Can't reach max rpm with stock prop!

[Zerex]

I have an old 17ft runabout with a small 50hp 4 stroke Mariner motor with a stock 12 pitch prop on it. The motor is rated to 6000rpm max but with the current combo it will only spin to 5200rpm. Can some recommend a different prop setup and what the pros or cons of changing my current prop may be.<br />Thanx

 

[Silvertip]

Re: Can't reach max rpm with stock prop!

17 Ft is a really heavy boat for a 50. But But before changing props, its always a good idea to make sure the existing setup is correct. 1) Check the position of the antiventilation plate on the motor in relation to the bottom of the boat. It should be even or slightly higher (an inch or so) above the bottom of the boat. Less drag. 2) Experiment with trim settings. 3) Experiment with boat balance (weight distribution). 4) Make sure the area under the floor is not water soaked or has trapped water. 5) Make sure the engine is performing as it should (tuned up). 6) Make sure your tach is reading correctly. Verify it with a known good tach. 7) Make sure the trottle is opening all the way. If all of these are ok, then you can consider a prop change. To increase RPM, you need to drop back in pitch. Every inch of pitch changes RPM by about 150 - 200 RPM. This would say you need to drop back roughly four inches but that would mean 8 inches of pitch and that is simply not right. I personally don't feel this is the answer. Something else is wrong.

 

[WillyBWright]

Re: Can't reach max rpm with stock prop!

There's no such thing as a stock prop on motors over 25Hp. Too many variables. The dealer is responsible for adding the right prop. The motor doesn't come with one in the crate.<br /><br />You might want to check the tachometer for the correct setting. 12p doesn't sound unreasonable. We put 11s on smaller pontoons, but that may also be a good choice for your runabout. Unless that's an incredibly heavy boat or has a badly fouled hull, I wouldn't go lower.

 

[WillyBWright]

Re: Can't reach max rpm with stock prop!

There's no such thing as a stock prop on motors over 25Hp. Too many variables. The dealer is responsible for adding the right prop. The motor doesn't come with one in the crate.<br /><br />You might want to check the tachometer for the correct setting. 12p doesn't sound unreasonable. We put 11s on smaller pontoons, but that may also be a good choice for your runabout. Unless that's an incredibly heavy boat or has a badly fouled hull, I wouldn't go lower.

 

[Zerex]

Re: Can't reach max rpm with stock prop!

Thanx for the replies so far.<br />Here are some answers.<br />Yes the boat is a heavier deep vee somewhat like a double eagle.<br />The engine which has under 50 hrs was just checked and given 2 thumbs up.<br />The Tack is a brand new Teleflex unit which I've double checked the hookup and seems fine.<br />The prop that is on the boat I believe the saleman said was good for boats up to 1600lbs.<br />I havn't tried trimming it up since I just got the boat so what's the best rule of thumb for doing this?<br />Thanx again