Page 192 - The Complete Rigger’s Apprentice

the monohull’s maximum negative stability is con- approach, the details of which I’ve barely touched
siderably less than that of the trimaran; the mono- upon. And there are plenty of boats in each category
hull will be easier to right. The stability curve for whose vices far outweigh their virtues. By studying
this boat as sailed will have a much steeper initial transverse stability curves, we can get an idea of
section, since such boats invariably carry large how to maximize the virtues and minimize the vices
crews, most of whose members spend much of their of each type of craft.
time perched on the weather rail to provide ballast. To build a better trimaran, for instance, we’d
Thus the boat can stand up to breezes without being work to make it still harder to tip over, whether
slowed down by extra in-keel lead ballast. from wave action, “tripping” on the outer hulls, or
The curve of the Deerfoot, also a monohull, any other reason. And we’d probably try to make
rises most gradually of all, but enjoys a long pla- un-capsizing easier, just in case. And this is what
teau, peaking at about 60 degrees. Then it drops good multihull designers try to do.
slowly, but look! It maintains positive stability right The cruising monohull can be improved by
through 135 degrees, and the maximum negative making the stability curve steeper in the first 30
stability is very small. Should this boat ever be degrees, if this can be done without sacrificing ulti-
capsized, she’ll be boosted back on her feet almost mate stability. This generally involves lowering the
immediately by any modest passing wave. vessel’s center of gravity and fussing with the amount
None of these curves is “bad” or “good,” and location of the ballast. Comparing Figures 5-22
although they are often interpreted as such. Mono- and 5-23, you can see that while the righting arm of
hullers, for example, never tire of deriding the spec- the Ohlson 38A—a conservative, wholesome cruis-
tacular range and intensity of multihull negative ing monohull—is 1.5 feet at 30 degrees of heel, the
stability, but fair consideration is hardly ever given 30-degree righting arm for Steve Dashew’s Sundeer
to the equally spectacular range of positive stability. is over 2 feet. Sundeer has almost no form stability,
Multihullers are just as nasty about putting but Steve sweated to get all the weight down low.
down monohulls. “Lead mines,” they call them, in This low center of gravity, combined with a perfectly
which you are always “sailing on your ear.” But this balanced hull, made for a very stiff ocean cruiser. It
is just a way of saying that, with a combination of can be done.
pendulum stability (the “lead mine” in the keel) and Racing monohulls are not so susceptible to
form stability (the buoyancy of the hull), a mono- improvements in seaworthiness. They could be, but
hull bends before the wind, spilling gusts instead their sailors are inclined to sacrifice everything to
of taking them full on. For a given displacement, going well to weather. That and rating rules (see
a multihull’s sails, rigging, and hulls must be much
more strictly and heavily engineered, since they Stability Attrition
literally have to stand up to every gust. It is good
practice to make multihull rigging at least 50 per- Yacht design since the 1960s has followed a steady
downward trend in terms of stability. According
cent heavier than rigging for a monohull of the same to C. A. Marchaj (Seaworthiness: The Forgotten
displacement. Factor, International Marine, 1987; Adlard Coles
Multihullers and racing monohullers do agree Nautical, 1986), “The positive area under the
on one subject: that cruising monohulls are too slow righting moment curve has been reduced to less
than half, and at the same time the negative area
and too heavy. But although the cruiser won’t sail as has increased dramatically.”
upright, it has a higher ultimate resistance to knock- Marchaj also notes that the angle of vanish-
down or capsize. And if it ever is capsized, it will ing stability has been reduced from about 180
degrees to about 120 degrees, and that the righting
tend come back upright. For some people, this can moment maximum is now “about half of that
be a compellingly attractive feature. characteristic of the traditional yacht form.”
So there are pluses and minuses to each design

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