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Ship Resistance

To move a ship, it is first necessary to overcome resistance, i.e. the force working against its propulsion. The calculation of this resistance, R plays a significant role in the selection of the correct propeller and in the subsequent choice of main engine.

A ship’s resistance is particularly influenced by its speed, displacement, and hull form. The total resistance Rт consists of many source resistances R which can be divided into three main
1) Frictional resistance
2) Residual resistance
3) Air resistance

The influence of frictional and residual resistances depends on how much of the hull is below the waterline, while the influence of air resistance depends on how much of the ship is above the waterline. In view of this, air resistance will have a certain effect on container ships which carry a large number of containers on the deck.

Water with a speed of V and a density of ρ has a dynamic pressure of:
½ × ρ × V 2 (Bernoulli’s law)

Thus, if water is being completely stopped by a body, the water will react on the surface of the body with the dynamic pressure, resulting in a dynamic force on the body.
This relationship is used as a basis when calculating or measuring the source resistances R of a ship’s hull, by means of dimensionless resistance coefficients C. Thus, C is related to the reference force K, defined as the force which the dynamic pressure of water with the ship’s speed V exerts on a surface which is equal to the hull’s wetted area As. The rudder’s surface is also included in the wetted area. The general data for resistance calculations is thus:
Reference force: K = ½ × ρ × V 2 × As

and source resistances: R = C × K

On the basis of many experimental tank tests, and with the help of pertaining dimensionless hull parameters, some of which have already been discussed, methods have been established for calculating all the necessary resistance coefficients C and, thus, the
pertaining source resistances R. In practice, the calculation of a particular ship’s resistance can be verified by testing a model of the relevant ship in a towing tank. 


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