Figure 5. shows open propeller efficiency curves for MAU propeller and PAI propeller. The full lines are PAI propeller's efficiency obtained by theoretical calculations, and the marks of circle and of triangle are PAI propeller's model test results; while the broken line is MAU propeller's efficiency. Fixing your eyes on the efficiency curves, you will see that PAI propeller's is higher nearly as much as 6% over the MAU propeller's. Besides, the efficiency estimated by theoretical calculations could be said to correspond very closely to the model test results. Generally speaking, in case of blade area ratio of around 0.1 per blade, the value of the efficiency improvement will be around 4% over the MAU propeller to be obtained from its designing diagram.
4. LOW EXCITING FORCE PROPELLER (HIGHLY SKEWED PROPELLER)
It is the "Propeller exciting force" to which remarkable attention has been paid during these 10 years as a secondary cause for the cavitation generation. The propeller exciting force is a vital cause for the hull vibration on the same level with engine's combustion vibration, being an important element related to the comfortableness aboard.
When propeller blades pass near hull bottom, the pressure fluctuation resulting from cavitation acts onto the area as the exciting force. The propeller cavitation will be generated in a concentrating manner around the hull bottom where the propeller blades draw closest to.
The cause of cavitation is the negative pressure over the blades' suction side, and each time the blades draw closer to the hull bottom, the area will greatly be influenced by the negative pressure.
