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Old 07-22-2011, 09:22 PM   #9
Delfin
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Join Date: Jan 2010
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RE: Fuel Economy Question

Quote:
Tony B wrote:
I think I finally figured it out.

With a given size semi-displacement hull, it does not matter whether you have a120HP or a 220HP engine. What matters is the RPM's. If for example, I found my most efficient speed to be 7 knots, I should be gettting pretty much the same consumption rate with either engine because the larger engine will be going much slower (RPM's).

Am I even close?
*Yes, of course you are close.* The output HP of any motor is a function of the fuel consumed.* Larger engines can burn more fuel, releasing more energy, but the most efficient engine will be one that is operating at 75% or so of its rated capacity delivering the HP you need for the specific boat, prop, trim, displacement, hull form, at the desired speed.

For example, a Cat 3408 displaces 18 liters, and burns*11.7 gph to produce*216 hp, max HP 536.

A Cat 3306 displaces 10.5 liters and burns 12.1 gph to produce 215 hp, max HP 270.

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