Prop Slip

[rlc3854]

I bought a sun tracker pontoon with a Mercury 40 HP 4 stroke last summer. The last time I had it on the water was christmas eve. While running every once and awhile it sounded like the prop was cavaiting, this was the first time I had the boat loaded with 8 people. Any way I recenty had the 20 hour service rx for a new motor. The shop discovered that a seal/washer was not in stalled from the factory and the lower end/gear case had to be replaced because water had gotten in. They checked this because I showed the service manager how the prop would slide in and out about 3/8 of an inch. He stated that is how they are on newer motors, does that right? I have not had the boat back in the water yet but after replacement of the lower end the prop still has the slippage and that just doesn't seem right. They also found a bad spark plug wire and the engine was missing. I never noticed a miss firing and the boat always ran great through out the throttle range and WOT. Hoping that the motor is ok and not lemon, the motor tag says made in China/Subru motors.

 

[steelespike]

Re: Prop Slip

true "slip" is the difference between the theoretical speed and the actual speed. all props have certain amount of slip
on a runabout probably 8 or 10% on a pontoon about 20% or more.
"Cavitation" you don't hear or feel it happens in the low pressure areas of the prop and can erode paint and metal usually
it is of no consequence.
"Venting" is what you are hearing and feeling.It is the result of air fouling the prop so it loses grip on the water.
It is fairly common on a pontoon from all the turbulence in the water. Might do it more with a load.
If you have power trim; try adjusting up or down the see if it helps.On most pontoons there is a very narrow range where the trim setting is best.If manual trim try a different notch.If venting is a pain you could lower the motor one hole. Also there are 4 blade "spitfire" props for the merc that are very effective.

 

[Silvertip]

Re: Prop Slip

If the motor is not already mounted in the lowest hole, just do it. There is far too much turbulent water in that area of toon and having the motor (especially smaller HP engines) raised in an attempt to make it faster is a waste. It needs water to get a bite!