Prop for DF-300

[Peter Werner]

Need to know sources of props for a DF-300. Present prop is 3B / 16" D / 17" P (R). Boat is 8000 lb Sea Sport, turning just under 6000RPM @ full throttle. Would like to go to 16"P, with 4 blades if possible. Prefer to be "under-propped" to reduce burden on engine.

[Harper]

Only 4 blade props I've seen advertised for the DF-300 with a 16" pitch are only 14 1/2" diameter. I think that would be way too small and would surely overspeed very easily. Seems to me that that's an awfully big boat for a single outboard, even a 300hp. But I don't think you're likely to find a better prop than the one you are now using. Full throttle RPMs are right where they should be.

[Peter Werner]

Thanks Harper. Most recent test run with light load: 5600 RPM @ FT, and 47% trim & 35 knots/40mph. I don't want/need that speed - would prefer quicker planing and less burden on engine with loaded boat. You are correct - closest 4-blade I have found so far is an Apollo SS: 14 5/8"D x 16"P. Question 1: Wouldn't the extra blade make up for the surface area lost by the reduced diameter? Question 2: Would the over-speed you refer to with a 14 5/8"D prop be due to cavitation, slipping, or increased thrust-advantage? My basic understanding is that the lower pitch provides more thrust-advantage and the extra blade results in less slip under load. Thanks Again - Pete

[Harper]

I don't know for sure, Peter, and my belief that it would overspeed so easily is not based on any knowledge of fluid dynamics, but more hunch, as in, "300 hp is just too much engine for the amount of resistance afforded by that small a prop."

Now, I could be wrong. Won't be the first time. But it's just my gut feelin'.

Then again, it IS a heavy boat....