The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 3 – Wood as a Structural Material 13
deck planking, and keel. This total outer skin, plus the
stringers and other longitudinal members, provides
support fore and aft. Frames and bulkheads, at right
angles to these, form a strengthening lattice athwartships
and provide torsional rigidity. If this lattice is
loose and weak and its skin flexible from poor design,
bad construction, or inappropriate choice of materials,
the entire beam may lose its shape. In boats, problems
of inadequate support most commonly show up as
hogging, sagging, leaking, and slow performance.
Adequate structural strength is required of any material
used in boats. Usually, enough material is used to
provide an adequate safety margin against material
fatigue and unforeseen loads; beyond this basic requirement,
ultimate strength is less critical. Stiffness,
however, continues to be important, and maximum
stiffness is very desirable. All materials must deflect—
either stretch or compress—under load, but usually
any deformation in a boat’s hull shape is undesirable.
While both strength and stiffness can generally be
increased by using more material in the form of a
thicker hull skin, this will increase hull weight. A basic
problem with boats is that skin weight alone becomes
the major factor in overall boat weight.
In the quest for more speed, reducing weight is of major
importance. Materials have been pushed to their limits
for many years. Recently, strong competition and the
desires of clients have caused many designers and
builders to go further and to sometimes test margins
of safety. <strong>The</strong> results of this—boats that never finish
races because of materials failure—point to the importance
of fatigue resistance in any boatbuilding material.
When a material loses most of its strength after a single
high loading, it may quickly fail, no matter how strong
it was originally.
Fatigue is an accumulation of damage caused by
repeated loading of a structure. When boats, wind
turbine blades, and masts are subjected to a continuing
series of loads, the fatigue behavior of the material of
which they are made is more important than its
ultimate one-time strength. If a boatbuilding material
loses most of its strength after a few thousand hours of
service, it may fail. Failure in fatigue is probably one of
the leading causes of breakdowns in racing boats.
Materials differ widely in their resistance to fatigue.
Some are very strong for a limited number of loads but
lose a high percentage of this strength as the loads are
repeated. Others, particularly wood, begin with slightly
lower one-time load strengths but retain most of their
capabilities even after millions of cycles of tension and
compression. One-time, load-to-failure figures may not,
therefore, be very representative of how a material
behaves under long-term cyclic fatigue stress.
Figure 3-1 illustrates the cyclic tension fatigue behavior
of some popular boatbuilding materials. <strong>The</strong> left side
of this chart displays the load capability of each material
expressed as a percentage, with 100% representing
the ultimate one-time strength of each. All of these
materials lose strength as a given load is repeated at
steadily increasing numbers of cycles. <strong>The</strong> plotted
fatigue curves show the relative percentages of strength
which remain after any given range of loads and cycles
to failure.
Millions of cycles of stress are difficult to imagine, but
they can be translated into operating hours. Boats at sea
have been carefully instrumented where cyclic load
increases associated with waves were measured once
every 3 seconds. At this rate, after about 833 hours,
the equivalent of about four years of seasonal weekend
sailing, a hull would experience about a million cycles.
<strong>The</strong> material used in a wooden boat would at this time
still have about 60% of its ultimate strength, while
aluminum would retain 40% and fiberglass composite
20% of their respective ultimate capabilities. Given the
anticipated life of a hull and the fact that human lives
depend on its strength over time, these figures deserve
serious consideration.
Figure 3-1 Tensile fatigue comparison. Fatigue strength of
various structural materials as a percentage of static strength.