On Tue, 23 Nov 2004 07:51:27 -0500, Harry Krause
wrote:
With a 225 four stroke, my Parker with a half load of fuel, two guys
aboard, and some "stuff" will hit between 39 and 40 mph at WOT,
according to my GPS. And that number is what's on spreadsheet Parker
sent me a year or so ago.

On the Yamaha "performance" site, the same boat with a 300 hp two stroke
DFI will hit 44.1 mph under similar load conditions...a 33% increase in
horsies only produces about 5 mph in top end. There are, however, more
worthwhile differences in the rpm/cruise speed curge.

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OK, let's do the math. An increase from 225 to 300 hp is a ratio of
1.33 (300 / 225). Taking the square root of 1.33, we get 1.15, and
39 mph times 1.15 is 45 mph. That agrees fairly well with the notion
that speed increases in proportion to the SQRT of hp at planing
speeds.

Theoretical hull speed for a Parker 25 is about 6 mph which you could
probably achieve with a 15 hp outboard in calm conditions. To go from
6 mph to 12 mph, double the speed in non-planing conditions, takes a
lot more power, perhaps 8 times as much if power required is varying
at the cube root of speed. That would imply 120 hp which sounds about
right, perhaps half throttle on your 225.

At some speed between 12 mph and 20 mph, probably around 18 or 19,
the boat should begin planing and the power increase required for more
speed should drop back more towards the first set of numbers. That's
the basis for my conjecture that a mathematical definition might have
some merit. The problem of course is measuring the transition without
a whole boatload of instrumentation. :-)

For another data point, I have a 12 foot inflatable which will plane
off at 13 or 14 mph when lightly loaded, using about half throttle on
a 15 hp Merc.