Wednesday, May 13, 2009

Design constraints of propeller diameter

by Charles Roring in Manokwari of West Papua - Indonesia


When designing a controllable pitch or fixed pitch propeller, a naval architect or propeller designer must face many design constraints which have to be compromised. One of them is the diameter of the propeller. After obtaining the lines plan and general arrangement from the ship designer, a propeller designer will check the stern detail drawing of the ship, to find out such data as stern draft measured at the after perpendicular of the ship or boat, and rudder dimension.



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Ideally, the diameter of the propeller should be as large as possible to obtain a higher propulsive efficiency, but it will not be possible to do that. Too large propeller diameter will result in air drawing to the propeller thus abruptly reducing the propeller efficiency itself. In addition, there has to be an adequate clearance between the hull and the propeller tip to ensure that the propeller can be installed and dismantled during ship construction, and can operate properly without creating vibration.



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The propeller whether it is controllable pitch or fixed pitch has to be fully immersed under water to give maximum propulsive efficiency during operation. Large ships such as bulk carriers or tankers often have to sail in ballast condition. To avoid air drawing, the diameter of the propeller will be smaller compared to other merchant ship type such as cargo or container ship.


The determination of propeller diameter is also based on the RPM of the main engine. The lower the RPM of the main engine, especially marine diesel engine, the larger can be the propeller diameter. As a general guidance, the propeller diameter and design draught ratio d/D of ships are as follows:


For bulk carrier and tanker: d/D < approximately 0.65


For container ships: d/D < approximately 0.74


Another factor which naval architect or propeller designer consider when determining the diameter of propeller is its strength. Too large diameter will create to large compressive and tensile stresses to the propeller. If the stresses are very large and close to or more than the maximum allowable stresses stipulated by classification societies, the propeller will brake during operation. Generally, for this reason, the diameter of propeller is limited to 10 meters. Also read: Propeller and ship's hull; Controllable Pitch and Fixed Pitch Propeller; Propeller design methods

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