Propulsion of a Ship

Jet propulsion of ship is found to be less efficient than propulsion by screw propeller due to the large amount of frictional losses in the pipeline and the pump, and therefore, it is used rarely. Jet propulsion may be of some advantage in propelling a ship in a very shallow water to avoid damage of a propeller.

Consider a jet propelled ship, moving with a velocity V, scoops water at the bow and discharges astern as a jet having a velocity Vr relative to the ship.The control volume is taken fixed to the ship as shown in Fig. 11.7.

Fig  11.7    A control volume for a moving ship

Following the momentum theorem as applied to the control volume shown. We can write

Where F_c is the external force on the control volume in the direction of the ship’s motion. The forward propulsive thrust F on the ship is given by

(11.18)

Propulsive power is given by  

(11.19)