Page 167 - The Complete Rigger’s Apprentice

stay pendants, which can be made of vinyl-coated determined that stays contribute another 85 percent
7 x 7, or my favorite, Spectra/Technora-blend rope to the compression load on the mast, added to that
(see page 50). Running backstays can be lighter generated by the transverse stays. So, revising our
than the forestay, since the mast itself and the inter- previous formula, we get:
mediates take up some of the load from the forestay.
These and other considerations will affect what RM 30 5 1.5 5 1.85 = total mast
wire you hang on a mast, but it all traces back to ⁄2 beam compression
1
that righting-moment curve. As long as your deci-
sions are based on that, you’ll at least have an idea Condensed, the formula reads:
of how close to the edge a racing boat will be, or how RM 30 5 2.78
reassuringly stout a cruising boat will be. ⁄2 beam
1
Spectra standing rigging requires higher relative
strength than wire. This is because, at loads much In the case of our sample boat, that comes to:
over 15 percent of break strength, you get acceler-
ated “creep” (see sidebar). 50,000 5 2.78 = 25,272.7 pounds
5.5
or 6,925 5 2.78 = 11,528 kg
MAST STRENGTH 1.67
Set that number aside for the moment; before
Getting to wire size takes time, but it’s only a matter we can plug it into one more formula, we need to
of matching expected loads to wire strength. Mast consider some strength and stiffness variables.
design, however, is a more complex, slippery design
challenge. The object is to come up with a column Unsupported Length
that is stiff enough to take, without buckling, the Earlier we touched briefly on the significance of
standing rigging’s compression loads, and yet light unsupported length—how a long column is more
enough that the boat won’t be top-heavy, and small likely to buckle under a load than a shorter but oth-
enough in diameter that it won’t offer unnecessary erwise identical column under the same load. Spe-
drag from wind. Unfortunately, these qualifications cifically, stiffness varies inversely with the square of
call for mutually exclusive responses. Fortunately, the unsupported length; double the length of a mast
it’s an old, old challenge; others have faced it in the without adding the intervening support of spread-
past, and have left us formulas to plug into. Again, ers, and you have to make it four times as stiff to
these formulas are meant to be patterns, not straight- handle the same load. Remember the uncooked
jackets; to use them creatively, it helps to understand spaghetti?
some of the thinking that went into them. For now, Spreaders are commonly viewed as a means to
let’s return to our 35-foot-LWL cutter. widen the angle of shrouds to the mast, but they
also serve to shorten unsupported length, allowing
Extra Load designers to make masts adequately buckle-proof
The shrouds put some compression load on the without making them massively heavy. Intermedi-
mast, but the fore-and-aft stays impose an addi- ate fore-and-aft rigging (running backstays, fore-
tional load—boats also have a fore-and-aft righting stay) also help with unsupported length, but they,
moment, and some of that gets transferred to the as well as spreaders, add complexity and thus vul-
mast via staysails and stays. Also, stays add com- nerability to a mast; more things to break. That’s
pression to the mast simply by trying to keep it in why you usually see more than two sets of spreaders
place against boat acceleration and deceleration and only on racing boats, who expect to lose the occa-
the pull of various sails. Engineers Henry and Miller sional stick.

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