A Study on the Three Bladed Propeller with Smaller Blade Area Ratio Designed by the New Method

Abstract

It is well known that the reduction of a blade area ratio and a number of blades are the effective means for improvement of propeller efficiency. On the other hand, the reduction of a blade area ratio and a number of blades cause problems such as a severe cavitation and a excessive propeller surface force. Due to these problems, it is not general for a merchant ship to have a three bladed propeller with smaller blade area ratio. The authors designed the prototypes of three bladed propeller with smaller blade area ratio (expanded area ratio of 0.25) by the new design method which can take into consideration of the variations of effective attack angle in a wake to make pressure distributions on blade surface desirable in view of cavitation. Model experiments were also carried out for the purpose of comparison between a newly designed propeller (SBA type) and a conventional propeller (MAU type). The results this time indicate that the new design method is useful to control and reduce a cavitation and a hull surface pressure of the propeller. The results in the research are summarized below. (1) The propeller efficiency of the newly designed three bladed propeller with smaller blade area ratio (SBA3-25) was improved by about 5% at Reynolds number R_<nD>=6.0 x 10^5 in comparison with the conventional four bladed propeller (MAU4-46). (2) Cavity extent on propeller blade surface for the new propeller (SBA3-25) was remarkably reduced to about a half of the case for the MAU type (MAU3-25). The fluctuating pressure on the stern hull for the SBA3-25 was reduced by about 20% as compared with the MAU3-25. This pressure was almost the same level as that in non-cavitating condition and same as that of the MAU4-46. (3) The characteristics of a smaller blade area propeller was found to be much influenced by the Reynolds number in model propeller open test. (4) It is necessary to make further study because the strength was not exactly considered in this investigation and the self propulsion test behind model ship was not performed. The evaluation of the performances for this kind of propeller should be investigated at full scale in future.