Unlikely WOT ever gives best economy unless it is severely underpowered and
required WOT to get the boat running right. Anyway it is not advisible to
run at WOT continuously, the engine's not gonna last. The Mercruiser
recommendation is not to run at WOT for more than 2 minutes.

Best economy is speed dependent (assuming engine has enough hp) than engine
or (this assumes planing hulls are on plane and displacement hulls are not
trying to plane) and is affected by the hull/load combination. Most modern
boat hulls and engines are already at their best according to the intended
design applcation, so there's not much the user can do here.

Drag, however, is the largest horsepower consumer (read as fuel), in the
fluid flow equation, drag increases as a square of speed. Fortunately, and
the boater can control this. If you have a fuel flow gauge, its the easiest
way to find the best speed but you can rely on your ears, the sweet spot is
unmistakable.

As an example of speed vs fuel burn. I have a 23ft planing hull with a 5.7
mercruiser i/o, following are numbers read off a flow guage.

1. 22 kts 48 litres/hr
2. 29 kts 118 litres/hr

The 7 kts increase is only one third of 22kts, but the fuel burn is 1.5
times more. Speed costs!

Tan PS


"Tony Thomas" wrote in message
news:NL0dc.217444$po.1077466@attbi_s52...
Yes, it is more fuel efficient with the throttle backed off slightly.
Best
fuel economy is usually found at 3200 to 3500 rpms as a general rule.
This
assumes your on plan at these rpms. Anything over about 4000 rpms starts
to
drink fuel. 5000+ is really bad fuel economy in general.
If you have to run the boat at WOT all the time then your underpowered.

--
Tony
my boats at http://t.thomas.home.mchsi.com



"RB" wrote in message
...
I've had several boats with big single and twin o/bs in the past. I
remember most as having a "sweet spot", which was just down from wide
open
throttle (WOT). Many of you are probably familiar with a sweet spot.
Cars
and trucks have been known to have them. Seems like going to WOT can
eke
out a little more speed, but things are straining, coming back down a
little
seems to let everything harmonize and get in the groove, and run more
smoothly.

The question I have is: assuming a boat with o/b has a sweet spot, is
my
fuel consumption more efficient at the sweet spot than wide open?
Sounds
like it would be -- a little bit---as you come back on the throttle to
get
to the sweet spot. Or, is it that there's no real reason to run at the
sweet spot, other than it "feels right", and good?

Just curious