Yanmar 3GM30F - Confused

[BobnLesley]

We've just bought a new boat that's fitted with one of these beasties. It starts easily, runs sweetly and pushes us along at a comfortably easy five knots (35' long-keeler) with the rev counter reading about 1900 - 2000 revs. However, when reading the owners manual, it refers to 'one hour power rating' being achieved at 3400rpm and cruising power at about 3100rpm; now I've not tried revving it that hard, but I suspect that it might well go 'bang' before it ever reaches 3000rpm. Having trawled threads here, I've seen references to 3GM30s being run at 2700 & 2900rpm, so why does ours appear to run so much slower?

 

[prv]

I have a 2GM20 whose rating is 3600 maximum (1 hour), 3400 maximum continuous, and for which the recommended cruise setting is 85% of max continuous, or 2900. In practice, on the rare occasions I motor "on passage" as opposed to chugging into harbour, I run it at 2800. That gives 4.5 knots in my long-keeled 24 footer.

Have you actually tried running the engine at these sort of speeds?

If you're used to slow-revving old-fashioned marine diesels they sound quite fast, but they're not really. Running at under 2000 all the time isn't good for the engine.

EDIT: I presume the revcounter is actually reading accurately? Is it a Yanmar one or third-party?

Pete

 

[Spyro]

There will come a time when you'll need those extra 1000 revs to keep the speed up e.g when you're motoring into a big sea and a headwind.
Any reason why you think it will go bang?
My 2gm20 seems to run well when pushed hard

 

[jimi]

I've got the yanmar 3gm30f as well and I cruise at 2000 rpm max revs is c 3000. I've been told I'm probably a bit overpropped.

 

[vyv_cox]

My 3GM30F will reach 3600 rpm with a Brunton's Autoprop. We normally cruise at between 1900 - 2200 rpm but we try to ensure that we run at maximum revs for a few minutes every couple of hours, as per Yanmar's advice.

No question of it going bang, it will either fail to reach 3600 rpm for one of several reasons (prop size, fouled hull, cable length, etc) or it will reach it and go no further. These engines are pretty much unburstable and will stand 3600 rpm for extended periods.

 

[BobnLesley]

One more question?

If you're used to slow-revving old-fashioned marine diesels they sound quite fast... That's us!

Thanks everybody, I'll be a bit more aggressive next time we go out.

Our Owner's Manual also tells us how to check the gear oil, but doesn't clarify the dipstick reading: The only mark on it, is a groove about 1/2" from the bottom, though the current level is about another 1/2" above said mark - is it overfilled?

 

[charles_reed]

<<Our Owner's Manual also tells us how to check the gear oil, but doesn't clarify the dipstick reading: The only mark on it, is a groove about 1/2" from the bottom, though the current level is about another 1/2" above said mark - is it overfilled?>>

Yes, but don't worry too much, it's only splash lubrication for layshaft and the cone-clutch.