On Wed, 08 Nov 2006 11:36:48 -0500, DSK wrote:

We burn about 1 3/4 gph cruising about 7 to 7.5 knots. The
boat will (in theory) go 8.5 but fuel consumption starts
getting up into the 5gph range and we make a loke of wake
and foam and noise, for not a lot of speed gained.

Here's another data point:

On our Grand Banks 49 we average about 1 gallon per mile on extended
cruises unless we make a really concious effort to economize which can
save an additional 10 to 20%. That includes about 4 hours per day of
generator time, running on both engines, and using active stabilizers
virtually 100% of the time. Our average speed is about 8.5 kts,
slowing to about 7.9 if we are really trying to stretch the fuel.

Wayne.B wrote:
On Wed, 08 Nov 2006 11:36:48 -0500, DSK wrote:

We burn about 1 3/4 gph cruising about 7 to 7.5 knots. The
boat will (in theory) go 8.5 but fuel consumption starts
getting up into the 5gph range and we make a loke of wake
and foam and noise, for not a lot of speed gained.

Here's another data point:

On our Grand Banks 49 we average about 1 gallon per mile on extended
cruises unless we make a really concious effort to economize which can
save an additional 10 to 20%. That includes about 4 hours per day of
generator time, running on both engines, and using active stabilizers
virtually 100% of the time. Our average speed is about 8.5 kts,
slowing to about 7.9 if we are really trying to stretch the fuel.

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.

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.

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.

On Wed, 08 Nov 2006 22:28:05 -0500, Jeff wrote:

1 GPH

If only it were true. :-)

Jeff wrote:
Sorry - I read 1 GPM as 1 GPH!


Wayne.B wrote:
If only it were true. :-)


Hey, you might get that if you go slow enough.

Our boat goes about 3.5 knots at idle. I have no idea what
the burn is, since we've never gone long enough at that to
register any fuel consumption, but I bet it's considerably
better mileage than at 7 knots.

DSK

On Thu, 09 Nov 2006 14:39:01 -0500, DSK wrote:

Hey, you might get that if you go slow enough.

Our boat goes about 3.5 knots at idle. I have no idea what
the burn is, since we've never gone long enough at that to
register any fuel consumption, but I bet it's considerably
better mileage than at 7 knots.

Ours idles at about 5 1/2 kts but it's not good for the engines to run
it at that speed for any length of time. I think we'd have to run on
a single engine to significantly improve fuel burn and that brings up
other issues with free wheeling props, transmission issues, shaft
locks, etc.

Supposedly there was a guy who brought a GB49 from HI to CA by
removing one prop and coming back on a single engine at slow speed.

Too dicey for me, and I sure wouldn't want to be in a position where I
had to wrestle that prop back on in mid ocean. We have an effective
range close to 1,000 NM running the way we do. That's good for just
about anything but crossing oceans and the boat was not really
designed for that in any case.