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Old 11-07-2010, 17:01   #1
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Differences Between Yanmar Engines

I am trying to make sure the prop on my '82 C & C 34 is correct. The mark on my engine says 3GM - 20 HP @ 3000 rpm. The only info I can find is for the 3gm 30 which varies between 27 & 30HP. Did this model supercede the 3GM and what is the difference between the 3gm & the 3qm. I would appreciate some help from one of you experts because the more searching I do the more confused I get.
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Old 11-07-2010, 17:25   #2
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3000 rpm marked on engine sounds wrong. Also when you sort that out just check the tacho is correct by an optical tacho. Both of mine were calibrated 500 rpm out when the engines were installed back in the 80's & previous owners never picked it up. I did after spending big on wrong props!! The figures out of the service manuals I have 3GM 22.5 hp/1hr @3600 & 20 hp/cont @3400 Just be careful with specs Kw's also get in there!!
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Old 11-07-2010, 17:46   #3
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3QM is older than the GM series. In the GM series the 3 cylinders were

3GM, 3GM-F, 3GM-D, 3GM30, 3GM30F, 3GM(30)(F)V

the HP ranged from a meager 21, up to 27hp. Some were rated continuous, some, 1hr maximum.
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Old 14-07-2010, 09:26   #4
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Thanks

Thank you guys. I found out more in a couple of posts than I did with hours of searching. the plate on the engine distinctly states. 3GMF. Cont rating HP20, RPM 3400. So I guess I have to go with that. I was misinformed by the original owners. The prop they fitted was a 16" 10. 2blade, so I will now have to more research.
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Old 14-07-2010, 10:18   #5
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I'm no expert ....but I do own a Yanmar 3GMC 22 1/2 hp ...
cont. rating 20 hp @3400 rpm's (mine is raw water cooled with a saildrive = "C") yours 3GMF should= fresh water cooled.
Hugo
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Old 14-07-2010, 12:05   #6
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I spoke to Tom Watts, tech. support at Mac Boring (distributor of Yanmar in eastern US) 908-964-0700...he was very helpful
explaining the evolution and ID of the GM series motors...but
they don't like to recommend prop sizes if I remember
correctly. Spoke to him 4-5 years ago give a call! Hugo
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Old 14-07-2010, 14:12   #7
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My 1981 C&C 34 has a 3GMD, raw water cooled, with about 2200 hours on it. The tag on the engine says 20hp at 3400 rpm.

The owner previous to me had a 16x10, two blade Martec on it. That worked okay but we do a LOT of motoring in Puget Sound so I replaced the Martec with a 14x7 Campbell Sailor.

I am very happy with the Campbell prop. It is much smoother, allows me to get to 3400 at full throttle, and it sails well. I cruise about 5.5 kts at 2600 rpm.
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Old 14-07-2010, 19:59   #8
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Thanks John. I don't know what prop it is yet, but I will certainly keep your change in mind. I have been thinking about 3 blade, I cruise in the same waters as you. I am berthed in Point Roberts.
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Old 20-07-2010, 13:09   #9
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Send a message via AIM to MitchM
1) there is a great yanmar dealer MER Marine on west side lake washington in seattle. 2) go to boatdiesel.com and look for your model of engine, it will tell you the HP and continuous and WOT rpms. get this info and your boat design data then 3) go to michigan wheel's website which has a propeller calculator on its web free of charge. 4) for curiosity you might also read Dave Gerr's book "propeller design' and you will then know more than you need to know about the proper propeller specifications for your boat...
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Old 01-08-2010, 00:25   #10
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... Also when you sort that out just check the tacho is correct by an optical tacho. ...
What is an optical tacho?
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Old 01-08-2010, 01:23   #11
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hand held optical tacho has a reflective tape or something to contrast a laser or light beam & reads the pulses optically. not too expensive to buy used to read the rpm at the crank shaft.
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