Saturday, May 9, 2009

Cavitation of Marine Propeller

Every naval architect or propeller designer knows what a propeller cavitation is. Cavitation happens when the pressure in the fluid flowing around the propeller blades drops and causes some fluid to change into water vapor. When cavitation of marine propeller happens, it changes the homogeneity of the fluid flow. This causes the racing of propeller shaft but the speed of the ship drops. Cavitation on propeller also causes the erosion of material particularly on the surface of the propeller blade. Read my other article Erosive Effect of Cavitation on Propeller to know more about it.
The phenomenon of propeller cavitation was first noticed by Thornycroft and Barnaby on their destroyer, Daring. When the ship was being tested, they expected it to reach a speed of more than 27 knots. In reality, it could only reach 24 knots. Finally, Thornycroft and Barnaby solved this problem by increasing the surface area of the blade up to 45%. They concluded that the loss of the propelling force in high speed propeller had been caused by the high loading of propeller blade. According to them, the phenomenon happened when the thrust/inch2 of the projected surface of the propeller was more than 11.25 lbf (77.55 kN/m2). After modifying the propeller, finally, Daring's speed could reach more than 29 knots.
Source: Marine Propellers and Propulsion, by John Carlton, pp. 214
High loading of propeller will create high negative pressure at the back of the propeller which forms cavities that will be filled by air and water vapor. It was then R.E. Froude who suggested Thornycroft and Barnaby that the phenomenon be called "cavitation."
When designing the propeller of a ship, I have to check whether the propeller, at the service speed of the ship, will experience cavitation or not. The tool that I usually use to check the cavitation is the Method of Burril.
This method is also my basis for determining the developed area ratio or AD/A0 of the ship's propeller. Before determining the ratio I have to calculate, according to Burril method, some such cavitation parameters as, thrust coefficient, the conversion factor of resultant speed at radius blade fraction of 0.7, static pressures at shaft axis, and static and dynamic pressures at the shaft axis of the propeller.
After determining the propeller blade area ratio, I then can continue the process of designing the ship's propeller by performing the strength calculation of the propeller's blade.
In theory and in practice, the thinner the propeller the better will be its efficiency. But, the chance of it to experience cavitation will be higher. In addition, the thin blade may fail or brake due to high stress during operation. But if the blade is too thick, it will be uneconomical and inefficient. Therefore, I have to perform the blade strength calculation, using Taylor method, to determine the most optimum thickness of the blade. I will discuss this matter in detail in my next posting. by Charles Roring in Manokwari of West Papua - Indonesia