Page 166 - The Complete Rigger’s Apprentice

these configurations, with the percentage of the load leverage advantage from a wider angle on the mast.
each wire will bear. Note that the numbers can add In addition, the uppers always pick up a significant
up to more than 100 percent; different combina- portion of the jibstay’s load, leaving less load on the
tions of sail and sea condition will put varying loads backstay.
on each wire. For our sample, the minimum wire sizes run
For our example boat, a cutter with a length on thus:
the waterline of 35 feet, we’ll use the second con- 1 x 19
figuration shown, a double-spreader rig with a pair % safety load wire
of lower shrouds on each side. The lowers share load factor (lbs) diam.
50 percent of the load, and the uppers and inter-
mediates take 30 percent each. Because the lowers Lowers 25 2.5 3,400 1 ⁄4"
share that 50 percent, many designers make them Intermediates 30 2.5 4,090 9 ⁄32"
smaller than the other shrouds, assuming they’ll Uppers 30 2.5 4,090 9 ⁄32"
take only about 25 percent each. But because the Jibstay 30 2.5 4,090 9 ⁄32"
mast can shift fore and aft significantly under sail, Backstay 25 2.5 3,400 1 ⁄4"
each lower may take a great deal more than it’s sup- Forestay 25 2.5 3,400 1 ⁄4"
posed share at times. It might be prudent to make
the lowers as heavy as the uppers. Again, it might be better to use stronger wire
The jibstay is usually made at least as heavy than noted here for the lowers. Note also that the
as the heaviest shroud, to take the big loads from last-entered wire, the forestay, is a little lighter than
the genoa. When in doubt, make this wire heavier; the jibstay. Although it will be the sail you’ll reef
it will fatigue more slowly and stretch less than a down to, its ultimate loads will be less than that
lighter wire. exerted by the most load-inducing sail that the jib-
The backstay can almost always be the next size stay must endure: a closely trimmed #2 genoa.
lighter than the jibstay, since it almost always has a Bear in mind that none of these figures is cast
in bronze. An extraordinarily tall rig, for example,
would have a very steep angle on its jibstay, which
Elastic Limit
means higher loads and the necessity for bigger wire.
Everything stretches when pulled, and every- Also, cruisers often make all their shrouds and stays
thing recovers its original length when the pull is out of the same size wire so that they only have to
released—within a limit, the Elastic Limit. carry one size of spare wire, turnbuckle, clevis, etc.
With most steels and steel alloys, the limit is
55 to 65 percent of the metal’s ultimate tensile An exception is sometimes made for running back-
strength. Once past that point the metal is perma-
nently deformed. On a sailboat, a rig tensioned
past its elastic limit will become untunable, Creep
because tuning is based on full stretch recovery
from tack to tack. In addition, overstressed A related form of permanent deformation to
wires suffer from accelerated fatigue, so become exceeding the Elastic Limit is known as “creep.”
untrustworthy. This involves continuing deformation under a
Therefore, in the design stage one must scale steady load. Spectra is notably vulnerable to creep,
the rig components to be hefty enough that there’s so that standing rigging made from it can slowly
no chance of an Elastic Limit–exceeding load. slacken over time. Not good. To counteract this,
Furthermore, one must make equally sure Spectra standing rigging should be scaled to keep
that the rig as tuned is not loaded past the Elastic typical loads low, preferably under 10 percent of
Limit. That is why, in a typical rig, the tightest breaking strength. This gives you a rope that is still
wires are tuned to at most 25 percent of the far lighter than wire or rod, and which has a truly
wire’s strength. massive safety factor.

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