Hi Larry, and merry xmas.<br><br>The hull speed
for your new launch is (theoretically) 1.34 times the
square root of the waterline length. But this is purely
a theoretical maximum (there's considerable debate
over whether it can ever be exceeded, and under what
conditions, but all of that discussion is irrelevant to your
boat). I assume the 23' measurement is overall length,
not waterline length, which is the number we really
need. Since she's a fantail, let's assume (for argument
and round numbers' sake) that her waterline length is
exatctly 20 feet. The square root of 20 is 4.47 near
enough. Multiplied by 1.34-that is 5.98 knots, or a
little better than 6 miles an hour. This means that with
a 10,000 horsepower nuclear powerplant on board,
your launch still can't go any faster than 6 knots. As
a practical matter, naval architects know that any
given hull shape will need to use a multiplier somewhat
below 1.34. In the case of a well-designed fantail, I
know for a fact that number would be around 1.2 due to
the efficiency of the hull. So your boat has a true
maximum hull speed of about 1.2 times 4.47, which is
about 5.36 knots. In the case of a round-bottom
displacement boat like a launch, this means there is no power
on earth that will move it through the water faster
than 5.36 knots.<br>What it also means is that you
need to perform all further hull/speed and horsepower
calculations using the 1.2 x 4.47 calculation as your base
number. Since not even a 900-horse jet engine will move
her faster than 5.36 knots, there is no sense in
using any value higher than 5.36 for hull speed. (To
some extent this whole discussion is faintly
ridiculous, since we're working in hundreths of a knot, which
is well beyond any measurable tolerance we'd be
working with in the real world, but out of habit we all
tend to work in two or three decimal places, as though
they meant something. Tenths would be more realistic.
But I always work to 2 decimal places out of
habit.)<br>You may need to know where and why the 1.2 is
important--it is the estimated coefficient of efficiency of
hulls shaped about like fantail launches, and is a
derivative number that comes from measurements and averages
of lots of boats. Yeah, you'll get a little argument
here and there; one naval architect will argue 1.18
and another will say 1.24 if she's really long and
flat...and yadda yadda yadda--this is where my footnote
about 2 decimal places comes in. The number 1.2 is
reasonable and close, and we're dealing with ballpark stuff
here anyway--nothing that can ever be measured and
calculated before. The only way to derive an actual number
is by actual testing--throw a 100-horse outboard on
her, go as fast as possible, measure the absolute
maximum flat-out speed, and work the calculation backward
(divide that speed by 4.47 to get your actual 1.2 (or
whatever) multiplier.<br>The "cruising speed" of your
launch is something else again. Everybody picks a
different multiplier for this, but it tends to go around 70
or 75 percent of maximum hull speed. This would be
the speed at which you and your boat can go
"comfortably" (whatever that is) before horsepower consumption
starts to rise on a graph. It is the speed you want to
cruise at without using maximum gas/electric/coal or
whatever. The number you pick is important, because it is
really the number you need to perform "ideal" or
"optimum" horsepower, shaft speed, power consumption, prop
diameter, etc., etc. calculations. In your case, I'd pick a
number around 4 knots as ideal cruising speed, and work
with that. I'd size the prop, figure shaft speed,
power usage, etc., all off of this base number--4
knots. Ya want 4.1? Fine. 3.9? Great. The presumption
is, this is the speed you'll be using the vast
majority of the time, so the engineering should conform to
it. There's no sense engineering your system to go
5.8 knots--you'll never see it happen, so why bother?
<br>Good luck. (Hey, post us all a picture.)
