Page 162 - The Complete Rigger’s Apprentice

of those on the helm, the quality of rig tune, hull
design variations, and many other considerations
mean that our figures can only be approximate. So
we come as close as possible and try always to err on
the safe side.
The process of determining the load that will
come on our rig begins by examining the hull.
The wind is always trying to knock the hull
over, the hull is always fighting to remain upright,
and the rig is caught in the middle. Accordingly, the
strength of the rig must be scaled to stand up to the
amount of knockover-resisting force the hull gen-
erates. Think of the rig as a big lever stuck into the Figure 5-22. Static stability curve for an Ohlson 38A
hull; you don’t want that lever to break. as calculated under the International Measure-
The hull also acts as a lever, and it gets its power ment System. The righting arm is simply the right-
from ballast and buoyancy. The more ballast it has, ing moment of the boat divided by its displacement.
and the lower it is mounted, the more the ballast Above the baseline the curve shows positive stability;
will help lever the boat upright. Likewise, the fuller below the line, negative stability. The larger the neg-
and more buoyant the hull, the greater amount of ative stability, the more disinclined is the boat to right
leverage buoyancy will exert. Stated in designerese, herself from an inverted position after capsize. The
we are trying to maximize the distance, for a given degree of heel at which the curve crosses the baseline
amount of heel, between the hull’s center of gravity from positive to negative righting moment is the angle
and its center of buoyancy. The two forces combine of heel at which the boat will capsize—134 degrees in
to give a vessel its stability. this case. The ratio of the positive area of the curve to
Leverage is a matter of force applied over dis- its negative area—here 8.997—is a measure of sea-
tance—the farther from the fulcrum one exerts a worthiness. Most boats measured by the IMS will cap-
force, the greater the effect it has. So we’ll measure size at about 120 degrees and have a ratio of around
our forces in “foot-pounds,” to translate our par- 4—minimal numbers for medium-size offshore sailing
ticular forces and distances into measurable effects. yachts; smaller seagoing boats need higher numbers.
The name for the amount of foot-pounds a lever can (From Sea Sense, 3rd Edition, by Richard Henderson.
exert is a “moment.” The moment that a hull exerts International Marine, 1991)
in trying to stay upright is a “transverse righting
moment” (abbreviated RM). This righting moment nearly straight line to at least 30 degrees, some-
varies with each degree of heel, depending upon the times up to 40 degrees. After that, RM increases
shape of the hull, the distribution and amount of more slowly, to its maximum (usually at around 60
ballast, the weight of construction materials, etc. degrees of heel), and then begins to decrease. Since
You can plot these shifting moments on a stability maximum RM indicates the maximum sustained
curve (Figures 5-22 and 5-23). This curve is as dis- load the rig will have to bear, designers need to use
tinctive as a fingerprint, and can tell you a lot about this figure as the basis for mast scantlings. Nowa-
the performance and safety of a hull. It’s also the days computer programs can spit this number out
fundamental mast and rig design tool, since it tells directly—along with the numbers for load at every
you the loads the mast and rigging will face. angle of heel. But riggers can also find the RM for
For some sample fingerprints, see Figure 5-23. 30 or 40 degrees, then multiply by a factor to take
In most monohulls, righting moment (RM) maximum RM into account. Why do this? For one
starts at 0 with the hull upright, then climbs in a thing, it’s very easy to find RM at a small angle of

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