Wayne.B wrote:
On Wed, 08 Nov 2006 16:51:33 -0500, Jeff wrote:

This doesn't seem quite right to me - my catamaran, which weighs about
15% of what your trawler weighs, uses almost as much fuel.

If you boat displaces 60000 pounds, then it will need about 60 HP to
get over 7 knots, 90 hp to get to 8.5 knots. Looking at specs for
various Caterpillar engines, the gallons/hour at 60 hp is 3+ gals.
Using two smaller engines doesn't help that much, though at least they
can idle with lower consumption.

Your number only make sense if you spend more time on average using
you engines to run the A/C than propel the boat.

I'm not entirely sure that I understand your point. The numbers are
what they are, and yes, the boat weighs about 60,000 lbs.

The engines are twin Detroit 6-71s which are way bigger than what is
actually needed to run at 8.5 kts. I'm sure that introduces some
inefficiencies. The port side engine also drives a hydraulic pump for
the stabilizer system. It wouldn't surprise me if that took an
additional 10 to 20 hp, or about 1 gph. The primary generator is an
oversized 20 KW unit that uses about 1 gph on average but we do not
run it continuously except in hot weather.

The best we have ever done is about 6 gallons per hour running 7.9 kts
in flat water, and using the smaller backup generator as little as
possible.

Sorry - I read 1 GPM as 1 GPH! Doing about 8 GPH when approaching
hull speed is in line with my rules of thumb.