The traditional agent employed to move a ship is a propeller. The necessary propeller thrust, T, required to move the ship at speed V, is normally greater than the pertaining resistance Rt. The thrust of a propeller depends on its size, the angle of attack of its blades and the speed at which it spins. By convention, propellers are described by diameter and pitch in that order. The diameter of propeller is governed by its speed of rotation and the power of the engine rotating it. For example a 20 H.P. engine would drive a 14 inch propeller at about 1500 rpm. If the propeller had too small surface area for the power provided it would over speed and cavitate at high revolution, providing little or no thrust. Pitch is dictated by the propeller’s speed of rotation, its percentage of slippage, and the speed required from ship.
The diagram power vs shaft speed, the so called propeller curve, contains useful information about the working point of the main engine and the propeller.
So, a propeller curve is a very much significance for chief engineer. From the curve he can easily make out that for the particular engine power what will be the maximum rpm at which he can run the engine to get good propeller efficiency. In case of fixed pitch propeller, when operating in heavy weather condition, the propeller performance curves i.e. the combination of power and speed (rpm); will change according to the physical laws and the actual propeller curve can not be changed by the crew. But in case of controllable pitch propeller the pitch can be altered to suite the conditions outside and best propeller efficiency can be achieved. For this the chief engineer will require propeller curve.