44 NICHOLLS S SEAMANSHIP AN1> NAUTICAL KNOWLEDGE

When a second tackle is hooked on to the hauling part of another
tackle the power gained by the combination of the two purchases is
approximately equal to the product of their powers.

Example. — A 10-ton load is being lifted with a two-fold purchase
rove to disadvantage, (power 4) with a gun tackle rove to advantage,
(power 3) secured to its hauling part. Required the stress on the
hauling part of the gun tackle

S is required. P=4x3=12. W=10 tons. w=6 sheaves

16

S= — =1J tons pull on the hauling part of the gun tackle.
12

Figures 2 and 3 shew a test load of 42 tons on a Mannesmann tube
derrick with a five-fold purchase rove to advantage. The fall leads up
from the lower block through a lead block at the derrick head, and
another at the masthead, and down to a winch. It is required to find a
suitable size of wire.

The power gained is 11. There are 12 sheaves, but as the two lead
blocks offer a straighter lead for the wire their resistance will be less
than the others and they may be taken as one block, so now we have
11 sheaves.

Tension on the hauling part is got from the equation —

Sxll=42+(llxl2-r-10)==884
S=88-2-Ml=8 tons
Special extra flexible steel wire rope is used for heavy lifts the
working load of which is about one-third of the breaking strength so
2C2
if the working load 8 tons = -
3
- then 2Ca=24:, and 0=Vl2=3i
A 3J-ineh wire would be suitable. In practice, however, a 3-inch
wire would probably be used, as the general formula is not quite so
applicable to higher purchases running over sheaves of big diameter.
Nor is it desirable, unless for strength and. smooth running, to use a