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"Rick Cortese" wrote in message
m...
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

18hp is a lot of motor for that sized boat. I have an old 6hp Johnson
pushing my 5,000 lb 24' Morgan at hull speed. Does just fine in the shelter
waters of the Indian Lagoon. If I were in a place without the shelter we
have I may go to an 8hp but no more. I've motored this boat in winds up to
20 kts without struggling. I plan my sailing around the weather instead of
getting my butt kicked or sitting with no wind at all. Sitting on flat water
has it's own theraputic benefits, but is not much fun in the middle of
summer in Central FL.
I'd say your prop may be pitched wrong for the boat. Shouldn't have to run
WOT. Another possibility is that one of the cylinders is not firing. Might
check the plugs.
Have a friend with a 6-9.9hp you can borrow for a trip?
Merlin

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