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.

"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? And how much current will you
ever motor against? -shrug again- Both those factors need to be added
to "load" but all you can do is guess at what might be the worst case
scenario and plan for that. I think that may be part of the point CWM
is trying to make. 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?

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.

Rick

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.

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.

On Sat, 17 Mar 07, sherwindu wrote:
I can't lift the engine into the
cockpit like I used to do with my Silver Century Seagull

Tell me about it. Or it's nice to set it in cabin to lock it up. Plus
the tendency to list, and/or squat the stern, and lift the bow,
instead of adding stability low and amidships where weight needs to be
(I've almost convinced myself to go inboard next time.... but not
quite).

I think Honda putting a
larger prop on it's 'high-thrust', high gear ratio engine is a poor
compromise.

For you and me maybe. But for them, it's a profit maximizing decision
that prolly works pretty well on their bottom line. But at least
they're honest about what it'll do. They're claiming only a 15%
increase in thrust. Sounds about right to me.

The only thing I don't like about
my T9.9 are the tiny jets in the carburetor, which are continually blocking
up.

Honda has the same problem. I spent 30 minutes yesterday cranking and
cussin' at mine. Never did get it started. So it's off to the shop
with it (again). I no longer have the patience. Hopefully I'll get it
back in a few weeks (as opposed to months like last time). Meanwhile
I'll putter along with my ol' ancient standby Evinrude. It ALways
starts, no prob.

I am
actively working on solutions for that

If you come up with one, please post it here.

Rick

Actually, repairing a blocked carb on the Yamaha is tiresome, but not that
difficult.
After dissassembly, just run a very fine wire through the jets to open them up.

A more pro-active way would be to buy only premium gasoline. Make sure your
fuel system is clean, hoses, tanks, etc. Use a stabelizer like Sea Foam or
Stabil in
the gas tank. Put a fine filter (about 10 micron) in line with your fuel
delivery system.
I am going to try all the above next season. Now that I hear that Honda's have
the
same problem, I regret even less my decision to go with Yamaha. Honda is
obviously catering to the power boat community.

Sherwin D.

lid wrote:

On Sat, 17 Mar 07, sherwindu wrote:
I can't lift the engine into the
cockpit like I used to do with my Silver Century Seagull

Tell me about it. Or it's nice to set it in cabin to lock it up. Plus
the tendency to list, and/or squat the stern, and lift the bow,
instead of adding stability low and amidships where weight needs to be
(I've almost convinced myself to go inboard next time.... but not
quite).

I think Honda putting a
larger prop on it's 'high-thrust', high gear ratio engine is a poor
compromise.

For you and me maybe. But for them, it's a profit maximizing decision
that prolly works pretty well on their bottom line. But at least
they're honest about what it'll do. They're claiming only a 15%
increase in thrust. Sounds about right to me.

The only thing I don't like about
my T9.9 are the tiny jets in the carburetor, which are continually blocking
up.

Honda has the same problem. I spent 30 minutes yesterday cranking and
cussin' at mine. Never did get it started. So it's off to the shop
with it (again). I no longer have the patience. Hopefully I'll get it
back in a few weeks (as opposed to months like last time). Meanwhile
I'll putter along with my ol' ancient standby Evinrude. It ALways
starts, no prob.

I am
actively working on solutions for that

If you come up with one, please post it here.

Rick