574 NICHOLLS^'S SEAMANSHIP AND NAUTICAL KNOWLEDGE

1 . Find the displacement of a vessel, length 500 feet, breadth 40 feet,
mean draught in salt water 20 feet, block coefficient *7.
Am.— 8000 tons. *

2. A vessel 400 feet long, 30 feet beam, and 25 feet mean draught
displaces 5143 tons in sea water. Find her block coefficient and state
what type of vessel you think she might be.

Ans. — Coefficient -6 A steam yacht or a very fine lined steamer.

3. Length 500 feet, breadth 50 feet, mean draught 30 feet, block
coefficient -8. Required the vessel's displacement in fresh water.

Ans. — 16,666 tons.

4. Length 420 feet, breadth 45 feet, mean draught 25 feet, block
coefficient -8. Required the vessel's displacement in salt water.

Ans. — 10,800 tons.

VARIATIONS IN DENSITY AND DRAUGHT.

The hydrometer is used to find the density of harbour water so that
the corresponding increase in draught may be computed. The buoyancy
of all floating bodies is dependent upon the law of Archimedes.

If the density of a body is exactly equal to the density of the water
it will neither sink nor float ; if its density be greater the body will
sink, and if less it will rise until the weight of the body is equal to the
weight of the volume of water it displaces. It is obviouss therefore,
that a ship of a given weight will displace more fresh water than salt
water because every cubic foot of fresh water can only support 62J Ibs.
weight of the ship whereas 1 cubic foot of salt water supports 64 Ibs.
The draught varies inversely as the density, and in the case of box-
shaped vessels this may be written

New draught old density
Old draught new density

Eaximple. — The mean load draught of Caledonian Monarch is 24 feet
*) inches in salt water, what should her draught be when loading in dock
water of density 1005?

Draught 1025

2_ -. -

wjiich a?=24 ft. 10 »8 ins.
24 ft. 5 ins. 1005

Draught in dock water 24ft. 10*8 ins
Draught in salt water 24 5-0

Difference in draught 5-8