2GM Engine- Max RPM?

kloudie1 · Nov 2, 2012

Notes: The 3400 RPM rating is for continuous duty.. The 3600 RPM rating is a one hour max allowable.. 2800 is fine for continuous service and is not "babying" the engine.. This from the official Yanmar shop manual for the 2 GM..

rardiH36 · Nov 2, 2012

Seems to be some difference in manual information? The Yanmar original service manual I have cites (for both the 2GM20's sea and fresh water versions) 16HP@3400 continuous and 18HP@3600 for one hour. Most info I've seen on this topic is that 80% of max rating is a good rpm to keep the engine and exhaust elbow happy. I've got a 2QM20 with a lower 2650 rpm spec than the 2GM20. I find that for my boat, about 2200 rpm which is about 80% also corresponds to a low prop/engine vibration node. So that is my normal rpm. But also, on most outings I need the engine only about 10 minutes out and 10 minutes in.

navigator97 · Nov 2, 2012

On my 2GM20F Yanmar engine I had been limited to about 2850 before starting to get smoke out of exhaust. A good diesel mechanic checked it out and determined that he felt it was over proped. In checking the manuals it stated that the prop size for my H-33.5 was 15 x 11. The prop on the boat (from the manufacturer) was 16 x 11. They apparently determined at Hunter that the 2GM20F engine was undersized for that size boat, and tried to compensate by putting on a 16 x 11 prop. In the following year they put in 3GM27F engine for that model boat.
This past winter I had a prop shop re-size the prop down to 15 x 11. Now I can run for hours at 3000 rpm with no problems (no smoke) and fuel consumption improved by about 4.5%.. I haven't yet run it over 3000 rpm.

gpdno · Nov 2, 2012

I have the same exact engine. I run mine at between 2800-3000 all the time. Most of my trips are short 15min runs to he gulf so occasionally I will run down the ICW for a couple of hrs to let it stretch it's legs.

Deadline · Nov 2, 2012

My 2GM is 18 HP and I run it sweetly at 2800. Don't push it.

H33CRich · Nov 2, 2012

Ed H said:
My 1981 Hunter 33 has a 2GM raw water cooled engine. The PO told me that he ran it at 2800 RPMs. I see the old Yanmar manuals list the 2GM as 13 horsepower at 3400 RPM and 15 horsepower at 3600 RPM. Owners of 2GM engines- at what RPM do you run your engines????
I wonder if I have been "baby-ing" the engine these last 2 years.
Thanks. Ed H.

I have the exact same engine. I tend to run at 22-25 hundred RPMs running in/out of the creak. It loves it in that range and from what I've read and heard from mechanics is where it should be run (our yanmars should not be babied). I make 5-5.5 knots in calm conditions at these revs. Pushing it harder will not produce a proportional increase in speed. You will decrease efficiency. I was in a 25-30 knot blow a month or so ago and had to head into the teeth of the wind to return to home port. I had the engine running at 2600 rpms and was making 3.8 knots and at 3000 rpms was making 3.9 knots (I have a 3 bladed prop). Consequently, I backed off the engine to just under 2k, let out the 130% jib and tacked 6 NMs home at 6.5-7.5 knots. Walking the wind ladder (tacking) 6 nautical miles turns out to 8.5+ NM. In other words, 1.5 hours motoring (plus pushing a lot of fuel out the exhaust) vs. 1.25 hours motor sailing and having a blast with water coming up into the cockpit through the scupper and the rail under water at times. Having the engine on just made tacking a lot easier in the high winds. I prefer the latter and feelin the heelin.

You can get more scientific by installing a digital fuel flow monitor and conducting some tests; record fuel flow, RPMS, SOG on a range of RPMS from idle to wide open.