In article ,
Messing In Boats wrote:

I have a boat like that with a similar powertrain. I get 1.3 nautical
mpg at best on plane (20 knots) and 2.3 nmpg at best at hull speed (6
knots). I use this boat 150 to 200 hours a year (mostly at 6 knots) and
buy a bit of gasoline. If/when I repower, I will probably go back to gas
because the motor is so much cheaper, it will bolt right on without any
modifications, it's quieter and I can buy a lot of gas for the
difference in price. ($10,000)

One issue for me is range, which would be better with the diesel, but I
have an auxiliary tank.

Capt. Jeff


??? I would have expected quite a bit better mileage at displacement
speeds, not less than double.

Then again, what size powerboat (LWL) are you doing this with? As I
recall, the original poster was about our size (28' loa, 22' lwl). Could
you be just at hull speed at 6 knots indicated? On our diesel sailboat,
we get about 7 nm per gallon at hull speed (6.6 knots) and 22 nmpg at
85% (5.8). Quite a difference for about a knot difference in speed.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/

Six knots is probably a bit faster than the true hull speed, but I don't
want to crawl quite that slowly. This is a planing hull, so it really
doesn't have a "hull speed" like your displacement hull. Using the 1.34
X sqaure root of the LWL gives me about that though. Fooling around with
the speed and a fuel management system shows me that the slower I go,
the better mileage I get. Going down to three knots gets me up to 5 nmpg.

I've thought about repwoering with diesel if/when my 5.7 goes bad on me
in about 2000 hours, but unless the price of fuel doubles and my hours
on the water double, I'm looking at about ten years for a payback. Not
worth it.

Capt. Jeff

Six knots is probably a bit faster than the true hull speed, but I don't
want to crawl quite that slowly. This is a planing hull, so it really
doesn't have a "hull speed" like your displacement hull. Using the 1.34
X sqaure root of the LWL gives me about that though. Fooling around with
the speed and a fuel management system shows me that the slower I go,
the better mileage I get. Going down to three knots gets me up to 5 nmpg.

I've thought about repwoering with diesel if/when my 5.7 goes bad on me
in about 2000 hours, but unless the price of fuel doubles and my hours
on the water double, I'm looking at about ten years for a payback. Not
worth it.

Capt. Jeff

In article ,
Messing In Boats wrote:

I have a boat like that with a similar powertrain. I get 1.3 nautical
mpg at best on plane (20 knots) and 2.3 nmpg at best at hull speed (6
knots). I use this boat 150 to 200 hours a year (mostly at 6 knots) and
buy a bit of gasoline. If/when I repower, I will probably go back to gas
because the motor is so much cheaper, it will bolt right on without any
modifications, it's quieter and I can buy a lot of gas for the
difference in price. ($10,000)

One issue for me is range, which would be better with the diesel, but I
have an auxiliary tank.

Capt. Jeff


??? I would have expected quite a bit better mileage at displacement
speeds, not less than double.

Then again, what size powerboat (LWL) are you doing this with? As I
recall, the original poster was about our size (28' loa, 22' lwl). Could
you be just at hull speed at 6 knots indicated? On our diesel sailboat,
we get about 7 nm per gallon at hull speed (6.6 knots) and 22 nmpg at
85% (5.8). Quite a difference for about a knot difference in speed.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/

I have a boat like that with a similar powertrain. I get 1.3 nautical
mpg at best on plane (20 knots) and 2.3 nmpg at best at hull speed (6
knots). I use this boat 150 to 200 hours a year (mostly at 6 knots) and
buy a bit of gasoline. If/when I repower, I will probably go back to gas
because the motor is so much cheaper, it will bolt right on without any
modifications, it's quieter and I can buy a lot of gas for the
difference in price. ($10,000)

One issue for me is range, which would be better with the diesel, but I
have an auxiliary tank.

Capt. Jeff