questionable performance limitation, the adaptation of an OGEE tip plan- 

 form to a marine propeller appears, at best, marginal. 



An increase in the lifting surface area is another planform change 

 which can improve tip vortex cavitation performance. The thicker viscous 

 boundary layer associated with a larger planform area increases the viscous 

 mass flow entering the vortex core, thus accelerating the vortex core decay. 

 The benefit of this concept is shown in Figure 7 which presents cavitation 

 inception data for several propellers — all with identical loading distri- 

 butions. As shown, the variation of cavitation index a as a function of 

 advance coefficient J is substantially lower for increasing blade area, 

 e.g., at design J = 0.833, the 4132 and 4133 propellers, with area ratios 

 of 0.303 and 1.212 respectively, show a o^lo^ ~ 8.8/4.2, which corresponds 

 to a ratio of free-stream speeds of 1.40. However, the increased blade 

 area also results in increased blade drag, and thus decreased propeller 

 efficiency. 



WING TIP EDGE DETAIL 



The chordwise edge of a wing tip can be finished by being rounded, 



squared, or tapered sharp. Investigations of wing tip edge details have 



11 12 

 reported ' varied and contradictory observations. However, the discon- 

 tinuity of a sharp or squared edge appears to disturb the tip vortex rollup 

 process in a somewhat favorable manner. Although, intuitively, little gain 

 would be expected, this simple modification could prove beneficial. 



SERRATED LEADING EDGE 



A serrated wing-tip leading edge (Figure 4) produces a much higher 

 level of turbulent flow entering into the tip vortex core. This increased 



turbulence destabilizes or accelerates the decay of the tip vortex. Exper- 



13 

 imental studies with serrated leading edges report a small increase in 



wing efficiency for small angles of attack and identify the serration size 



and density as important parameters. The adaptation of this concept to the 



marine propeller would likely produce additional local cavitation. 



14 



