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Old 26-10-2009, 00:26   #1
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Cruising RPMs

I have a 3 year old Yanmar 2GM20FC. I understand that the maximum rpm's are ~3500. How do you determine what is the best cruising rpm's.
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Old 26-10-2009, 01:13   #2
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We use the lowest possable revs to get us going at the least speed we can enjoy.

Ours is a 4JH3E and is meant to max at 3,200

We idle at 800,
charge at 1,200
Slow ahead 1,500 about 4.5 kts to 5 kts great for motor sailing etc
Cruise 1,800 to 2,000 5.5 to 6.0ish
Fast cruise 2,500 but not for long 6.5 to 7kts
2,800 total max I would have seen once or twice only. 8kts!!!

The dills ho say that Yanmars should only be used flat out at max revs are dills
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Old 26-10-2009, 01:45   #3
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Many have used their engines any way they want and have gotten years of use out of their engines. I tend to run my engine closer to the recommendations given by the dills that made the engine in the hope that they know more about it than the average consumer.

This web page is operated by a Yanmar dealer and has recommendations that expand on what the manual for the engine says on operation.

Yanmar Marine Engine Help

John
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Old 26-10-2009, 02:08   #4
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Fouling, age, hours, gearbox, maintenance, propeller...

There seem to be quite a few schools of thought, most of which would be fine on newish engines. It's when the engine starts to get a little old that the serious questions begin.

If the propeller "perfectly" matches the engine then it may be OK to run the engine a little harder when it's new (but run in). However if the propeller is overpitched (like mine) then I can hear (and feel) the engine complaining if I try to get full revs.

If the engine (and gearbox) are completely maintained to manufacturer's specifications then a little harder may be OK.

If the gearbox is oversized for cruising then a little harder may be OK, however some marine gearboxes may be undersized for continuous use. Check your handbook. We seem to get almost as many threads on gearboxes as on engines.

If the engine is old, has a lot of hours and maintenance is overdue then, in my humble opinion, it's starting to be time to baby it a bit.

If the boat has significant fouling the engine/gearbox is going to have to work harder to give you usual speed so it may be time to back it off a bit.

If the exhaust hose is a tad undersized for the engine then it may be time to baby it a bit.

And finally it's nice to listen and feel how the engine, gearbox and propeller are going, how they're working against swell, chop and wind and to adjust the throttle so it all feels "sweet".
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Old 26-10-2009, 06:27   #5
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Most diesels have a "safe" cruising RPM of 300 less than max. In your case 3,500 - 300 = 3,200. Now having said that, that doesn't mean that is the most effective cruising speed. How hard do you have to run it to get hull speed? Anything over that is mostly just wasted fuel.

Lost of other good advice above.

George
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Old 26-10-2009, 06:49   #6
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I believe most of us have a fear of running engines hard. I guess it just goes against instinct. Common sense tells us that things you run hard will wear out faster. But most diesel mechanics and manufacturers know the old proven saying "Kill a diesel with kindness" There are always exceptions, my neighbor has an old Ford he hasn't changed the oil in for years. It still runs fine and he balks at frequent oil changes. Diesels if maintain properly for the most part are hardy beasts, designed to run at the rpms the manufacturers recommend for hours on end. You can attempt to be smarter than the engineers that designed your engine if you like. Diesel mechanics in the long run will thank you for being such good customers.
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Old 26-10-2009, 06:49   #7
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Most diesels have a "safe" cruising RPM of 300 less than max. In your case 3,500 - 300 = 3,200. Now having said that, that doesn't mean that is the most effective cruising speed...
I'd say that the most common safe continuous speed is about 80% of maximum (often specified as 1 hour rating); hence
3500 x 0.80 = 2800 maximum continuous RPM.

The prime reason to not “baby” a diesel is to ensure that you’re running at operating temperature (not “cold”).

The most economical cruising RPM will be determined by fuel consumption & speed curves, particular to your application.
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Old 26-10-2009, 06:55   #8
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My 3JH2E is rated at 3600 max rpm, 3400 continuous rpm. I have been told that 75% of max power is the most efficient rpm for a diesel engine...FWIW
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Old 26-10-2009, 07:17   #9
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I believe most of us have a fear of running engines hard. I guess it just goes against instinct. .
Nope. It goes against the wallet. The last 1,000 revs just burns fuel in an ever increasing logrithymic scale.
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Old 26-10-2009, 07:26   #10
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From Yanmar Help

For prolonged running, a pleasure boat engine a good operating speed is around 85% of maximum continuous rated RPM (see the engine data plate), assuming the propeller size is correct and the engine can reach maximum rpm).

For the GM - 4JH - 3/4JH2 series ≈ 2900rpm
For the 3/4JH4 series ≈ 2400rpm

Goto ➥ Yanmar Marine Engine Help
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Old 26-10-2009, 07:39   #11
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Nope. It goes against the wallet. The last 1,000 revs just burns fuel in an ever increasing logrithymic scale.

At a constant 1200 rpm to charge, a constant slow ahead 1500 rpm, a long constant 1800-2000 rpm for the most part, and the occasional not for long 2500 rpm. The wallet is relative. You may be very lucky and be the exception to the rule. But the odds are against it and chances are good you'll find yourself proving another old saying. "Pay now or pay later".
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Old 26-10-2009, 07:48   #12
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"Pay now or pay later".
The Yanmar guy said 1,200 is high enough to stop the cylinders being polished.
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Old 26-10-2009, 08:02   #13
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The Yanmar guy said 1,200 is high enough to stop the cylinders being polished.

My Yanmar guy just laughed. But we have to judge these things ourselves.
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Old 26-10-2009, 09:29   #14
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Calculate your hull speed by the waterline length and that speed will likely be your most efficient rpm. Your engine should maxout at about 85% or so of the rated rpm. For maximum travel speed find a sweet spot where the propulsion system is smooth and sounds good near your hull speed.
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Old 26-10-2009, 10:33   #15
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1200 rpms for charging bateries in a yanmar sounds to me wear and tear in the long term, what about 1500 rpms??? Cheers.
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