Engine- Yanmar 3YM30 At max throttle on no-load, the RPM is 3880; at max throttle in fwd gear, the RPM is 2995. The propeller is the original fixed 3 blade and has just been cleaned and has no damage. Fuel filters are clean. Having just read the workshop manual, it should read closer to 3600 on full load. Any suggestions as to the reason for the low RPM? Thanks.
2006 Beneteau 343 full power RPM
- Thread starter Methane Mick
- Start date
More than likely the propeller is over pitched. It might not be the original factory installed propeller. That is a rather significant difference. I have a prop on my 323 that is pitched two inches over the stock prop and am losing about 200 RPM at full throttle.
There is a lot of discussion about the subject on this site. The prevailing opinion is that this is not a good situation. I tend to disagree somewhat on the theory that if the boat reaches its designed hull speed at a lower engine RPM, it isn't going to go any faster at full throttle anyway. It is just going to waste fuel. What you get is black smoke and part of that is probably because the engine cannot get enough air..
I really don't believe everybody who motors while cruising is running at wide open throttle, and if you have to travel in a no wake zone to open water you cannot be running at hull speed or even close.
There is a lot of discussion about the subject on this site. The prevailing opinion is that this is not a good situation. I tend to disagree somewhat on the theory that if the boat reaches its designed hull speed at a lower engine RPM, it isn't going to go any faster at full throttle anyway. It is just going to waste fuel. What you get is black smoke and part of that is probably because the engine cannot get enough air..
I really don't believe everybody who motors while cruising is running at wide open throttle, and if you have to travel in a no wake zone to open water you cannot be running at hull speed or even close.
How are you determining the RPM? Via the engine panel tach? If so, then before you do anything else--and especially before you start throwing any significant money at the problem--get yourself one of these: http://www.harborfreight.com/digital-photo-sensor-tachometer-66632.html
The tach on my Yanmar 1GM is off by a good 250 RPM or more on the upper range. Using the optical tach you can make yourself a "deviation table" to note the actual RPM vs. what shows on the engine panel tach.
It could be that you are indeed overpropped. But even here, you should get an accurate reading of your RPM so that if you have a shop re-pitch your prop they can have another piece of info that will help them in determining how much pitch to relieve.
Thanks Philip, It is the original prop but I have never run the engine higher than 2600 (in my ignorance) because the sound and vibration seemed fine at that speed and I thought I was conserving fuel.
[Thanks Doug, it is the original prop and I have verified the tach reading as correct. My trials were done at the dock and have been told now that the readings may change when actually underway. The manual advises to run the engine at full speed for thirty minutes every so often.QUOTE=DougM;1094218]More than likely the propeller is over pitched. It might not be the original factory installed propeller. That is a rather significant difference. I have a prop on my 323 that is pitched two inches over the stock prop and am losing about 200 RPM at full throttle.
There is a lot of discussion about the subject on this site. The prevailing opinion is that this is not a good situation. I tend to disagree somewhat on the theory that if the boat reaches its designed hull speed at a lower engine RPM, it isn't going to go any faster at full throttle anyway. It is just going to waste fuel. What you get is black smoke and part of that is probably because the engine cannot get enough air..
I really don't believe everybody who motors while cruising is running at wide open throttle, and if you have to travel in a no wake zone to open water you cannot be running at hull speed or even close.[/QUOTE]
There is a lot of discussion about the subject on this site. The prevailing opinion is that this is not a good situation. I tend to disagree somewhat on the theory that if the boat reaches its designed hull speed at a lower engine RPM, it isn't going to go any faster at full throttle anyway. It is just going to waste fuel. What you get is black smoke and part of that is probably because the engine cannot get enough air..
I really don't believe everybody who motors while cruising is running at wide open throttle, and if you have to travel in a no wake zone to open water you cannot be running at hull speed or even close.[/QUOTE]
Thanks Alan, the tach reading was verified with a hand held but the trial was done at the dock. I am told that that may make a difference so I'm waiting for it to stop raining and take the old girl out to sea.
Before changing my gear box and prop (and ending over prop; but that's a different story) I could achieve 3400-3500 max rpm in the water and around 3000-3100 max rpm at the dock. Worth trying out in the sea.
I assume, but to clarify, you mean at the dock in gear- as well as in the sea in gear ?
Mick,
I would think that there would be a substantial difference in RPM's underway vs RPM's with the boat tied to the dock and transmission engaged. I usually run my Yanmar at 2800- 3000 RPM's while underway. I rarely run it at 3400 -3600 even though Yanmar advises.
I really don't know what the max RPM is on my 343. Will likely run it this weekend and let you know; however, I do have a folding 3 blade Volvo prop. I am sure that that would be different than the stock 3 blade prop.
I would think that there would be a substantial difference in RPM's underway vs RPM's with the boat tied to the dock and transmission engaged. I usually run my Yanmar at 2800- 3000 RPM's while underway. I rarely run it at 3400 -3600 even though Yanmar advises.
I really don't know what the max RPM is on my 343. Will likely run it this weekend and let you know; however, I do have a folding 3 blade Volvo prop. I am sure that that would be different than the stock 3 blade prop.
You can not achieve max rpm with the transmission in gear while tied to the dock. You have to be moving.
Yes, in gear both times. I think (but can be wrong; I am not a mechanic) the reason you cannot achieve full rpm at the dock is due to the very heavy load.
Off topic but you probably know the answer. What kind of fuel consumption are you seeing on your 343?
I currently have the original 3 blade fixed prop on and can get about 3500 rpm at wot in fresh water with a clean hull tanks at normal levels etc. Dirty hull, full tanks, heavy seas etc can knock that down to 3100. Kiwi prop would hit about 3600 wot but move the boat about .5 to .7 knots slower.
I cruise at quietest level between 2500 and 2800.
I cruise at quietest level between 2500 and 2800.