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Old 27-05-2013, 09:30   #1
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Carver 500 450 Cummins optimal cruise RPM

I do not have flow meters and given non consistent current conditions, I do not have significant data to optimize my fuel consumption. Does anyone have data or recommended optimal running RPM for long distances.
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Old 27-05-2013, 14:42   #2
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Re: Carver 500 450 Cummins optimal cruise RPM

You can get Cummins fuel flow curves for those engines from their website. Off the top of my head, I think 2200 RPMs is 13 GPH (per engine) or thereabouts, and you get whatever speed your boat will give you from that (wind, current, tide, load, etc. dependent).

Rated WOT is 2600 RPMs, and you should check to be sure yours will do that under typical load conditions (if they won't, you might have some subsequent prop research to do).

You'll see various recommendations to run this type of diesels at 80% of WOT, 85% of WOT, "200 off the top," etc... but generally (if you don't have fuel flow gauges) there's some experimentation involved, often with fuel/trip logs and so forth.

FWIW, when planing, we cruise somewhere between 2100 RPMs and 2400 RPMs, usually closer to 2200-2350, depending... when we're not running at hull speed, maybe 3 GPH/engine....

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Old 27-05-2013, 15:17   #3
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Re: Carver 500 450 Cummins optimal cruise RPM

For the newer Cummins Diesel (Tier two and higher), Cummins recommends running it at 100 percent for one-half hour 10 percent of the time. This burns the carbon out of the combustion chamber. Otherwise I run my Cummins B-series turbo diesels at 2400. Diesels do not last as long when run at slower speeds for most of their lives. But on the other hand, the faster you run your Diesel the worse your miles per gallon. So you have to figure out for yourself which is the best compromise between these two factors. Even with Diesel at $4 to $5 per gallon at most fuel docks in the US, I think it is more cost effective to run your Diesel faster than to run it slower and shorten its lifespan. I'm burning around 20 GPH total (18 knots) on both engines at 2400 RPM.

If your healthy engine cannot reach it's maximum governed speed then you are over pitched or over diameter or both, which means you are lugging your engine which will shorten its lifespan. Contrary to some peoples opinions doing this is not more fuel efficient. A Diesels specific fuel consumption does not improve by lugging the engine.

You can get your engines Specific Fuel Consumption curves at Cummins website. The specific fuel consumption is the rate at which fuel is consumed divided by the horsepower produced. The units are grams per second of fuel consumed divided by watts (power) produced.

You can purchase aftermarket fuel flow meters. When interfaced with your GPS it can give you gallons per mile. I think the meters might give you a better grasp on what is going on with your consumption versus speed and eventually pay for themselves. My engines are 5 years old and came standard with fuel flow meters. This information has been very helpful to know saving thousands of gallons of fuel.

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Old 30-05-2013, 12:38   #4
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Re: Carver 500 450 Cummins optimal cruise RPM

thanks for the responses! helpful and reconfirms what I had believed. Just hard when running gas most of life where you want to vary the RPM and keep lower as percent of max RPM where as the diesels need the load.

Agree with comments, better to spend more on diesel than a premature teardown!
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cruise, rpm

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