5 - Performance

Abstract

This chapter presents an overview of a ship's machinery performance. Many variables affect the performance of a ship's machinery at sea so the practical basis for a contract to build to a specification and acceptance by the owner is a sea trial where everything is under the builder's control. The margin between the trial trip power and sea service requirements of speed and loading must ensure that the machinery is of ample capacity. One important variable on the ship's performance is that of the propeller efficiency. Propellers are designed for the best combinations of blade area, diameter, pitch, and number of blades, and are matched to a given horsepower and speed of propulsion engine. It is important that the engine should be able to provide heavy torque when required that implies an ample number of cylinders with ability to carry high mean pressures. When selecting a propulsion engine for a given ship a suitable power allowance for all factors such as weather, fouling, wear and tear, as well as the need to maintain the service speed at around 85 per cent of the maximum continuous rating, should all be taken into consideration.