Development of New Propeller for High Speed Vessels, Effective for Prevention of Root Cavitation Erosion
Yoshihisa 0KADA*, Masaru YOSHIOKA* and Hironao KUB0*
The root cavitation erosion, which is frequently sustained by high speed vessels, is the problem involved not only with deterioration of the propeller's durability or life but also with hindrance of the vessel's operational safety. Despite the fact that the study on its generation mechanism was made, and several countermeasures with some appendages against the phenomenon were devised, no development has been made yet on a method for an effective prevention of the phenomenon with any designing improvement applied to a propeller itself. If the root erosion generation peculiar to high speed vessels could be prevented, we understand that the relative method will surely comply with the needs by the shipping industry.
Under such a background as above, this study aims at development of the new propeller effective to prevention of root erosion. Our concept of the new blade section for the root erosion prevention is described in the concrete, to be followed by the description about verification of its effectiveness by means of model test and actual vessel test, and successful development of the new propeller with preventive effectiveness against the root erosion for high speed vessels.
Key Words: High speed vessel, Propeller, Blade section, Cavitation, Erosion
1. CAUSE OF ROOT CAVITATION GENERATION AND STUDY CONCEPT
The root cavitation generation is said to result from unsteady pressure variation to be produced at the blade root due to angle of attack there to cyclically be varied when an inclined propeller is operated in water flow. Figure 1. shows production of a cyclic variation of angle of attack against an inclined propeller operated in water flow.
On the other hand, in the studies of propeller antisinging devices etc. made by others, we find out some examples with decreased effect of angle of attack variation on lift variation in a part of the blade section tests with its blade trailing edge shape intentionally revised1); wherefrom it is estimated that if a blade section with decreased effect of angle of attack variation on lift variation can be obtained, the root cavitation generation could be restrained with the pressure variation in accordance with the angle of attack variation to be reduced thereby.
In this study, the following procedures were carried out, in order to verify the effectiveness of the blade section with decreased effect of angle of attack on lift variation adopted for the reduction of the root cavitation generation.
(1) "Test concerning static characteristics of the special two-dimensional blade section" in order to study the relation between blade's trailing edge shape and its lift characteristics.
(2) "Test concerning dynamic characteristics of the special two-dimensional blade section" in order to study the lift characteristics of the special two-dimensional blade section when angle of attack is cyclically varied.
(3) "Cavitation test by model propeller" in order to study the effectiveness of the special blade section applied to a propeller.
Fig.1 Angle of attack variation of propeller on inclined shaft operated in water flow2)