On Mar 13, 1:09 pm, Charlie Morgan wrote:
On 13 Mar 2007 09:04:11 -0700, "Wm Watt" wrote:

Given the walterline length and displacment of the boat you can
calculate the horsepower required to go at any speed.

...

Not really. You are leaving out a whole lot of variables in your
theory.

The other variables important when designing the hull, but not when
computing the power requirement. Assume it's a well-designed monohull,
then all you need is the displacment to calculate the amount of sail
to put on the boat, and therefore I claim the size of outboard engine.
At 17-21 kt windspeed each sq ft of sail produces 0.04 hp (beam
reaching). I'd multiply the area of the sails on the Thunderbird by
0.04 to get the hp requirement. As I mentioned I don't know how to
convert that effective hp to the nominal hp of the outboard
manufactuer's rating.

I was wrong to suggest the waterline length is needed to compute the
pwer requirement. Only the displacement is needed. There square root
of the sail area divided by the cube root of the dispalcement should
be just above 1.0. For dingy's it's 1.3. The ratio is called the
Bruce number.

Rather surprizingly, wind tunnel tests show there is not much
difference in the type of sail on a boat when beam reaching. It's in
upwind saiing that some outperform. So the above formula works quite
well for all sail plans. And that's why I suggested using the hp
rating for beam reaching when sizing the outboard motor.


That's the speed in still water in a dead clam. Wind, tides, waves,
and current will effect the actual speed. You might need more speed to
overcome local tides but a more powerful engine won't make much
difference because it's the waterline length and displacement which
are the biggest influences on boat speed.

Incorrect. A more powerful engine will allow you to keep moving in
adverse conditions where a minimal engine will not. That's why you'll
find that tugboats (displacement hulls) are pretty much ALL engine
below decks. It doesn't make them go faster. The high thrust outboards
under discussion produce a lot of thrust by use of a very large, slow
turning propeller. Standard outboards use a comparatively tiny prop at
High RPM.

I doubt tugboats have huge engines to overcome adverse conditions,
rather to be able to pull and push heavy loads. For "load" read
"displacement". Take away the load and the tug's top speed is
determined by wetted surface and waterline length, just like every
other displacement hull. Because the power requirement quickly rises
to infinity additional engine power is useless. At a speed to length
ratio of about 1.5 (can't remember exactly) any hull is plaining. Ever
see a tugboat plane?

Because I don't know how to convert nominal to effective horsepower
for boat engines I bow to your superior knowledge of rpm's, propellor
size, pitch, etc.