On Mon, 15 Sep 2008 08:25:21 -0700, Rick Cortese
wrote:

wrote:
On Sun, 14 Sep 2008 18:46:52 -0700, Rick Cortese
wrote:


Slow in the group so I figured I'd waste some bandwidth. Checked the
mileage and my boat with an 18hp two stroke only makes 2-3 mpg. I figure
it is mostly because I usually go sailing with my son so WOT for 7-10
miles to get where the winds are to his liking and same drill coming
back after the winds die.

Just my guesstimate but we seem to make ~5 knots at 1/3 throttle and ~6
knots at WOT. My GPS went down for the count so I have to get a new one
before I can get good data.

Is this about right for a hull that is in sore need of a bottom
cleaning? I saw some info that seemed to say engine displacement is more
critical then other factors. Specifally I would be much better off with
a smaller perhap four stroke of ~5 hp if I motor like this? I mean it
seems like the 18 hp is mostly overkill and just makes 13 hp worth of
bubbles.

Rick


We need at least a little more information.

What kind of boat? Length? displacement? Just how bad is the bottom?



24' Islander Bahama. Bottom was brushed to remove loose algae, mussles,
what have you, but surface still uneven. Displacement, about 4,000-5,000
I think.

What trigged my interest in MPG was an article in Latitude 38 a couple
of months back where someone took their boat, looked to be a 30-36' boat
with probably a 12-18hp diesel, on a trip from ~Tiberon to the
Sacramento Delta and said it was ~$90 total cost including fuel. A lot
of their trip was under power IIRC. I'm guessing it was about 100 mile
round trip. I use $30 just getting out of the Oakland estuary to the Bay
and back.

Rick

Sounds like something is wrong. I have a C&C 27 that weighs about the
same as your boat. I have a 9.9 hp high-thrust Yamaha 4-stroke and I
figure I average about 10 miles per gallon when motoring at hull
speed.

Are you getting it in deep enough?