Figure 7 and Photograph 4 show the typical configuration of the highly skewed propeller. The receded angle as you see is the peculiar characteristics of highly skewed propeller, and the angle is called skew angle. The skew angle is defined, as shown in the figure, to be the angle made by the two sides with the mean line on the blade being put between them, viewed from the boss center. Usually the propellers with skew angle of 25 degrees or larger is called highly skewed propeller. Incidentally, conventional propeller's skew angle is 10 degrees or smaller.
Fig.7 Typical configuration of the highly skewed propeller
Figure 8 shows the conditions of the cavity volume being varied at each blade element during the propeller in revolution for conventional propeller and highly skewed propeller. The total cavity volume by all the blades will become the propeller exciting force. Although there is almost no difference in the maximum volume of cavity at each blade element between conventional propeller and highly skewed propeller, the conventional propeller with uniformed peak positions has apparently larger total volume of cavitation all over the blades. You will see that the peak positions differ greatly at each radius with highly skewed propeller. That is to say, the generating/collapsing manner of the cavitation over the whole blades with highly skewed propeller is averaged, with each timing of cavitation generation being deviated smoothly, even though the generated cavitation volume itself might not be different. In this way, the smooth generation and smooth collapsing of the cavitation phenomenon can be controlled.
