Friday, June 5, 2009

Pitch diameter ratio of a propeller and engine performance of a ship

by Charles Roring in Manokwari of West Papua

The performance of marine propulsion machinery is much influenced by the propeller installed behind a ship or a boat's hull. If a propeller is well designed, the engine will work well without suffering overloading or underloading. Propeller designer knows that the selection of pitch diameter ratio plays a very important factor.


If the pitch diameter ratio is too high, the main engine will suffer overloading. When the ship is moving on its service speed at sea, the sign of overloading can be seen from its exhaust pipe. A lot of black smoke will be released by the engine whose propeller's pitch diameter ratio is too high. Further inspection to the inside parts of the engine shows that the valves are burnt. Sometimes we can find cracks in the cylinder heads.

If the propeller's pitch diameter ratio is too small, then a sure sign for this is that slow speed of the ship, and cavitation. Cavitation is usuall solved by increasing the developed blade area ratio and diameter of the propeller. We may feel that the fuel consumption is small but in reality the speed of the ship or boat is lower compared to other similar ships.


The solution for too low or too high propeller pitch diameter ratio is repitching. Repitching has to be done by experienced propeller manufacturer with the help of an experienced propeller designer or a naval architect who understands the resistance and propulsion system of ships or boats. Also read: Ship displacement calculation, and The propulsion system of submarine

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