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From: (no name) (no email)
Date: Fri May 25 2007 - 08:26:21 EDT
In a message dated 5/25/07 12:01:40 AM, Bill writes:
> To this end, I just looked up enough factors to calculate fuel burn for a
> given weight boat at displacement speeds. Although there are many
> variables, a close approximation for diesel burn is to use .035 gph per
> horsepower.
>
> The assumptions I used:
> 130,000 BTU/gal for 2D diesel
> 1 hp = 2544 BTU-hr
> therefore 1 gph = 51 hp (perfect world)
> diesel engine efficiency = 56% (sadly)
> so, 1 gph yields 28.6 hp (real world)
> or, 1 hp uses .035 gph
>
> For my prospective future boat, a DeFever 44, at 44000 lbs displacement and
> 500 lbs per hp, I will use 88 hp at hull speed. That figures to about
> 3.1gph on a clean bottom with clean props. About right, with hull
> speed of
> about 7.5 kts.
>
Good logic but your estimate of fuel burn per horsepower per hour is about
half as much as that usually used. Most diesel engine experts estimate that an
average small diesel engine burns .06 gal/hp/hr. The reason your hypothetical
engine is so economical is that your estimate of diesel engine efficiency
(56%)
is far too high. Most trawler sized diesels have an efficiency in the 30%
range. If your engine is burning 1 gal/hr it is producing about 15 hp. There
are
some diesels used in stationary power plants that have higher efficiencies but
they are about the size of a house.
Larry Z
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