With my little knowledge, here's my 1 cent:

Bryan kirjoitti:

For those of you who have used the fuel flow meters what do you think about
fuel efficiency?

I assume that you mean to ask which speed zone is most efficient in the
terms of miles per gallon (or nm per liter).

You could take a look at boat/outboard tests - in the Internet and
magazines - as some of those list fuel consumption at various speeds.

It might be a bit difficult to generalize. What makes boating so
interesting is that you have usually many parameters to consider, and
this is no exception... For example you have different engines (and
even different engine types), propulsion types, differences between the
hulls at displacement speeds as well as planing speeds, different loads
for the boat, etc

For example, I gather that a long, narrow hull makes for good
displacement speeds but a flat, wide hull is more efficient for planing
speeds. Are we assuming that engines or propellers are not changed for
the given hull, to get better values at a certain speed?

For a planing boat, which is more efficient, motoring at 3-5 mph (no wake)
or motoring at the lowest speed that allows the boat to plane?

I understand it's a solid general rule of thumb that going at a
displacement speed is very efficient. Planing hulls are not really
designed for this, but the physics of water still favour these speeds.
I looked at some boat tests and it seemed that droning on at 4 knots or
so gave indeed the most mpg of the listed values in the test. But I
doubt this speed would be a good choice for travel in practice because:
1) for starters, it's dog slow by today's standards 2) the engines will
not like it 3) typical small planing boats with efficient design are
not too much worse off at planing speeds. If you get a boat that is
designed for planing speeds, why on earth would you want to run it
mostly at displacement speeds?

There is some theoretical confusion on when exactly your boat is going
at a "planing speed". But I think you'd need to go a few knots faster
than that to get on the plane cleanly (clear of the boat's wake?).

Is the lowest speed that allows the boat to plane the most efficicient
planing speed or is there a sweet spot somewhere between planing and WOT?

You may have noticed boats usually have a cruising speed listed. I'm
not sure how exactly that is determined (and I doubt there is a solid
common standard) but I'd think it takes into account speed, fuel
efficiency and what's best for the engine. Your sweet spot for the fuel
efficiency (when planing) would probably land somewhere between the
cruising speed and the lowest planing speed, I think...

I'll need to look at a few more tests... But it's a good question.
However you should not forget to consider this in relation to what is
best RPM in terms of engine lifespan and what are one's needs as a
boater.

Risto