is the limiting value.
<strong>The</strong> following example shows how to determine the
diameter and depth of an oversize hole so that the force
required to extract the fastener is approximately equal
to the breaking strength of the bolt:
A 3 ⁄8" diameter 18-8 stainless-steel bolt with an ultimate
tensile strength of 85,000 psi and an effective crosssectional
working area of 0.0775 square inches has
a breaking strength of 6,587 pounds. <strong>The</strong> hole depth
should be the breaking strength (6,600 pounds)
divided by the epoxy shear strength (800 psi), divided
by the circumference of the hole (0.625" � 3.1416 =
1.9635") to equal 4.20". In actual practice, we used
a 5 ⁄8" diameter hole with a depth of 41 ⁄2" for a 3 ⁄8"
diameter bolt with 41 ⁄4" inches of bonded length.
<strong>The</strong> above calculations need to be modified when you
are limited in the depth of the hole allowed or when
a short or oversized bolt is specified in the hardware
installation. You do not want to cut away large portions
of a keel or floor by using a grossly oversized hole.
Likewise, you may be limited in a deck application by
blocking location or overhead space limitations. In such
cases, it is best to use the 1 ⁄4" larger diameter, epoxyfilled
hole to assist in stabilizing and securing the keel
bolt, but use floors or metal pads to distribute the load.
How the surface area of the hole affects small and large
fasteners is discussed below.
Small fasteners in tension
In 1978 and 1979, we conducted tests to determine
the relative effects of surface area on fasteners. We
compared tension withdrawal of 11 ⁄2" long flathead
wood screws, ranging in size from No. 8 through No.
14, in dry pilot holes and oversize holes from 3 ⁄16" to 3 ⁄4"
Flathead machine screw
Figure 14-2 Typical screw types.
Self-tapping sheet metal screw
Flathead wood screw
in diameter. <strong>The</strong> screws were buried in 1 ⁄4" plywood
and Sitka spruce samples which had been constructed
to simulate a deck and blocking. All of the screws,
from No. 8 through No. 14, had about the same load
capacity when they were bonded in 1 ⁄4" diameter
holes—approximately 1700 pounds. At this load,
the fastener/epoxy plug pulled out of the wood. Larger
diameter holes often resulted in a tension failure of
the fastener.
Large fasteners in tension
Failure mode for 3 ⁄4" diameter and larger fasteners can
also be controlled by varying the surface area of the
hole. Withdrawal resistance is affected by a number of
variables, including fastener aspect ratio, length, and
shape. <strong>The</strong>refore the figures given here should serve
only as general guides. Depending on the number of
load cycles, R ratio, and other variables, safe allowable
design loads should not exceed 1⁄2 of the ultimate
withdrawal resistance loads.
In 2002, our Materials Test Laboratory tested some
large fasteners potted in epoxy as part of Lloyd’s
Register certification for a 154' wood/epoxy sailboat
project built by Hodgdon Yachts. <strong>The</strong>re were two test
programs, one with 3 ⁄4" diameter stainless steel threaded
rod and another with 11 ⁄2" diameter silicon bronze bolts.
All fasteners were bonded into laminated 3 ⁄4" clear
Douglas fir. Figure 14-3 shows how the bolt was
oriented relative to the grain of the wood. <strong>The</strong> annulus
was 1 ⁄16" per side, as specified for the 3 ⁄4" bolts—smaller