Yanmar 1983 1GM

kloudie1 · Jun 16, 2017

From the manual;
The maximum no load speed for the 1GM is 3750, (+50, -0) (that is 3800 MAX)
Continuous max rating is 3400 RPM
One Hour max is 3600 RPM
These should be read with an optical tachometer and not the Yanmar tacho.
Age of the engine should not affect the unloaded max RPM IF everything is in serviceable condition. Not reaching max unloaded could be governor setting, valve lash setting, injector condition, timing, etc.. Not reaching max power under load could be all of the above plus a bit too much pitch on the prop, or some growth on the prop. If there is not a cloud of black smoke at wide open throttle, it might be more engine setting related.. rather than propeller pitch.
I would clean the prop and bottom, set the valve lash, clean/replace filters , carefully bleed all air from all lines. Have mechanic set max unload speed after previous. Then check loaded RPM.. ya should be able to reach 3500-3600 if everything mechanical is working AND the prop pitch is correct.
5-6 knots by GPS in calm water doesn't sound that bad for a 27 foot boat, given that the theoretical hull speed is about 6.3.

Benny17441 · Jun 16, 2017

And besides all that how are you measuring speed? GPS will give you speed over bottom while a knot meter will give speed over the water. Speed over the water is influenced by course, currents and wind (example a boat can be showing 6 knots in the knot meter but is travelling against a current of 3 knots so its forward speed by GPS will be 3 knots). When you talk boat speed and performance you want "speed over the water" and to get the best results two runs in opposite directions should be made to average out condition factors. Also don't trust an old Yanmar Tachometer, had one that read 500 RPM less than the engine was putting out.

Alan Gomes · Jun 16, 2017

johnm5858 said:
Thank-you for replying, all should be good with what is mentioned - still waiting to get the tran ratio spec to give to the prop guys to determine if my prop is correct. Also I have a mechanic going on-board next week, to look over. Still hoping if someone with a similar set-up with a 1gm can advise what they are getting, I thought it might be helpful.

thank-you again,
JO

I have a 1GM and also a 1GM10. (Interestingly, I just pulled the 1GM this morning and will be replacing it with a 1GM10 a week from today. So right at the moment I have TWO of these little beasts sitting in my garage!) The 1GM10 is the same engine except for a slightly larger cylinder bore and a few extra hp. Anyway, the info that Kloudie gave you is correct and if your engine is running properly you should be seeing numbers close to these. (What the surveyor told you is wrong.) In neutral I can rev both engines to about 3750 or 3800 (only for a momentary burst; obviously you don't want to run it that fast for any length of time). In gear, and with a properly pitched prop, you should get close to the the max rpm of 3600. If you are within about 150 rpm of that you are probably OK.

As others have pointed out, the Yanmar tachs are not accurate. You should confirm the actual engine rpm using a digital optical tach, which you can obtain inexpensively. Make yourself a deviation table so you know the true rpm at various tach readings. The discrepancies are not linear, hence the need for the table.

If you have a 1GM (not the 1GM10) then you most likely have a KM2-A transmission. If so, your gear ratio is 2.62:1. There is a plate on the starboard side of the transmission that gives the model number and the gear ratio.

kappykaplan · Jun 17, 2017

Benny17441 said:
Speed over the water

You mean speed through the water?

Ron20324 · Jun 19, 2017

kappykaplan said:
You mean speed through the water?

Haven't you been watching the America's Cup?

Yanmar 1983 1GM

johnm5858

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kloudie1

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johnm5858

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Benny17441

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Alan Gomes

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kappykaplan

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Ron20324

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