Pulsing Power

[SeaFeret]

I'm helping a buddy with his "new" older boat. It has an older Yanmar... 30+ years and it starts and runs ok.. for an older engine. Issue is that when the boat is in forward and under load the motor rpms don't change but the thrust does. If this was a car I'd say the clutch is slipping.

Transmission shifts gears ok.

Any ideas ?

Thanks in advance!

Sea

[Sailmonkey]

How are you deciding that the the thrust increases?

How are you determining that the engine speed does not change?

This is not normally possible unless you have a controllable pitch propellor.

Is the prop clean?

Does the engine throttle normally in neutral?

[SeaFeret]

Originally Posted by Sailmonkey

How are you deciding that the the thrust increases?

How are you determining that the engine speed does not change?

This is not normally possible unless you have a controllable pitch propellor.

Is the prop clean?

Does the engine throttle normally in neutral?

Thanks for getting back to me.
I had the boat tied up at a slip to stress test it. I was watching the wash from the prop. It was strong then it would fade.

The engine sound did not change. if there was variation in the rpm it was very minor.
The engine does throttle normally in neutral.

Is the plate between the engine and transmission a wear item? I was under the impression it is not.

[Sailmonkey]

That sounds like the transmission is slipping.

The damper plate doesn't slip, it can go bad, but sounds like hell when it does, but will almost always still turn.

Is the prop clean?

Have you watched the shaft while doing this? If it slows down while in gear with the engine running then you've got a confirmed case of slipping transmission.

[jamhass]

Current/surge through the slip making the "wake" vary in appearance?

[hobopacket]

I would get it out of the slip onto open water on a flat calm day.

Have a tach?

Again, check for growth on prop.

Start easy and narrow from there

[SeaFeret]

Originally Posted by jamhass

Current/surge through the slip making the "wake" vary in appearance?

The area is protected from the current. But good thought Thx !

[SeaFeret]

Originally Posted by hobopacket

I would get it out of the slip onto open water on a flat calm day.

Have a tach?

Again, check for growth on prop.

Start easy and narrow from there

A tach would the be great I'll check the prop.

Thx