"Gene Kearns" wrote in message
...
On 4 Oct 2003 07:21:34 -0700, (roy) wrote:

Am looking at a new boat approx 20-21 ft in length with 15 degree
deadrise, sort of a flats type hull shape for shallow water. The
manufacturer rates the transom for up to 200 HP, the dealer claims
that 150 is plenty and that anything larger is a "waste of money" in
both initital cost and operating costs. He also claims a speed
increase at WOT of only 2-3 MPH.

I have looked over the Yamaha website for performance on specific
power packages but can't seem to find the same hull with two
different
HP performance tests.

My previous experience indicated that by having maximum rated HP on
the transom that one could turn a larger prop, say a 21 inch pitch
while the less powerful engine would force the prop pitch down to a
19
or less.

I realize that at WOT a 200 HP will burn more fuel but it is my
intent
to operate for the most part in the 30-35 MPH range on a hull that
will offer in excess of 50 MPH. Will the reduction in RPM on the
larger motor to achieve a given cruising speed provide adequate
additional fuel economy and performance increase to justify the
increased initial expense?

50 HP isn't going to make a difference of a mere 2-3 mph. Bear in
mind that not only will the HP change, but so will the pitch of the
screw. You will most likely be turning fewer RPMs at your desired
cruise with the larger engine due to this.... and, again, burning less
fuel. If you really want to burn max fuel and go flat out... you have
the option....

You are in the "driver's" seat.... if the dealer wants to sell the
boat, he must let you sea trial both models. My guess, extrapolating
from your description, is that he has a smaller engine he doesn't want
to get stuck with through the winter.

Be from Missouri...... say, "Show me....."

The rule of thumb is that max speed is proportional to the square root
of hp. If you increase the HP by a factor of 1.333, you increase the
top speed by about 14 %. If the top speed is about 50, that is 7 mph.

But if you are only going to go 35 because of waves or other factors,
then that should be taken into consideration. The bigger motor won't
burn less gas at the same speed fer sure. Maybe not much more, but not
less.

del cecchi

"del cecchi" wrote in message news:NTKfb.139

The bigger motor won't burn less gas at the same speed fer sure.
Maybe not much more, but not less.


Bzzzzzzt.

Anyone who reads this NG can tell you that I had FOUR different motors on
the back of my (dearly departed) Four Winns last year.

76ci 80hp - 89ci 100hp - 99ci 115hp - 99ci130hp (crankshaft ratings to keep
this simple)
All tested with a 14, 15, and 17 pitch stainless props.

I can tell you *unequivicably* that in the case of 2-stoke outboards, output
is governed by two factors - spark advance and throttle openings.

If you have to run a 100hp with spark at max advance and the throttle body
3/4 open to achieve a speed of 25 mph but you can take a 130 and do the same
thing at max advance with the throttle body just cracked a bit - guess which
motor will deliver better mileage??

And that's not even accounting for the propping up you can do with more HP.

I'm sorry, but that sentance could not be *more* wrong.

If I ever wanna help this world deplete it's gas supply - I'll start by
putting the 80hp (I kept it) on the PM-II and pulling tubers all day - OK?


-W

"Clams Canino" wrote in message
. net...
"del cecchi" wrote in message news:NTKfb.139

The bigger motor won't burn less gas at the same speed fer sure.
Maybe not much more, but not less.


Bzzzzzzt.

Anyone who reads this NG can tell you that I had FOUR different motors
on
the back of my (dearly departed) Four Winns last year.

76ci 80hp - 89ci 100hp - 99ci 115hp - 99ci130hp (crankshaft ratings
to keep
this simple)
All tested with a 14, 15, and 17 pitch stainless props.

I can tell you *unequivicably* that in the case of 2-stoke outboards,
output
is governed by two factors - spark advance and throttle openings.

If you have to run a 100hp with spark at max advance and the throttle
body
3/4 open to achieve a speed of 25 mph but you can take a 130 and do
the same
thing at max advance with the throttle body just cracked a bit - guess
which
motor will deliver better mileage??

And that's not even accounting for the propping up you can do with
more HP.

I'm sorry, but that sentance could not be *more* wrong.

If I ever wanna help this world deplete it's gas supply - I'll start
by
putting the 80hp (I kept it) on the PM-II and pulling tubers all day -
OK?


-W


My guess would have been the 100. That is the way it works for 4 stroke
motors in cars, and if one believes the fuel consumption curves in
magazine tests of outboards, it would appear to be the same for
outboards based on a little extrapolation.

However experience trumps extrapolation. It certainly seems true that
WOT blows the fuel economy.

del

Yes, once the carbs start to open, the closer you get to WOT - the more gas
it'll suck. What's more unclear to me is if the 115 and 130 (both 99ci) had
much of a difference. I only ran the 115 for a week or so. But it felt to
me like I needed less throttle to get the same speeds even then.

Another thing I could add - is that if you pull a lot of skiiers and toys.
Holeshot sucks gas. The shorter holeshot the better off you are. In that
respect, I'm *sure* I'm doing better with the higher output 99ci than the
old one. That's why I advise people to get the highest output version of
any one particular block. Since they weight the same anyway - might as well
get the most bank for the buck.

-W


"del cecchi" wrote in message news:rv2gb.198

However experience trumps extrapolation. It certainly seems true that
WOT blows the fuel economy.

del