"Gould 0738" wrote in message
...
The reason the first one didn't make sense to you is that you're
forgetting
that the point of maximum drag is at the speed you're running right
before
you jump on plane. At 9 mph, that boat is pushing tons of water. At 25
mph, that boat is displacing much less water.

I'm not forgetting anything. Even though I gravitate toward semi and full
displacement hulls, I have been on plane in many dozens of boats and the
numbers don't make sense.

Let's apply some logic.

If wew agree for purposes of discussion
that the boat is not on plane at 9mph, (and it most likely would not be),
and
that we have only 500 rpm to "spend" to get the boat from 9 mph to 25,
observe
what has to happen:

We will need to spend at least 100-200 RPM (maybe more) to climb the bow
wave
and maintain a minimal plane. For purposes of discussion, let's say when
we
throttle up enough to climb the bow wave we achieve 14 mph. The unspent
300 RPM
are going to take this boat from 14 mph to 25? Almost doubling the speed?
No,
they are not.

Chuck,
I apologize for confusing you. You're absolutely correct.

Numbers should read:
1500 rpm (approx. 6mph): 6.2mpg
2000 rpm (approx. 8mph): 4.5mpg
2500 rpm (approx. 9mph): 3.5mpg
3000 rpm (approx. 16mph): 6.5mpg
3500 rpm (approx. 25mph): 6.2mpg

From the graph, it appears that the boat comes on plane at a little bit over
2500rpm. The 500 rpm increase from 1500 to 2000 rpm gives a speed increase
of only 2mph. The 500rpm increase from 2000 to 2500 gives only a 1mph speed
increase. The 500rpm increase from 2500 to 3000 rpm gives a speed increase
of 7mph. The 500 rpm increase from 3000 to 3500 gives a speed increase of 9
mph.

As I was stating earlier, the worst efficiency is at the point right before
the boat jumps on plane...and then there's a very rapid rise in the boat
speed per 100 rpm increase after that.

As Wayne B. said earlier, the best mileage would come at 1000 rpm or less
(but the boat would be going less than 5 mph). The second best mileage
comes at an engine speed on 3000rpm...and a boat speed of 16 mph.