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From: (no name) (no email)
Date: Sun Apr 27 2008 - 00:55:24 EDT
In a message dated 4/27/08 12:02:20 AM, Peter writes:
> I am presently a sailboater but ready to make the switch to the dark
>
> > side-POWER boats. My primary concern is fuel usage. Can the members
>
> > give me some insight into the fuel costs and or fuel usage rates.
>
Here is a quick and easy way to calculate fuel usage. It will probably get
you within 10 or 15% of the correct value.
First get the LWL and the displacement of the boat you are considering. Your
hull speed will be 1.34 times the sq. root of the LWL. It generally takes 1 hp
per 600 lb of displacement to move a boat at hull speed. Most naval
architects use 1 hp per 500 lb but that's being unduly pessimistic. That should give
you a reasonable estimate of the power required. A diesel engine burns .06
gallons of fuel per hp per hour. A gasoline engine about .09 gallons per hp per
hour.
Here is a sample calculation. Assume you have a 30 ft. trawler with a 27.5
ft. LWL with a displacement of 16,000 lbs. Your hull speed will be 7 knots. The
power required to move a 16,000 boat at that speed will be 26.7 hp. If a
diesel, fuel consumption will be 1.6 gallons/hr. If gasoline engined, fuel
consumption will be 2.4 gal/hr. At slow speeds, the hull shape is not too critical and
fuel consumption should be in the calculated ballpark for most boats.
Speed is the most critical factor in fuel usage. The power required to drive
a hull rises almost as the cube of the speed. If you drop your cruising speed
a couple of knots below hull speed, you will use half the fuel. Twin engined
boats use a bit more fuel than single engined boats because of additional
weight and mechanical losses but there is no denying their convenience in docking.
Larry Z
**************
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