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Bill Mc 09-26-2007 08:00 AM

Yanmar 3GM30F
Friday night we arrived at the boat and stowed our gear. The better half and I were discussing leaving the next day in the morning and not like our last adventure when I quaffed 151 Rum into the wee hours and we wound up spending the weekend in the slip, I thought is was a damn good weekend, go figure, but I digress. To end the Discussion we immediatly cast off the dock lines and began motoring to one of our favorite little harbors (Bath) to spend the night on the hook as we motored along at 2250 rpm it was getting dark so I kicked the rpm up to 3100 after about 45 min the overheat alarm rang out so we secured the engine and set sail. after tacking for about 1 1/2 hrs just to let the engine cool we dropped sail and made our way into Bath creek and made fast to the hook. I forgot to mention our nav lights failed to work, whew. So the question to this long winded explanation is why would I overheat and what would the max rpm be for this engine? I hear that 100% is OK, others state 70-80% of max rpm. Then I get all the what if explanations. ie prop pitch, STOCK, plastic bag/ dirty strainer, NOT strainer was clean. So I am totally open to some more suggestions short of a complete overhaul..

Fair Winds,


Melrna 09-26-2007 08:37 AM

I would check a few things. First, your water impeller to see if it is missing any blades or blades damage. Second your heat exchanger. It could be blocked. Zinc anodes are usually the culprit. From there I would check your exhaust elbow. I know on some boats the exhaust elbow is too small and the next size up eliminates the overheating at higher RPM. Another cause can be barnacle build up in the intake, restricting flow. It is just a start but I am sure the mechanics here will chime in. Now back to my pink jobs! Cinnamon rolls coming up guys!

JohnRPollard 09-26-2007 09:37 AM

Melrna hit all the obvious points, pink jobs notwithstanding.

The only other thing I'd mention is that 3100 rpm sounds a bit high for that engine for continuous operation. I know it's within the rated rpms, but subject to all the caveats relating to prop pitch and size, we have the same unit in our boat and we cruise in the 2700-2800 range. We occassionally will do brief bursts into the low 3000s during docking manoeuvres, etc, but the engine just sounds like it's really working hard at those rpms so we don't stay there long. Maybe consider running at lower cruise rpms, i.e. < 3000.

Now about those cinnamon rolls....

jrd22 09-26-2007 09:46 AM

I would agree with John(above) that 3100 is too high for continuous operation. Max RPM is 3600, we cruise at 22-2500, anything above that and we start to see more smoke/steam out the exhaust. If you run it again at normal RPM and don't have an overheat alarm come on you probably don't have any problems.

camaraderie 09-26-2007 10:17 AM

3100 rpms on a yanmar designed for 3600 rpm is WELL within limits and should be able to be run indefinitely at this speed. I had a 4JH with 80-90% of rpm limit and ran for years with NO problem whatsoever. MackBoring will tell you that running at 80-90% is ideal & they should know. Of course it is possible that the boat is over propped but if you are not seeing smoke normally at this speed then I would say to check the things Melrna mentioned before I throttled back permanently.

NOLAsailing 09-26-2007 10:52 AM

I would also like to chime in and say that a motor rated at 3600 Max RPM should have no problem at 3100. Normal cruising RPM should be around 2900; 2250 is under-reving the motor.

In addition to the other thoughts - you may want to check your hoses. As they age, the suction created by the higher pressure at higher RPM's can cause them to restrict flow. If the hoses are older, replace them as part of normal maintenance and you may see an immediate improvement.

Bill Mc 09-26-2007 05:20 PM

Still scratch'n me noggin..
Prop is original size and pitch and clean, no barnacles.
Fresh water cooled engine, so no internal zincs.
Impeller is fresh, Just replaced by yours truley so I know at least this is right.
Only smokes a little when started after a week long layup, this has been improving since the marina in their infinite wisdom used :mad: EITHER :mad: to get her cranked to do my bottom job. ** I will no longer beat that dead horse}.
I believe we were doing about 6 kts at 3100 rpm and 5.3 kts at 2250 rpm.
70% = 2520 rpm, 80% = 2880 rpm, 86% = 3100 :confused: 3100 = BAD???

So I'm still scratching my head on this one. Thank ya'll for the feedback. I'll get this figgered out if it kills me. Financially that is...:D

Fair Winds,


Rockter 09-26-2007 05:52 PM

If it's an older, raw cooled motor, scaling can be a problem, as for my olde Volvo MD17C.
Sometimes, whan scaled, the poor motor has trouble getting the coolant to reach those regions that would be cooled easily when new.
Though I have never used them, some owners recommend the use of de-scalants. make sure the motor is hot before you circulate them, or the thermostat will not let them into the motor coolant jacket.
The last rtesort is to take off the exhaust manifold and de-scale all that you can see.
In my humble opinion, I would not run an older motor that hard... 3100 rpm is pushing it a bit... that's 86% of max.

Vasco 09-26-2007 06:36 PM


I would check the impeller. I know you said you recently replaced it and that may be the problem. It may have been an old dry one or got squished a bit going in and now has lost a vane. On the 3GM30 they're pretty easy to get at, unless you have a C&C where it's real hard to get at that side of the engine. :)

sailvi767 09-26-2007 06:49 PM

If your max RPM is 3600 on that engine it is important that the engine is able to run at a minimum of 90% of rated RPM. If it won't then Yanmar will deny all warranty claims as the boat is overpropped. The newer yanmars run at a lower RPM. I actually thought your max is 3800. The engine should be able to run forever at 90% of rated power. Running a diesel long term at lower RPM's (under about 80%) leads to wet stacking which is very harmful to the engine.

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