On Mar 14, 6:30 pm, wrote:
"Wm Watt" wrote:
For "load" read "displacement"

Herein lies the problem. There's more to "load" than "displacement"
and some of it is unpredictable. For instance, how much wind will you
ever motor against? -shrug- Who knows?

Very true but wind is what makes sails work. (The questioner wanted an
outboard for when there was no wind.) Auxilliary power on sailboats is
almost exclusively for windless conditions and restricted waters
(approaching and leaving moorings, narrow channels, canals, lift
bridges, etc.). Wind and waves aren't a good reason for sizing the
auxilliary.

And how much current will you
ever motor against? -shrug again-

Yes, but useful power is limited by the boat's speed to length ratio,
where speed is relative to current. Trying to make headway against a
current which is about equal to "hull speed" is futile. Thankfully few
currents are quite so fast. There is only so much power you can
reasonably apply to a displacement hull. Any more and you're just
heating the atmosphere and wasting fuel.

Another part is, no matter how much horsepower you
generate with an engine, if you don't transfer it effectively to the
water with a proper prop, the hp is of no use. If I'm reading you
correctly though, your theory is for calculating only hp and leaves
other "pushing parts" to be calculated by some other theory. yes/no?

Yes, I've noted there has to be a conversion from nominal or rated
horsepower to effective horsepower.


Whatever the case, my theory is.... you can never have to much power.
You can have to much weight though, which is why I wish someone would
market a 2 cycle high thrust outboard.

Too much weight and fuel consumption, sort of an SUV of a sailboat.
Interesting to note that heavier engines increase displacement and add
somewhat to power requirement.