Wm Watt wrote:

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.

I think you should add conditions where tacking is impractical. I have had
to do my share of motor sailing into headwinds, and there, the sails were
used
more for stability than forward thrust.

Also, I agree entirely with Charlie Morgan about the effectivitiy of high
thrust
engines for sailboats. When shopping for a new engine for my 2 ton sailboat,

I originally bought a Honda. When I discovered it had a high gear ratio, I
returned
it and bought the High Thrust T9.9 Yamaha. It is a heavy engine, but I
installed an
assisted motor mount, which helps a lot. However, I can't lift the engine
into the
cockpit like I used to do with my Silver Century Seagull, another great
sailboat
engine with an even higher gear ratio than the Yamaha. I think Honda putting
a
larger prop on it's 'high-thrust', high gear ratio engine is a poor
compromise. It will
still churn up a lot of water, with reduced thrust. I prefer having an
engine with extra
reserve power. I have had too many occurences where my boat could not make
headway into very strong winds and/or currents. The only thing I don't like
about
my T9.9 are the tiny jets in the carburetor, which are continually blocking
up. I am
actively working on solutions for that, such as fine 10 micron inline filters
and changing
out my fuel system with newer hoses. Could also be the gasohol we get now,
despite
doctering it up with stabilizers. Yamaha has not come out with a good fix
yet, so I
hope my measures will get rid of the problem.

Sherwin D.



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.

Guess you have never had to negotiate one of the many 'tidal cuts' in
Florida and the Bahamas, or ever tried to go upstream against a strong
current on the Mississippi River ( I have).

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.

Not if your boat is going slower than it's hull speed.



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.