"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.

In an automobile, slower is almost always going to give you better fuel
economy, assuming that you remain in the same gear. There is a direct
connection between the pavement and the engine such that the engine will
rotate the same number of times to cover a mile regardless of the speed. If
the wider open the throttle is the more gas it will consume to cover that
mile.

On a boat things are not so easy. On a displacement hull there is a certain
"hull speed" that can be achieved with minimal power and best economy.
Attempting to go any faster will result in burning much more fuel with
little or no increase in speed.

On a planing hull there is a tremendous increase in economy once the boat is
up on plane. Depending on the hull design, there can be further reductions
in drag with increased speed as the hull lifts and less hull is in contact
with the water. At some point the increase in "lift" is offset by the
increase in water drag and wind resistance and going any faster will
decrease your economy.

Add on top of all this the performance of the engine. At lower RPMs the
engine is making best use of the fuel it is taking. As the RPMs increase
there isn't sufficient time for the fuel to completely burn and create the
maximum amount of torque. The result is a lot of waste heat out the exhaust
and a loss in efficiency.

Put it all together and it becomes impossible to come up with a generic
answer. It would be rare (and certainly not in a recreational boat) that
WOT was the sweet spot, but it is possible. I would expect that the most
likely sweet spot is the speed right after the boat has come up on plane and
the bow has settled into a more even keel.

Rod