Fishing Craft Technology

Unit 3 - Design procedure

3.5. Displacement

a) Definition
The amount of water displaced or put aside by a freely floating ship is termed as displacement. Archimedes principle states that the body wholly or partially immersed in a fluid loses weight, equal in amount to the weight of the fluid it displaces.
This is applies whether the body is heavier, lighter or equal in weight to an equal volume of fluid in which it is immersed. For a ship to float freely in water the weight of the ship must equal to the weight of the amount (volume) of water it displaces. The amount of water displaced expressed in cubic feet is called the volume of displacement. The corresponding weight of this water is known as the weight of displacement or displacement and is usually expressed in tons.
A cubic feet of sea water weights 64 lb
35 cubic feet of sea water weights one ton.
A cubic feet of fresh water weights 62.4 lb
36 cubic feet of fresh water weights 1 ton.

This is because of the actual weight of any given material is affected by its density.
Sea water – 1025 kg/m³
Fresh water – 1000 kg/m³
For this reason, a vessel draft alters in going from fresh water to sea water and vice versa.
The displacement of a box shaped vessel is easily obtained by multiplying the volume of displacement by the density of water in which the body floats.
In metric system Displacement = Volume of displacement x density

b) Displacement curve

Displacement curve can plot as displacement in X-axis and draught in Y-axis. Displacement curve for a vessel of a box form 100 ft long, 20 ft beam and floating in sea water at a draught of 10 ft. The displacements may be calculated at series of draughts of say 2,4,6,8 and 10 ft and set off against the vertical scale of draught. The displacement curve in case of a box-shaped vessel is a straight line may then be drawn through the points obtained and the displacement at any intermediate draught can thus be found.
Displacement at draught of

c) Light weight and Dead weight

Light weight: This is generally defined as the weight of the ship complete and ready for sea but with no fuel, feed, water, fresh–water, store provisions, ballast, other than permanent ballast – passengers and baggage or cargo on board.
Dead weight: It is the actual amount of weight in tons that the ship can carry when loaded to the maximum draft permitted by international law. It is also defined as the total dead weight of a ship is the difference between the load displacement and the light weight. The latter comprises the net steel, wood and outfit and propelling machinery consequently the total dead weight includes 1) Cargo 2) Fuel 3) Feed and fresh water 4) Stores and provisions 5) passengers and baggage.
Displacement = Light weight + Dead weight

d) Calculation of displacement of a ship floating at a given water line
Method (1)

• The bottom of the ship is assumed to be flat and the load water line is parallel to it.
• The immersed depth of a ship is divided into equal number of intervals by water planes parallel to load water plane.
• The area of each water plane is found out by applying Simpson’s rule
• The areas are treated by Simpson’s multiplier. The sum of the product is multiplied by 1/3 of the common interval, if the first rule is applied and the result will be the volume of displacement.

Method (II)

• The immersed portion of the ship is divided by transverse vertical planes or sections into number of equal intervals suitable for the application of Simpson’s rule.
• The areas of these up to load water line are calculated applying Simpson’s rule.
• These areas are put through Simpson’s multipliers.
• The final result will give the loaded displacement (volume)