My came with a 60HP Merc Bigfoot with a 10 pitch prop.
When I run over 3500 rpm, the prop begins to cavitate and the stern begins to vibrate. My Outboard dealer has tested the motor and found
no motor vibration...he stated that a 10 pitch prop spinning faster than
3500 rpm will cavitate!
At less than 3500 rpm the boat runs great with a lot of thrust available to push through high chop...but is unable to get the boat above its hull speed.
What would be a pitch,which would give me a more optimum compromise
between speed and low speed thrust.
Cavitation normally means the prop is too small in diameter, and stating a prop is 10 pitch is only half the equation. What is the prop diameter? Your outboard dealer ought to know this, and simply stating a 10 pitch prop will cavitate at 3500 RPM without a recommendation as to what you need to do to fix it betrays lack of interest, stupidity or both. Perhaps you need to find a new dealer.
I can't help with specifics, but I believe a 14" diameter x 10" pitch prop for a Bigfoot on a Mac is pretty standard.
Yes you really need the diameter as well but I also think I read that some with the 50BF are using the 14 X 10, perhaps a 14 X 11 might be in order. One thing is for sure the prop must get you up into the proper RPM's range which is usually around 5500 rpms @ WOT. On my Honda it is 5725 @WOT for max HP according to the manual.
I wonder if you have a bent blade that is causing problems.
One more thing, some folks tend to get confused between Cavitation & Ventilation the latter being air drawn from the surface into the blade and making it spin faster, happens during turns alot as the skeg tends to lift out of the water and expose the prop to air. I also notice it happens in chop a bit when the stern lifts too high.
IN addition to checking for the necessary 14" dia. be sure the anticavitation plate above the prop is even with or about 1" below the lowest point of the hull bottom. Any closer to the water surface and it will ventilate even with the boat flat, terribly so in a turn.
If the motor is low enough on the transon to preven ventilation from the surface, the prop is a 14" dia., and allows you to reach the recommended WOT rpm, then look for a prop change, going from a three to a four blade with big fat ears (wider prop blades).and a trailing edge cupping.
When your motor can't reach at least WOT......your engine is lugging.........no matter what rpm you run at.....this leads to excessive wear on the engine.
Catigale wrote:My 14x11 prop on my 50 HP Merc BF is slightly overpropped - I cant quite reach WOT as recommended by Mercury.
I dont ever cruise at WOT so I dont worry about it - my max revs for any length of time are 3500 range or so.
True, but its a matter of degree and intended use. I put it on as an experiment to see how it would work in the 2000-2500 range where i cruise and was really happy - i get no pinging or knocking so Im not worried about it. Im am loading the engine more than the 14x10, I agree.
If you are consistently running the motor at top rpms I would not recommend this.
Prop far enuf below water surface to avoid ventilation?
Prop out of balance? Damaged blade?
Engine have high speed miss or the like?
Dealer suggestion, that a 10 pitch prop spinning at over 3500 rpm will cavitate, may or may not be true if we are talking prop rpm, but that seems to me to be a moot point, in that your rev counter is measuring 3500 engine rpm and hence 1800 or less prop rpm. Consider the vast number of OB's on other people's boats running 10 pitch props at over 3500 engine rpm without cavitation. The dealer's comments seem at least unhelpful.
Good luck.
Ron
came with a 60HP Merc Bigfoot with a 10 pitch prop.
