Page 163 - The Complete Rigger’s Apprentice

Figure 5-24. A graph of righting moments at 30
degrees. Note that a small increase in vessel length
Figure 5-23. Three more stability curves. Here the results in a large increase in righting moment. (From
righting-arm scale has had to be expanded to accom- Skene’s Elements of Yacht Design, Dodd, Mead
modate the much higher stabilities, both positive and and Co.)
negative, of a multihull. Note Sundeer’s exceptionally
high capsize angle (135 degrees) and ratio of posi-
tive to negative stability; even if it does capsize, it’ll
right itself immediately. The exceptionally high initial ballast. But the total load for most monohulls of a
stability of a multihull dictates sturdy rig scantlings. given waterline length varies so little that you can
(Courtesy Jeff Van Peski) get a fairly accurate RM 30 figure from the graph
in Figure 5-24. If you are working on a multihull,
brig, or other atypical vessel, consult a designer.
heel, then extrapolate along that nearly straight line Note the dotted lines on either side of the solid line
to find RM 30 or RM 40 , then work in the extra factor. in the chart. These indicate likely limits to extra-
You can do this with your own boat at dockside, as solid or extra-flimsy boat RMs. Note also that the
a way to get your boat’s actual RM 30 , instead of rely- RM 30 for a 32-foot-waterline boat is about 37,000
ing on a generic formula (see sidebar, “Inclining”). foot-pounds, while that of a 40-footer is about
For another thing, almost all of a vessel’s sailing 80,000 foot-pounds; a little length adds a lot to the
is done within 30 degrees of heel; you can use that moment.
as a benchmark, then safety factor (see below) gen- The trick, once you have your RM 30 , is to
erously or stingily, depending on how safe and solid convert moment into actual pounds of tension on
for cruising—or skinny and scary for racing—you the standing rigging. Remember, foot-pounds is a
want your rig to be. convenient way of describing force over distance,
The steepness or flatness of any part of a given whereas when we go to buy the wire, we need to
vessel’s curve will vary depending on hull shape and know the load in plain old pounds. To make this

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