Page 153 - The Complete Rigger’s Apprentice

Spreader Length

A quick way to determine good spreader length for a adjacent side, and the spreader being the opposite
cruising sailboat is to measure the vertical height— side. The tangent of 12 degrees is about .2125. In
the distance from the base of a spreader to the upper the above example we’d multiply 16 feet 5 .2125 =
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attachment point of its wire—and divide by five. This 40 ⁄4". That’s a couple of inches less than our quick
will give the wire an angle to the mast of approxi- method gave us. The difference in angle is about
mately 12 degrees. It will actually be just a little less, .7 degrees, minimally significant in terms of loading.
but it will get you in the neighborhood. But for the pickily inclined, adding a couple of inches
For example, on a 36-foot mast with spreaders to the quick method result is a good practice.
attached 19 feet up, the distance from the spreader to As a further rule, spreader lengths for a single-
the upper shroud throughbolt might be 16 feet. One- spreader rig should be a minimum of 40 percent and
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fifth of that distance is about 38 ⁄8", and that’s the a maximum of 50 percent of the vessel’s beam at the
minimum spreader length you should have. chainplates (see USSA sidebar, page 148). On a dou-
You can also figure spreader length using the trig ble spreader, they can be 32.5 percent—or .8 times
functions on a calculator. Most often this involves the half-beam for lowers, and .65 for uppers.
the tangent function, with the mast portion being the

A Crane

A simple crane, as shown in the illustration, has com- it than for a mast, but the relationships are the same.
pression loads on boom, mast, and the ground or deck No matter what the level of complexity, rigging
beneath the mast. There are also loads on the halyard, tends to be invisible to most people’s eyes. If they
topping lift, and guys. And unlike the fixed forces on notice it at all, it is to comment on “all those lines,”
a sailing vessel’s standing rigging, all these loads and they seldom see beyond to the relationships that
change as the boom is raised, lowered, and swung make it work. The next time you play with rope or
side to side. So crane design must take into account a wire, pay appropriate attention to materials, intended
range of possible configurations. It might sound intri- use, and durability. But first and foremost, look to
cate, and the stress diagram does have more pieces to those essential angles.

The compression loads on a crane’s mast and topping lift
boom vary as the boom is raised or lowered.

1

3 2
halyard

steepest guy mast 1 1
boom boom at
2 lower angle

3

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