The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 3 – Wood as a Structural Material 23
Figure 3-11 Equilibrium moisture content as a function of
relative humidity.
percentages of the moisture content of wood as a
function of relative humidity. <strong>The</strong> graph is good for
that temperature only. Relative humidity throughout
the United States is such that the 8% to 12% moisture
range is attainable.
<strong>The</strong> speed with which a piece of wood reaches equilibrium
depends on the span between existing moisture
content and the percentage at equilibrium, and on the
ratio between exposed surface area and volume. Wood
tends to reach equilibrium faster when the difference
between equilibrium percentage as deduced from atmospheric
data and moisture content is greater. Extremely
wet wood expels moisture rapidly in an oven, and very
dry wood absorbs moisture quickly in a steam bath.
Thin veneer, which has a very large surface area and
relatively little volume, dries to brittleness or steams
until it is very pliable within hours. Whole logs, which
have huge volumes and little surface area, can take
years to reach equilibrium.
If you have any doubts about the moisture content
of lumber, test it. Tests are necessary on newly
acquired lumber or lumber that has been stored under
questionable conditions. We use an electronic tester,
and battery operated, hand-held moisture meters are
now relatively inexpensive. Some local building
inspectors and lumberyards have moisture meters
and will test specimens brought to them. On thicker
stock, moisture meter readings may not represent the
true moisture content inside the stock. You can determine
the actual moisture content by tests performed
in your kitchen.
To do this, weigh wood samples on a gram or postage
scale before and after placing them in a 225°F (107°C)
oven. It’s impossible to predict how quickly a sample
will lose all of its moisture because this depends on
variables such as sample size and original moisture
content. Continue baking and checking every halfhour
until no further weight reduction takes place
and moisture level is therefore reduced to 0%.
Apply the following formula to your results:
100 (W1 – W2) = P
W 2
W 1 is the weight before drying, W 2 is the weight after
drying and P is the percentage moisture content of the
specimen before drying.
Dry Rot (and Other Vermin)
While some of the effects of moisture in wood may
be eliminated by careful selection of quarter sawn
and sliced boards or veneer and by careful control of
moisture levels, others cannot. When its moisture level
is at or near fiber saturation point, seasoned wood
becomes susceptible to fungus damage, often called
dry rot or brown rot. This causes millions of dollars
of damage each year to wooden structures of all kinds;
more wooden ships may have been lost to it than to
all of the storms and naval battles of history.
Neither very dry nor totally submerged wood is likely
to decay. In fact, wood sealed for 3,000 years in Egyptian
tombs at a constant temperature and humidity level has
lost none of its physical properties. But wood used in
boats is usually damp and therefore, if oxygen is present
and temperatures warm, subject to the types of fungi that
cause rot by feeding on the cellulose in cured lumber.
When this happens, the wood becomes browner, and it
may crack, shrink, and collapse. Attempts to eradicate
the fungi have usually revolved around poisoning their
food supply. Most of the commercial wood preservatives
that use this approach have only limited success on
boats because marine moisture levels are so high that
the poisons are quickly diluted.