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Old 30-08-2012, 15:14   #1
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RPM's

On a trawler cruising speed, 80 hp 4 cylinder engine, how many rpm's should it be turning? Anyone? Thanks
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Old 30-08-2012, 15:26   #2
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Re: RPM's

What engine?
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Old 30-08-2012, 15:33   #3
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Re: RPM's

There no such thing as a definitive cruising speed. Cruising speed is where you feel comfortable with the speed you are getting for the fuel consumption. It is somewhere between dead slow ahead and full ahead.

What is your engines maximum governed speed? Cruising speed is less than that. If you want numbers then perhaps 70% to 85% of max RPM.
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Old 30-08-2012, 15:36   #4
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Re: RPM's

The answer is it depends on the engine and the reduction gear ratio. Each engine is different, from old large tugboat engines that redline at 300rpm, and fast gas engines that are meant to run up to 6500 rpm.

Diesel engines in boats are installed, and then tested, to see if they over-rev or under-rev the engine with the hull and propeller. It is important to the longevity of the engine to have it max out at the proper rpm's.

The other factor is how much fuel do you want to burn. While the engine may be capable of turning over at 3500 rpm maximum, with the boat at 9 knots; quite often running 2700 rpm will push the boat at 8 knots using half the fuel. The key is to experiment. Usually around 70-75% power is the sweet spot, but it depends on the boat.

If you give us the engine make/model and the reduction gear ratio, we should be able to give you some idea.
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Old 30-08-2012, 16:41   #5
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Re: RPM's

It should be turning at rpm close to the ones required to deliver the power required to sail at the selected cruise speed. This power can be interpolated from Skene's.

It is fine when you have an engine that delivers this at .5 to .75 of max revs.

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