"Rick Cortese" wrote in message
m...
mmc wrote:

"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

The WOT is more a problem with my son use the boat. He really should be in
a Ski Nautique or something similar when I consider his patience and need
for speed.
Ha! Understood!