I have a Yanmar 3GM30 diesel that has always worked great. The top RPM for this engine is 3600 and mine has always been able to reach this, including after I had a 3-blade feathering prop installed last summer. I usually cruise the engine between 3000 to 3200 RPM, on the advice of my mechanic. This weekend I noticed that it seemed to be topping out around 3100 and could not reach 3600. I fooled with a few things and nothing made a difference - the engine would not exceed around 3100. I have gone under the boat to check the prop and all is fine in that area so it seems the focus is the engine. I have not been experiencing any loads or black smoke, and no overheating. Any thoughts on what might be causing this or what I could check before I have to place a call to the mechanic? Rob
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Rob, I do not have diesel but have been around
many and I find that 3100rpm is alot more than most advise. 2500 to 2800 cruising rpm on a diesel is what I have always been taught/told. Is the bottom of the boat extra fouled? Have you throttled up to max or dethrottled to an extreme? If you have , the throttle cable may have slipped through it's locking devise. just some ideas.... r.w.landau
many and I find that 3100rpm is alot more than most advise. 2500 to 2800 cruising rpm on a diesel is what I have always been taught/told. Is the bottom of the boat extra fouled? Have you throttled up to max or dethrottled to an extreme? If you have , the throttle cable may have slipped through it's locking devise. just some ideas.... r.w.landau
Follow-up...
I appreciate your thoughts r.w. The bottom is clean and completely unfouled, and it seems that the throttle cable is tight. I adjusted the rpm manually from the engine itself and it still maxed out around 3100.
I appreciate your thoughts r.w. The bottom is clean and completely unfouled, and it seems that the throttle cable is tight. I adjusted the rpm manually from the engine itself and it still maxed out around 3100.
Check the mixing elbow!
Rob: A common problem is the mixing elbow. Once they start to clog the RPM's will dimish.
Rob: A common problem is the mixing elbow. Once they start to clog the RPM's will dimish.
Rob, have you taken on more baggage,
passengers, or stores? If the boat/engine preformed in one manner one year and another manner the next year there must have been a change. Perhaps you are carrying a larger tax burden this year and the boat knows but you haven't noticed. You could blame it on the war, or the rising cost of fuel, or the chance of inflation. There is also the posibility that the pitch of the prop has increased with a little wear. It really wouldn't take much change to mage a big difference if you were pushing the margins anyway.
passengers, or stores? If the boat/engine preformed in one manner one year and another manner the next year there must have been a change. Perhaps you are carrying a larger tax burden this year and the boat knows but you haven't noticed. You could blame it on the war, or the rising cost of fuel, or the chance of inflation. There is also the posibility that the pitch of the prop has increased with a little wear. It really wouldn't take much change to mage a big difference if you were pushing the margins anyway.
One more thing
One thing I have noticed is lots on here see an RPM drop over a period of time, or even instant drop, and automatically assume there is something wrong with the engine, prop, boat or whatever. One of the first places to look is the tach. Before you start all the other stuff, make sure the tach is reding properly. An easy way to check is with a direct reading, hand held tach. You can maybe borrow one of these from someone who has one.
One thing I have noticed is lots on here see an RPM drop over a period of time, or even instant drop, and automatically assume there is something wrong with the engine, prop, boat or whatever. One of the first places to look is the tach. Before you start all the other stuff, make sure the tach is reding properly. An easy way to check is with a direct reading, hand held tach. You can maybe borrow one of these from someone who has one.
The same happened to me,
when I went from a fixed to a feathering prop. What I have learned is that feathering prop manufacturers ofter tell owners to over pitch the prop, as they would rather lean on the high side than on the low side. I am told that each degree of pitch reduction will increase your RPNs by 200. If your prop can be repitched in the water without a haul-out, then you should consider yourself fortunate, as many feathering props will not allow 'tuning' without a haul-out.
when I went from a fixed to a feathering prop. What I have learned is that feathering prop manufacturers ofter tell owners to over pitch the prop, as they would rather lean on the high side than on the low side. I am told that each degree of pitch reduction will increase your RPNs by 200. If your prop can be repitched in the water without a haul-out, then you should consider yourself fortunate, as many feathering props will not allow 'tuning' without a haul-out.
Why?
If the original poster is correct, the engine would operate up to 3600 RPM, "and ... has always been able to reach this, including after I had a 3-blade feathering prop installed last summer. " So unless he's wrong, it's unlikely that an over-pitched prop is the cause, unless it was a spontaneous pitch change. Could the pitch change due to a loosening prop adjustment?
If the original poster is correct, the engine would operate up to 3600 RPM, "and ... has always been able to reach this, including after I had a 3-blade feathering prop installed last summer. " So unless he's wrong, it's unlikely that an over-pitched prop is the cause, unless it was a spontaneous pitch change. Could the pitch change due to a loosening prop adjustment?
You are right,
I totaly missed him saying that it reached top RPM after the feathering prop was installed.
I totaly missed him saying that it reached top RPM after the feathering prop was installed.
Update
Sorry to be late in responding since this was my original post... My prop was installed back in June 2005 - about 14 months ago - and all has been well. I used the engine on Saturday and it ran fine, but then on Sunday I noticed the top-end problem. I have since gone under the boat and all looks well, including the prop and related bolts. The bottom is also clean with no debris anywhere to be found. I hope to do a bit more troubleshooting this weekend and will look at some of the issues you folks have described. I apprecaite all of the feedback! Rob
Sorry to be late in responding since this was my original post... My prop was installed back in June 2005 - about 14 months ago - and all has been well. I used the engine on Saturday and it ran fine, but then on Sunday I noticed the top-end problem. I have since gone under the boat and all looks well, including the prop and related bolts. The bottom is also clean with no debris anywhere to be found. I hope to do a bit more troubleshooting this weekend and will look at some of the issues you folks have described. I apprecaite all of the feedback! Rob
Check your Fuel Filter
Try changing your fuel filter...you may have picked up some trash in it which would slow down the fuel going to the engine. That would slow down your RPMS. That is a quick, cheap attempt to fix the problem.
Try changing your fuel filter...you may have picked up some trash in it which would slow down the fuel going to the engine. That would slow down your RPMS. That is a quick, cheap attempt to fix the problem.
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