mercruiser 120 hp high engine rpm during turns

[pwheeler]

I have a 1971 mercruiser 120GM. When making sharp turns the engine rpms revs up to a very high rpm that seems unsafe and not good for the motor. I can straighten the steering and the rpms come back down while at the same time pulling up on the throttle. What causes this and how can I fix it? Thanks for any input.

 

[walleyehed]

Re: mercruiser 120 hp high engine rpm during turns

The prop is blowing out in the turns...you need to trim down some, and maybe consider a prop with better bite.

 

[pwheeler]

Re: mercruiser 120 hp high engine rpm during turns

what does prop blowing out in the turns mean? I think the trim is down most of the way.

 

[ziggy]

Re: mercruiser 120 hp high engine rpm during turns

i think it is called ventalation. it's when the prop literaly comes out of the water and lose's it's ablilty to push water and starts pushing air. which it will do easy. so it rev's up. triming down will ususally help but there's a point where ya turn so tight it still breaks loose. maybe don't turn so tight as to get it to ventalate or slow down prior to the turn a bit. just my 2 cents... jim

 

[mikecjn]

Re: mercruiser 120 hp high engine rpm during turns

It means that when you are cutting the turns your prop is actually coming out of the water somewhat. Like walleyehed said, maybe try trimming the motor down a bit more or slow down just a bit before taking turns.

 

[walleyehed]

Re: mercruiser 120 hp high engine rpm during turns

Yes, Prop blow-out, is Ventilation, and if you are trimmed all the way down when this happens you have 2 possibilities: #1 turning tighter than boat was designed to turn..#2, the hull is not clean.<br />Come to think of it, there is another possibility...do you have a transducer or speedo tube, paddlewheel etc, mounted within 16-18 inches of the center-line of the boat???<br />If you do, you may be "self-inducing" the ventilation when you turn by putting disturbed waterflow into the prop.

 

[cobra 3.0]

Re: mercruiser 120 hp high engine rpm during turns

A hydrofoil will all but eliminate this if you "need" to make such tight turns.

 

[walleyehed]

Re: mercruiser 120 hp high engine rpm during turns

The foil is a great tool as long as the items I mentioned above are not an issue such as transom mounted hardware...