Prop Issue? RPM's Too High

[runningm01]

I have a 94 Sport with the Pro Boss that has been converted to carburetion. It has an 4-blade OJ Legend 13x16 but will run well past 4400 rpms. At 3500 rpms I'm doing 32 mph based on a GPS. Any thoughts?

[TRBenj]

That powertrain performs best when propped to turn 4800-5000rpm at WOT. What are you seeing for RPM and speed (verified with GPS)?

A 300rpm differential at skiing speeds is a little much for that 13x16 OJ, which makes me think it may be worn out. Your WOT numbers will tell the story, but my experience is that your prop should turn closer to 1:1 in the 30's.

[runningm01]

Thanks TRBenj! Sounds like it might be time for a new one. . .

[Zach@n3]

Get Tim some real data. Like he said the HO Fords and gt-40s tend to be happy at 4800-5000 WOT. If you're going new you might as well pick up a 422 and make yourself happy.

[runningm01]

I didn't test it over 4400 RPM's (the manual says the max RPM for this boat is 4400 so I didn't want to go over that), but at 4400 I was around 38-39 with the GPS. We live and boat at around 3,000 to 3,500 ft above sea level and will put some weight in the boat (probably no more than 800-1,000 lbs of water ballast). Should I still go with the 422 or should I look at the 654?

[TRBenj]

I like the 1442 more than the 654, but propping down 1/2" is probably wise based on your elevation and desire to run ballast.

A 500-600rpm differential between speed and rpm at that low of a speed indicates that the prop is tired or has been repitched significantly. Either way, I'd put it on the shelf. With a stock prop at normal altitude, you should be closer to 45mph at 4800rpm. Do not worry about spinning the engine above 4400. As stated above, best performance will be achieved if youre propped to turn 4800-5000.