mass ejection, especially linear, may prove to be an effective means of 

 delaying tip vortex cavitation on marine propellers. Practically, the 

 concept will be limited by the required delivery power, which will be aided 

 by the centrifugal action of the propeller. 



OTHER CONCEPTS 



There are other tip vortex dissipation concepts which are not reported 

 in the literature, e.g., elastic and flow separation tips and large tip 

 skew. All of these ideas address the propeller nonhomogenous wake environ- 

 ment and would tend to delay tip vortex cavitation by averaging the unsteady 

 blade loading — i.e., by avoiding the unsteady high lift conditions, the 

 unsteady tip vortex cavitation would be reduced. Both an elastomer tip sec- 

 tion designed to deform to reduce camber and a flow separation tip section 

 designed to have separation, at angles of attack larger than design, would 

 present a more constant loading for a given wake variation. However, flow- 

 separation-related cavitation may be a limitation here. Similarly, large 

 tip skew, applied to that area of the blade span which directly controls the 

 vortex rollup, would also tend to average the unsteady propeller loading 

 due to wake nonunif ormity. This idea could suffer possible structural 

 limitations as a result of increased blade stresses. 



One final thought, a very recent, but not yet reported, concept aimed 

 at delaying vortex cavitation involves the application of a localized arti- 

 ficial surface roughness in the area of the wing tip. Earlier qualitative 

 studies have shown that a roughened surface on the pressure side of the 

 wing tip can reduce the tip vortex cavitation inception index O by approx- 

 imately 20 percent. This lends support to the earlier hypothesis that the 

 thickness of the wing tip viscous boundary layer plays an important role in 

 the occurrence of tip vortex cavitation. Although no supporting performance 

 data are available, this idea may warrant pursuit. 



SUMMARY AND CONCLUSIONS 

 As mentioned earlier and reinforced in the above discussions, the large 

 body of literature dealing with tip vortex delay offers limited guidance 



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