when considering the effectiveness of a particular device to delay tip 

 vortex cavitation on a marine propeller. The primary problem is that most 

 of the studies are performed in air and involve investigations of the wing 

 far-field wake. The crucial cavitation inception and performance data and 

 the wing near-field wake data are generally not available. However, even 

 with these limitations, the aircraft tip vortex alleviation work can provide 

 some insight. For example, the trailing edge devices (splines and honey- 

 comb) designed to mechanically destroy the tip vortex structure are subject 

 to high drag and reduced efficiency; similarly, planform changes designed 

 to thicken the tip boundary layer and increase tip vortex decay may, if not 

 carefully designed, alter the spanwise loading and result in decreased 

 efficiency; and, practically, all of these aircraft "add-on" devices are 

 susceptible to local cavitation. One additional consideration which deserves 

 mention: the marine propeller usually operates in a nonhomogenous wake and 

 experiences a large angle of attack variation which results in rather 

 dramatic changes in blade loading. Thus, any potential concept should also 

 provide a reasonable degree of effectiveness for off-design operation. 

 This requirement would tend to render less attractive such devices as the 

 OGEE tip and endplates. 



In view of the foregoing discussions of the various devices and the 

 additional requirements imposed for marine propeller application, three 

 concepts appear to warrant further consideration with regard to their po- 

 tential for delaying marine propeller tip vortex cavitation. They are 



• the bulbous tip 



• the porous tip 



• the linear mass injection tip. 



All of these candidates have been shown to be effective and reasonably 

 efficient. 



The bulbous tip which represents the only concept with supporting cavi- 

 tation inception data, has been shown to delay tip vortex cavitation on 

 marine propellers with a small- to-modest efficiency loss. Optimization of 

 the bulb design should result in additional improved performance. 



Similarly, the porous and linear mass ejection tips, with supporting 

 data based only upon air studies, have been shown to significantly alter 



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