Yanmar6lya-stp

My Mainship Pilot 34 single was running perfectly until yesterday. When I came up to cruising speed of 2800 rpm I was only getting 10 knots instead of 15 knots. I dove under but all looked fine. I checked all fluids all looked good. Does anyone have an opinion where to check next?

Any other symptoms...sight, sound, smell, touch/vibration?

There was some vibration, no smoke or any other symptoms observed.

vibration at idle too?Rev up to rated rpm at idle? If vibration only at load then bent prop is suspect despite your dive. vibration at idle a bad cylinder is suspect.

If no vibration my first suspect would be fuel filters and pumps

No vibration in neutral from 650rpm thru 2800. I am quite certain the prop and shaft are not the issue. I will check pumps and also change all filters this week to see if that solves the issue. Thank you for you're response. I will post results hopefully positive.

If I read your question correctly you are now getting 10 knots at the same 2800 RPM as 15 knots before - yes? If so suspect either your tachometer or speed measurement devices are incorrect. Or a very dirty bottom and fouled prop.

I have same engine...
When I got rpm but not speed it was fouled bottom.
When filter plugged I couldn't get rpm or speed

I have the same boat and engine. Most, maybe all of the suggestions above are valid.

I would not discount black smoke at higher rpms. The underwater exhaust tends to mask black smoke. Black smoke is an indication of overloading, or not enough air getting to the engine.

Low speed at a given rpm almost always means a fouled prop or bottom or maybe a sudden increase in boat weight or as a longer shot, a change in calibration of the tach or knot meter.

Remember props move boats and engines only turn the prop. So if the prop is being turned by the engine at 2,800 rpm, then there is nothing wrong with the engine, it is doing what it should be doing.

The low speed has something to do with the prop or boat that is causing the lower speed than at the same rpm as before.

David

It could be a frozen or fouled turbo but I think you be getting black smoke as well in that scenario. Are you sure there is no black smoke??? Even if it was a fouled bottom or prop and the "engine is doing what it is supposed to", you would likely have a black smoke scenario if you are only getting 10 knots would you normally get 15...

When the weather clears I will run boat to see if the issue happens again

30% reduction in speed at same RPM could be Transmission slip ?

gaston said:

30% reduction in speed at same RPM could be Transmission slip ?

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Photo tach check of engine speed and prop shaft speed should be next step

djmarchand said:

Low speed at a given rpm almost always means a fouled prop or bottom or maybe a sudden increase in boat weight ........

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I have that same model engine and my money is on the above.