The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

18 Fundamentals of Wood/Epoxy Composite Boatbuilding
Deflection in inches
with 200 with 500 Pounds
gram gram Specific per cubic Cost per
Materials2 weight weight gravity foot pound3 Glass fiber/polyester (50% fiber volume) 10 Failed 1.52 104 $ 2.58
Aluminum (5054-H34 sheet) 91 ⁄2 Failed 2.7 170 $ 2.50
Kevlar/epoxy (50% fiber volume) 61 ⁄4 111 ⁄2 1.18 81 $ 10.45
Graphite fiber/epoxy (50% fiber volume) 111 ⁄16 4 1.54 105 $ 24.40
White ash (select) 111 ⁄16 4 .64 42 $ .77
Sitka spruce (select) 13
⁄16
13 1 ⁄16 .38 26 $ 1.48
Western red cedar (select) 11
⁄16
5 1 ⁄8 .31 21 $ 1.63
1 Test conducted by <strong>Gougeon</strong> <strong>Brothers</strong>. August 1976.
2 All samples weigh 25 grams and measure 24-inches � 1/2-inch.
3 Prices as of September 1985.
Figure 3-8 Stiffness comparison of cantilever beams made of seven different engineering materials.
When weight is a major consideration, as it is in boat
hulls, there are advantages to choosing low rather than
high-density materials to generate stiffness. On a weight
per square foot basis, a thicker, lighter material has a
natural advantage over a thinner, heavier one. In a beam,
longitudinal stresses are better carried as far as possible
from the neutral axis, provided that the loads do not
exceed material strength. A wooden beam may be thicker
than a steel one for the same weight and may also offer
some thermal and acoustic insulation. It is only in areas
where space is limited and strength requirements are
high that strong, high-density materials become preferable.
Masts, booms, and centerboards may require more
strength in smaller dimensions than unreinforced wood
can offer.
Our test does not reflect the fact that both the strength
and stiffness of glass-reinforced composites may be
manipulated with the use of core materials. <strong>The</strong>
technique of bonding skins of fiberglass over wood
is enormously successful for small boats, most notably
stripper canoes, as we will discuss more fully in
Chapter 23.
Trees into Lumber
To understand wood as a structural material, it’s necessary
to know a little about its growth. While certain
of its properties, most notably resistance to repeated
buffeting of wind and water, stem from its functions
in supporting and feeding the tree, some of wood’s
problems also result from its original role in life.
As a tree grows, cellulose molecules are organized into
strands, and these become parts of the cell walls of
wood fiber. Cellulose, lignin, and hemicellulose give
wood its physical properties. New wood is laid around
a core of older wood and pith and reflects the growing
seasons in which it was made. Early wood cells, produced
in the spring, may have thinner walls and larger
cavities than latewood cells. <strong>The</strong> transition between the
two is visually apparent in annual growth rings, which
vary in size by tree species and by the general conditions
in which the tree grew. In areas like the tropics,
where trees grow year round, distinct rings do not
appear. By contrast, trees that grow in northern climates,
where seasonal changes are extreme, have distinct rings
of early and late woods.
Two kinds of wood develop in the tree trunk. Sapwood
or xylem, recently formed, stores food and transports
sap. When a tree has grown so large that its innermost
sapwood is no longer close to the cambium, its cells
undergo chemical and physical change to become heartwood.
Material deposited in heartwood cells may make
it darker than sapwood in color and usually increase its
durability.
<strong>The</strong> terms hardwood and softwood describe neither
strength nor toughness. Softwood trees bear cones and,