"Steven Shelikoff" wrote in message

So I completely disagree with your assertion that all the
engine/prop should
have to do is just reach hull speed when loaded to it's maximum RPM.
IMHO, that is an unsafe setup because you have no margin for bad
weather.

That's because you don't fully understand it after a very brief post
on a complex subject. I was not talking about the selection of total
horsepower for the vessel which is a different parameter. The
horsepower / speed curve is going pretty close to straight up at hull
speed so the difference between an engine with just enough power to
get to hull speed and one with power margin will not change the prop /
hull relationships that much.

99.98% of sailboats and most powerboats are going to have what is know
as a "free running" wheel which is optimized for the upper speed
range. There is a limit to how much power the prop can produce which
is dependent on the area of the blades. If this is exceeded, the
pressure differential that drives the boat becomes high enough that
the pressure on the front of the blades will be low enough to boil the
water at ambient temperature. The resulting bubbles drive the
efficiency of the blades way down, erode the metal when they collapse,
and causes vibration. Power increase is marginal and can even go
down.

Blade pressures also increase as the speed the prop is moving through
the water goes down as with an external drag such as a headwind or a
tow. This means that the prop has to get bigger. Just look at the
prop on a towboat and a regular powerboat of the same horsepower.
When designing a towing wheel you input a very low figure like 4 knots
into the propeller calculation instead of the vessel's potential
speed. The result is a very large prop and higher fuel consumption
when running free. It would also be a lot of drag for a sailboat.

--

Roger Long