1*2 d ICE DLLS' s SEAMANSHIP AND NAUTICAL KNOWLEDGE

An approximate connection between the weight being lifted and
stress on the hauling part of a purchase is given by the formuJa

where S is the stress or pull on the hauling part
P, the theoretical power of the purchase
W, the weight being lifted
n, the number of sheaves in the purchase

nW

- is the allowance for friction al resistance

The theoretical power or mechanical gain is equal to the number of
sheaves when the hauling part of the rope comes off the standing block,
but it is increased to the number of sheaves plus 1 when the hauling
part conies ofi the moving block. In the above equation P=n, if the
purchase is rove to disadvantage, and P=n+l when it is rove to
advantage.

Example. — A 12 -ton weight is to be lifted with a three-fold purchase
rove to advantage; find the tension on the hauling part of the fall and
the size of manila rope to use. There is a lead block at the masthead.
S is required. P=7, TP=12 tons, n=l sheaves

S '7 - 12 T * 12 =120+S* - 2Q4

/S = —— X — = 3 tons, the pull on the hauling part
The size of rope 0 = V?Xload 1t

= 4J inch j

Ans. Tension on hauling part is 3 tons and use a 4J-inch manila j

rope. !

Example.—The Single Purchase.—See Fig. 1.
A. The pull on hauling part is given as 12 cwt.

S=12 cwt. which is the same-as given.
. B. The stress on the top hook when hoisting is given as 33 cwt. it
is the load plus the pull on the hauling part plus the weight of the tacjde,