Motor RPM and prop size on 24' pontoon

[bheston]

Hello, I have a 1984 24' Aqua Patio pontoon with a 50 HP Mercury outboard. The prop that is on it is a Quicksilver stainless steel 3 blade, 48, 76228, A5, 13P. When I open the motor up the boat picks up speed for about 5 seconds, then I hear the motor sound change, it sounds like the motor revs up a few hundred RPM all of a sudden. The boat doesn't slow down, but doesn't seem to run quite as fast as it should be running, doesn't seem to be much load on the motor. At that point I can run it all day, it doesn't ever change at WOT. If I slow the boat down then go back to full throttle, same thing again, gets to a certain speed then motor sounds like it is over-revving. I'm wondering if I'm getting cavitation, and if the prop is not big enough for the motor and boat combination?

No tach on the motor, so I used a laser tach and it's difficult to get a constant reading laying on the back of the boat while running but I think after the motor revs up I'm running about 5100-5200 RPM. I don't know what speed the boat runs, forgot to check with GPS when I had it out last.

Does this sound like caviation? Any ideas if this prop is about right size? Other info you need to be able to give some suggestions?

Thanks, I appreciate the great info everyone shares on this board.

 

[jestor68]

Re: Motor RPM and prop size on 24' pontoon

Is this behavior something new, or has it been going on as long as you've owned the boat?

The term you're looking for is "ventilation"; i.e, air being sucked into the propeller stream.

Or it could be cavitation; if you're running too much pitch to the point that the prop blades actually "stall"; inducing cavitation severe enough to cause the prop to lose thrust. If it's cavitation, you might be able to see "burn" marks/lines on the prop blades if it's been going on for a while.

I would think that a 10 or 11 pitch prop would be more appropriate for a 50 hp on that boat.

Some folks report using a Doel Fin helps reduce/eliminate ventilation on their pontoon boats (if that's the problem).

 

[bheston]

Re: Motor RPM and prop size on 24' pontoon

jestor68, thanks for the reply. Yes it has been doing this as long as I've had the boat, behavior hasn't ever changed. Would you be more inclined to change prop first to a lower pitch or put a Doel Fin on?