5.2 TEST METHOD
The test conditions were the vessel operation for about 2 hours at her top speed, 20 times of abrupt speed acceleration from zero speed. In order for the test conditions to be conformed, the waterways for the maritime phenomena to be stabilized as much as possible and the time were selected.
Ship speed, propeller shaft speed, and trim and others were measured at the test. In every 15 minutes the propeller blade surface condition was observed with the naked eye. Furthermore, the surface roughness of the propeller blade root was measured, and a macro photograph was taken prior to and after the vessel operation. The surface roughness measurement was at 20%, 50%, and 80% position from the leading edge at 0.35R, three times for each.
5.3 TEST RESULTS
Figure 10. shows the surface roughness on the suction side at the blade root prior to and after the vessel operation in comparison, being representative of the test results.
While after the vessel operation, with the conventional propeller the surface roughness was increased at the blade's leading edge and the central blade, with the new propeller, on the other hand, the roughness development observed was limited on the blade's leading edge only and was reversibly decreased at the blade's trailing edge. On an average, at every measured point the development of the roughness was slower with the new propeller.
From these roughness measurement results, the root erosion prevention effectiveness by the new propeller with the improved blade section was well verified to satisfactorily be valid for the actual propeller.
Fig.10 Comparison of surface roughness at blade root on suction side (after the vessel was operated for 2 hours)
6. CONCLUSION
As the result of the study so far described, the following facts were found out.
(1) For the special blade section with insensitive or thicker/non-sharp shape, the effectiveness was verified that the angle of attack variation, being the cavitation generation cause, shall be reduced as expected initially. As the result of the blade section with thickness/width ratio of 12.5% having been vibrated at 10 Hz, the lift amplitude was reduced by about 11%. This effectiveness varies with the blade's thickness/width ratio and the vibratory frequency. The bigger the thickness/width ratio is, and also the higher the vibration frequency is, the bigger the reduction of the vibratory amplitude will be.
(2) With the new propeller, to which the specially shaped blade section applied at the blade root, the rotated range for the blade surface zone of the root cavitation generation was reduced by about 30%, compared with a conventional propeller. Such a tendency was observed irrespective of the number of blades and the difference of the blade's suction side or pressure side, as far as we perused the test results this time obtained.
(3) By the verification test using an actual full-scale propeller, development of the surface roughness at the blade root of the new propeller with the specially shaped blade section was below one half of that for a conventional propeller. Such a tendency is remarkably outstanding from the central blade chord toward the trailing edge.
From the study this time made, the initially intended root erosion prevention concept has now been verified in either of the model test and the actual vessel test. It could now be said that the new propeller for high speed vessels meeting the need of the era has successfully been developed with corroborative evidence.
Acknowledgement
In conclusion, this study was carried out in cooperation with The Foundation of Ship and Ocean, with the subsidy from their technology development foundation granted by them, which is an incorporated foundation operated on the funds from motor boat racing managed by an incorporated foundation of The Japan Ship Propmotion Association.
Sample Reference
[1] "Experimental Study on Karman Vortex Street Shedding from the Trailing Edge of the Wing "by N. Sasaki, Transactions of The West-Japan Society of Naval Architects, Mar., 1990.
[2]"Propeller for High-Performance Craft" by John L. Allison, Marine Technology, Vol. 15, No. 4, Oct., 1978.
