prop problem?

[malagaoth]

i have a 20foot boat of unknown origin, it weighs about 2000lbs and is currently powered by a 60 yamaha. the problem is that once above 3/4 throttle - sorry no rev counter - the prop seems to lose its grip on the water, the boat stops, the engine spins furiously but only suceeds in throwing water high into the air only by lowering the revs and allowing the prop to bite once more can any motion be achieved. not too sure of the prop spec but is this the likely cause?

 

[Silvertip]

Re: prop problem?

You have the engine trimmed up way too high and are apparently experiencing blow-out. Tilt the motor down more. another cause may be that you have a short shaft motor on a boat that requires a long shaft. Lastly, you may simply have the engine mounted too high on the transom.

 

[malagaoth]

Re: prop problem?

the engine is trimmed down as far as it will go and is a long shaft - the prop sits below the hull. this leaves the engine being too high how do i go about dropping it?

 

[umblecumbuz]

Re: prop problem?

Spun hub?

 

[Silvertip]

Re: prop problem?

You need to check where the antiventilation plate is located in relation to the bottom of the boat. Flush or slightly above is ok. Too high and the prop will blow out. In addition you run the risk of the engine not getting enough cooling water as the intake ports are out of the water stream. I doubt that a spun hub will spray water into the air like you are describing. You very likely have a setup problem (or a very wrong diameter prop is a possibility). Also, a 20 foot boat may require a 25-inch (XL) motor length.

 

[malagaoth]

Re: prop problem?

now that the problem as been identified i will check other posts on the topic - thanks to every one!

 

[walleyehed]

Re: prop problem?

Measure your transom close to the center where the engine would be from the top of the transom to the bottom of hull. You can do this with the engine on, just to the side of the engine mounting bracket, and straight down...estimate what the flush measurement is with a tape.