Page 150 - The Complete Rigger’s Apprentice

line. But neither will tell us the compression load on be picking up a pallet or crate that could be crushed
the object the sling will lift—remember, the ends are by excessive compression. There’s a related diagram
being pulled toward each other. This wasn’t an issue (Figure 5-10D) to show the load on each leg and
with the lifeline; the compression load wasn’t going the resulting compression. The wider the angle of
to buckle the deck of the boat. But the sling might the sling legs, the higher the tension and the com-
pression.
Boom Vang Again, we can get to the same place with a for-
mula: Compression = Load 5 One-half the horizon-
A boom vang, like a sling, imposes a compression tal span of the load ÷ Vertical length of the sling.
load on the object it’s attached to, the load varying Marvelous.
with angle.
Older-style boom vangs run from the boom So far we’ve let the load dictate things. But let’s
down to deck instead of from boom to base of now say that you have an existing two-legged sling of
mast. They impose little or no compression load 1 ⁄2-inch (13 mm) 7 x 19 wire, rated strength 22,800
on the boom, but they must be cast off and reset
with each gybe or tack. Worse, they can result in pounds (10,364 kg). You want to put a spreader bar
a folded boom, if the boom dips into a wave. This between the legs of the sling (Figure 5-10C). What
form often doubles as a boom preventer, but it’s is the maximum load that can come on this spreader
not so good for that, either; being so far forward,
leverage is minimal, so preventing the boom bar before the wire breaks? If you know the answer,
requires a lot of effort. And because the lead is you’ll know how strong to make the spreader bar.
vertical when the boom is right off, the boom can You could take the long way around, first determin-
travel aft a bit, even when the preventer/vang is ing what kind of load would put a 22,800-pound
tight. This makes it easier for the wind to catch the
sail aback. (10,364-kg) strain on each leg of the sling, then cal-
The modern vang swings with the boom, but as culating the compression load. But it’s quicker just
with so many modern conveniences, it brings with to juggle the formula again: Total Compression =
it complications. To start with, it ideally should
attach to the boom so that the horizontal distance Tension 5 Width ÷ Length of one leg. In this exam-
is about twice the vertical distance from the vang ple, that’s 22,800 pounds 5 6 feet ÷ 8 feet = 17,100
gooseneck to the boom. The resulting angle will pounds (10,364 kg 5 1.83 m ÷ 2.44 m = 7,773 kg)
give you about 90 percent of the available control
for a given height boom (100 percent is achieved that bar has to take, half that amount pushing on it
at the end of the boom, but that would not exactly from each side.
be practical). And once again, for the graphically minded, a
A high cabintop, low gooseneck, or extra-long
boom can conspire to produce a narrow vanging little picture tells the same story.
angle, minimizing leverage and maximizing com- Use this “prescriptive” method whenever you
pression. You’ll be very lucky to get a 45-degree have some of a system’s loads worked out and want
angle and will probably have to settle for more like to fit other components in.
30 degrees. If you can’t get even that much, you
might be better off either raising the gooseneck or Edging back toward boat rigging, you might
learning to live with a boom-to-deck vang. have noticed that widening sling angles produce
No matter what angle you get or what kind of higher loads, just the opposite of what happens
vang you use, bear in mind that vangs are hard
on booms, deflecting the midsection by downward with standing rigging. This is because a sailboat rig
pull as well as compression. Therefore, vanged
booms need to be appreciably stiffer than vangless
ones, particularly when the angle is tight. Before Slings on a Hook
installing a vang, get boom dimensions (depth,
width at several points, length, wall thickness, Extremely wide-angle sling legs are bad practice:
sheet attachment point, and projected vang angle), the load is excessive (see sling formulas), and there
sail area, and vessel size and displacement figures. is a likelihood that one of the sling legs will jump
Take them to a sparmaker to see if your boom will off the hook. Mousings, whether string or mechan-
be up to the loads you want to add to it. ical, are not designed to stop fleeing slings.

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