The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

376 Appendices
B-2 Effect of Curvature on Bending Strength
Stress is induced when laminations are bent to curved
forms. <strong>The</strong> following formula, drawn from the Forest
Products Laboratory's Wood Handbook, p. 10-8, gives
the ratio of allowable design stress in laminated curved
members to that in straight members. You can use it as
you plan and loft to determine the appropriate thickness
of each layer of laminations for stems, frames,
bulkheads, knees and other curved structural members.
2000
1.00 – 2 (R/t)
where R is the radius and t is the thickness of the
laminating stock. <strong>The</strong> values must be expressed in
the same units.
Suppose, for example, that you wanted to laminate a
frame for a midsection of a Folkboat. On the lofting,
the smallest radius of the bilge at this section is
24 inches. If you bonded a 11 ⁄2 inch thick frame with
B-3 Bending to Small Radii
In tests conducted in our lab, we attempted to
determine the smallest radii to which selected dry
boatbuilding wood and plywood would bend without
breaking. Test samples were 24 inches by 6 inches.
Material
Thickness
(in inches)
Radius
(in inches)
Dark red meranti 1
⁄8 8
Veneer
Douglas fir
Sitka spruce
1
⁄8
1
⁄12
12
11
Red cedar 1
⁄8 10
Okoume
plywood
5-ply Parallel 1 ⁄4 24
5-ply Perpendicular 1 ⁄4 16
3-ply Parallel 3 ⁄16 16
3-ply Parallel 5 ⁄32 8
3-ply Perpendicular 5 ⁄32 6
3
⁄8 inch thick laminating stock, the strength of the
curved laminate would be about 50% of the strength of
a straight piece of similar cross-section, calculated as
follows:
2000
1.00 – 3 2 (24/ ⁄8)
2000
= 1.00 – 4096 = 0.512
If you used 1 ⁄8 inch laminating stock, the curved
laminated section would have about 95% of the
strength of a similar straight piece.
In applying this formula, it's important to realize that
more laminations require more work and result in
greater waste of materials. Use common sense to
balance the increase in strength gained by very thin
laminating layers against their cost.
Average moisture content of the samples was 7%. <strong>The</strong>
relation between the grain of the face veneer and the
long axis of plywood samples is indicated below.