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Old 24 September 2006, 08:22   #11
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some technical info

here
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Old 24 September 2006, 09:07   #12
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Excellent. 3600 cruising is the most economical. Above 3800 and it begins to drink twice as much fuel. Unfortunately, at that speed you've only got about 65hp. At a guess, say, in a medium loaded 6mtr boat 20 - 22kts.

Cummins pleasure use is a little more strict on max rpm use but a little more lenient on cruise rpm.
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Old 25 September 2006, 14:29   #13
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Quote:
Originally Posted by jwalker
Excellent. 3600 cruising is the most economical. Above 3800 and it begins to drink twice as much fuel. Unfortunately, at that speed you've only got about 65hp. At a guess, say, in a medium loaded 6mtr boat 20 - 22kts.

Cummins pleasure use is a little more strict on max rpm use but a little more lenient on cruise rpm.

Don't agree when talking about a 6.0- 6.3 mtr rib and WOT. Ask Mike C he's had one for a few years
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Old 25 September 2006, 19:22   #14
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But, Andre, I wasn't talking about WOT. The curve to look at is the one that gives the power at the correct propeller loading. Cummins use the exponent of 2.7.

I wasn't very clear when I said twice the fuel. If you look at the difference in fuel consumption for each 200rpm rise in engine speed you'll see that after about 3700-3800 the motor consumes about twice what it was consuming before for each 200 rpm rise. 3600rpm would seem to be about the sweet spot for cruising fuel consumption. These figues will only apply if the prop fitted allows the engine to rev to 4400rpm with the load it is carrying...that's when the prop load will match the exp.2.7 curve.

Of course, it's possible to pitch up the prop and open the throttle more and cruise at a higher speed for the same revs but then you are using the engine above the loading of the pleasure rating.
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