Page 57 - The Complete Rigger’s Apprentice

sheave; by using an oversize sheave you reduce the this is no longer practicable. For example, take a
relative load, and thus further reduce friction. (This modern block with a 2-inch (50-mm)-diameter
is the source of that old rigger’s blessing, “Big blocks sheave and a breaking strength of 2,500 pounds
and small lines to you.”) (1,136 kg), designed for ⁄2-inch (13-mm) line.
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Considering all the above, it obviously makes That line has a breaking strength of 6,300 pounds
sense to use the biggest sheave you can, no matter (2,864 kg). Apply just one-third of that line’s
what your halyard material. But there are practical breaking strength, and you’ll have a load of 4,200
limitations. For one thing, bigger blocks cost a lot pounds (1,909 kg) on the block, 2,100 pounds
more than smaller ones. For another, an oversize (955 kg) on each side. You’d have to go down to
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block might just plain not fit in a tight space, espe- an uncomfortable-to-grip ⁄16 -inch (8-mm) rope
cially at the masthead, trapped between mast and before block strength would be in scale with rope
stays. And of course, big means heavy. Altogether, strength. But nowadays rope size is scaled for low-
we’re left with the compromise of using the biggest est stretch range, ease of handling, and fitting into
sheave that will fit, is light enough, and won’t bank- cam stoppers and winch-tailer caps. You might
rupt us. never put even a 1,000-pound (455-kg) load on
that ⁄2-inch (13-mm) line. So instead of trying to
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Block and Rope Strength match block and rope strength, the procedure is to
One more consideration: block strength relative match block sizes to design loads—the actual loads
to rope strength. In the days of manila and hemp, they’ll bear.
blocks were built to match the strength of the rope
that fit them. But since synthetics are at least twice
as strong as natural fibers and can be used safely Figure 2-18A. The effect of turning angle on block
at a lower safety factor (they are less prone to rot), loading.

200 lbs 173 lbs
(91 kg) (79 kg)

141 lbs
(64 kg) 120°

100 lbs (45 kg)
180°

90°
100 lbs (45 kg) 100 lbs (45 kg) 100 lbs (45 kg) (45 kg) 30° (45 kg)
100 lbs
100 lbs
45°

Figure 2-18B. Airblock Spectra grommet. Blocks like this one use rope instead of hardware as the means of
attachment. The result is a block that is considerably stronger, as well as lighter and more versatile than shack-
led blocks. (Left illustration by Margaret Wilson-Briggs)

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