Charlie Morgan wrote:
The sailboat motor (high thrust) is a "T" series motor.

PhantMan wrote:
This is gettin' a little too confusing for moi.

One last thing.....
Charlie's right in that the "T"series 9.9 has a 25" shaft. The
"FT9.9DEL" comes with a 22.3" shaft.
For a sailboat, longer is better.

Rick

Given the walterline length and displacment of the boat you can
calculate the horsepower required to go at any speed. The waterline
length and displacement can be found by taking measurements off the
hull, or they might be avaiable for the Thunderbird somewhere. I've
only done it for sail power, not outboard motor power. I don't know
the difference between rated engine horsepower and thrust or whatever
number measures the actual power applied to the hull. The point is you
can calculate to find the best outboard for a boat.

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.

Of two engines using the same engine block, the one which produces the
required horsepower at the lowest rpm's will last longer and burn less
fuel. On second hand engines the other main factor is the cylinder
compression which tells how worn the engine is. Measureing the
compression is cheap and easy to do, but find out if it has a bypass
to make pulling the starter rope easier. Lawn mower engines have this
which makes it impossible to measure the cyliner compression.

There is a well-known exponential relationship between power and speed
on a dispalcement hull. The point of maximum curvature where the power
requirement takes off is the "hull speed", equal in knots to 1.34
times the square root fo the walterline length. Commercial operators
run their boats at about 60% of hull speed for best fuel economy.

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.

On Mon, 12 Mar 07, Matt Colie wrote:
I push a 4800# centerboard sloop with the board halfway down
a 6.0k at just over half throttle at about 1/2g/hr with a Honda 9.9. WOT
gets me to 6.2 and empties the tank way faster. A friend bought the new
TH and you can tell the difference just as he cranks up, but his cruise
is at a higher engine speed and his fuel rate is not as good as mine and
on flat water I can be at cruise at the same time.

On Mon, 12 Mar 07, PhantMan wrote:
Just looking at your friend's Honda HT gearcase, can you tell a
difference between it and yours? I'm wondering if the distance between
the prop shaft and the cavitation plate is greater on his HT.... or if
Honda just sticks a larger diameter prop and lower ratio gears on the
same case.

ooookie doke. Getting no response, I finally broke down and called the
dealer to ask my question. He tells me, not only are the lower units
the same, but Honda doesn't even put lower ratio gears it's high
thrust model. The only difference is the prop diameter (9.25" vs 10")
and the decal on the cowling. Who'd a thunk it.

Rick

PhantMan wrote:
not only are the lower units
the same, but Honda doesn't even put lower ratio gears it's high
thrust model. The only difference is the prop diameter (9.25" vs 10")
and the decal on the cowling. Who'd a thunk it.

Charlie Morgan wrote:
Yamaha models:
F9.9 (regular thrust) gear ratio: 2.08 prop size and pitch: 9.25x10.5
T9.9 (High Thrust) gear ratio: 2.92 prop size and pitch: 11.75x11
That's a pretty substantial diofference, wouldn't you say?

I would say, uh huh.
The reason I asked about Honda is because I have the 20hp model and
wondered if I have room to swing a 10" prop. Apparently I do. Might
come in handy someday although chances are I'll never use it for
anything but scootin' my skiffs around. Standard 9.25" prop works fine
for that.

Back to Yamaha: I was thinking the 8hp model would be better for a
smallish sailboat, say a 22'-25', but now I see it's only 6lbs lighter
than a 9.9. Not enough to matter imo. I think I mentioned in some
other post that I wish somebody would produce a 2 cycle high thrust
25" shaft engine (I like "light" even more than I like "power").

I once had a 6hp 25" Evinrude on a Catalina 22 that was light (60lbs)
but not high thrust. It worked ok in most situations but a large dia,
low pitch, four wide bladed prop sure would've been nice.

Rick

Rick,
You Missed a line Look below:

lid wrote:
On Mon, 12 Mar 07, Matt Colie wrote:
I push a 4800# centerboard sloop with the board halfway down
a 6.0k at just over half throttle at about 1/2g/hr with a Honda 9.9. WOT
gets me to 6.2 and empties the tank way faster. A friend bought the new
TH and you can tell the difference just as he cranks up, but his cruise
is at a higher engine speed and his fuel rate is not as good as mine and
on flat water I can be at cruise at the same time.

On Mon, 12 Mar 07, PhantMan wrote:
Just looking at your friend's Honda HT gearcase, can you tell a
difference between it and yours? I'm wondering if the distance between
the prop shaft and the cavitation plate is greater on his HT.... or if
Honda just sticks a larger diameter prop and lower ratio gears on the
same case.

ooookie doke. Getting no response, I finally broke down and called the
dealer to ask my question. He tells me, not only are the lower units
the same, but Honda doesn't even put lower ratio gears it's high
thrust model. The only difference is the prop diameter (9.25" vs 10")
and the decal on the cowling. Who'd a thunk it.

Rick
Standard BF model 9.25*8
High Thrust Model 10*6.5

The older TH Hondas had a different gear ratio, I guess they have
decided it wasn't worth their while.

Matt Colie wrote:
Rick,
You Missed a line Look below:

You mean:
"Yamaha apparently does this with gears, Honda does it with
a larger D and smaller pitch on the prop with the same gears
(this year was different two years ago)" ?

Yep, I read it but I guess it just didn't register somehow. Maybe I
still had the old specs in my head... including the XXlong shafts.

The older TH Hondas had a different gear ratio, I guess they have
decided it wasn't worth their while.

Apparently. Who knows what goes into a marketing decision like that.

Rick

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.