Page 149 - The Complete Rigger’s Apprentice

A Here there’s minimal load on the wire, no shear on
the deck fastenings—and your crewmember is 200
feet to leeward. So you back up, working out a com-
angle A
promise between minimum tension and minimum
deflection. Playing with tether and lifeline configu-
vertical length of sling wire size that will provide you with a comfortably
ration can make the compromise less painful.
After you’ve settled on deflection, choose a

massive safety factor. In our case, a combination
1
of short tether, 2-foot deflection, and ⁄4-inch wire
⁄2
1
load
kinking) gives us a 10:1 safety factor. Now that’s a
span (7 x 7 construction, because it resists damage from
lifeline. But we can do even better. After seeing what
load a dramatic effect that angles have on loads, we can
lead our line through a fairlead secured to an upper
shroud, about chest-high. Now we have a much
larger angle, thus much lower loads. And that larger
angle also results in a very tall lifeline (jackline), so
leg length 8 ft; 1 ⁄2” 7 5 19 wire, Slings and Compression
B C we can use a very short tether. No long tail dragging
along behind us. No tether long enough that we can
fall into the water.

angle A
load borne The lifeline gave us a compelling example of the
load by one leg 22,800 lbs strength effect of angles on tension; now it’s time to see the

The strain on a two-legged sling can be calculated
angle A equally significant effect of angles on compression.
using the same formula or diagram as for the life-
6 ft
spreader
bar Figure 5-10. A load hangs from a simple sling in Dia-
gram A. If the load is 10,000 pounds, one-half the
load span of the load is 3 feet, and the vertical measure
of the sling is 7.42 feet, then the compression force is
10,000 5 3 ÷ 7.42 = 4,043 pounds, or 2,022 pounds
pressing inward at either end of the load. Diagram B
vertical length 7.416 ft load @ 8 ft = 22,800 lbs, each leg of the supported load, or 5,600 pounds. In Diagram
D shows a pictorial method to solve for the load borne
by each leg of the sling—in this case about 56 percent
C we’ve added a 6-foot spreader bar to protect a load
from compression. If the load were heavy enough to
strain the 7 x 19 wire to its rated strength of 22,800
pounds, the spreader would have to withstand 17,100
pounds of compression. Diagram D shows this graph-
span = 6 ft ically.

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