272

NICHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE

thrust on the gooseneck of the derrick when a weight of 4 tons is hanging
from its top end.
Construction.—Draw the figure to scale by making XY=4£) feet,
then with centre Y and radius 24 feet describe an arc, and with centre
X and radius 48 feet describe another arc cutting the first one at A.
Jojn AX and A Y. Draw AB vertically downwards from A and make
AB equal to 4 units from any convenient scale to represent the downward
force of the suspended weight. Draw BC parallel to the span and CD
parallel to AB; ABOD is the parallelogram of forces.
The tension on the span is given by the length of AD (2£ tons),
and the thrust on the gooseneck by AC (4| tons).

Fig   16.—A Hanging Weight.
A weight simply hanging from the end of a derrick is not the usual
condition on board ship. The load is lifted by means of a wire fall
which leads through a cargo gin at the top end of the derrick and down
through a leading block at the heel of the derrick and thence to the
barrel of a winch on deck, the thrust on the gooseneck of the derrick
being thereby more than doubled, as, obviously, when a wire fall is
osed the pull exerted by the winch to hold the weight must be equal
bo the weight itself and the cargo gin has to bear the double weight.