Selecting HP for given hull (outboard)
 

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?

Selecting HP for given hull (outboard)
 

One issue to consider is the engine displacement of the two outboards you
are considering.

Many companies use the same block for many HP configurations. This means
that you might see a 150, 175, and 200 with the same block. In *this* case
both outboards weigh the same so the 200 would give you better performance
and perhaps slightly better gas mileage at throttle settings under WOT, but
prolly get slightly worse mileage up at the top-end.

When comparing outboards check the displacement and weight - those being
equal, I'd opt for the higher output version of the block.

-W




"roy" wrote in message
om...
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?

Selecting HP for given hull (outboard)
 

"Clams Canino" wrote in message
news:iXAfb.677583$Ho3.144064@sccrnsc03...
One issue to consider is the engine displacement of the two outboards you
are considering.

Many companies use the same block for many HP configurations. This means
that you might see a 150, 175, and 200 with the same block. In *this*
case
both outboards weigh the same so the 200 would give you better performance
and perhaps slightly better gas mileage at throttle settings under WOT,
but
prolly get slightly worse mileage up at the top-end.

When comparing outboards check the displacement and weight - those being
equal, I'd opt for the higher output version of the block.

-W


One more thing, I have never heard anyone complain of too much horse power!
Greg

Selecting HP for given hull (outboard)
 

Yanno - actually - I have. sigh

I knew some guy with a '87 115 on a little bass boat - said it was "way too
much motor" for the boat. LOL. Now, I'm using the same basic block on the
PM-II and I could think of a thousand things I wanna do to trick it out to
make about 175hp "when I have the time". LOL

-W



"Greg O" wrote in message
...

"Clams Canino" wrote in message
news:iXAfb.677583$Ho3.144064@sccrnsc03...
One issue to consider is the engine displacement of the two outboards
you
are considering.

Many companies use the same block for many HP configurations. This
means
that you might see a 150, 175, and 200 with the same block. In *this*
case
both outboards weigh the same so the 200 would give you better
performance
and perhaps slightly better gas mileage at throttle settings under WOT,
but
prolly get slightly worse mileage up at the top-end.

When comparing outboards check the displacement and weight - those being
equal, I'd opt for the higher output version of the block.

-W


One more thing, I have never heard anyone complain of too much horse
power!
Greg

Selecting HP for given hull (outboard)
 

Old proverb: "Where there is loss there exists gain, Where there is gain
there exists loss" I think it's merely a question of hair splitting. Opt.
for the Max. HP. once on plane, back the rpm's off and just cruise. Max HP
might be an edge in the resale side of the equation.
"roy" wrote in message
om...
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?

Selecting HP for given hull (outboard)
 

"roy" wrote in message
om...
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?

Not on the lightweight hull you are describing. What's the difference in
weight between the two engines, not if you are planning to run the motor
in the 30s...

I'd guess the difference in top end would be between 3 and 5 mph, a
little more than your dealer sez.


--
* * *
email sent to will *never* get to me.

Selecting HP for given hull (outboard)
 

"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

Selecting HP for given hull (outboard)
 

"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

Selecting HP for given hull (outboard)
 

You did fine Gene. He was comparing apples and hammers.

-W

"Gene Kearns" wrote in message

Perhaps I did a poor job of making my point. Your rule of thumb works
when one compares apples and apples....

Selecting HP for given hull (outboard)
 

"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

Selecting HP for given hull (outboard)
 

So you disagree with the rule of thumb that speed varies as the square
root of HP? I got the formula out of a Mercury High Performance Prop
book a few years ago. :-) How much do you think the speed will
increase with an extra 33% or 50 HP? I know you think a 200 will use
less gas putting out 100 hp than a 150 will. I'm open minded on that
question.

del

"Clams Canino" wrote in message
news:H9Yfb.45272$%h1.29399@sccrnsc02...
You did fine Gene. He was comparing apples and hammers.

-W

"Gene Kearns" wrote in message

Perhaps I did a poor job of making my point. Your rule of thumb
works
when one compares apples and apples....

Selecting HP for given hull (outboard)
 

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

Selecting HP for given hull (outboard)
 

I'm sorry - I wasn't dissagreeing with the speed part - just the fuel part.
But again I stress that getting the highest output version of *any
particular block* is "free hp" since the weight is the same. If you have to
get a heavier motor to go up - then you might be wasting time.

That's why I run the 99ci inline 6 as opposed ot the V-6's. My outboard
even with power trim is well under 350 pounds. If I want more HP, I'll soup
up my 99ci as opposed to going V. Those things were rigged to put out 90,
115, 130ish, 140, 150, and 160 at the crank. (with race stuff getting to
200hp) Anyone that bought that 90hp was just silly. LOL.

-W



"del cecchi" wrote in message
...
So you disagree with the rule of thumb that speed varies as the square
root of HP? I got the formula out of a Mercury High Performance Prop
book a few years ago. :-) How much do you think the speed will
increase with an extra 33% or 50 HP? I know you think a 200 will use
less gas putting out 100 hp than a 150 will. I'm open minded on that
question.

del

"Clams Canino" wrote in message
news:H9Yfb.45272$%h1.29399@sccrnsc02...
You did fine Gene. He was comparing apples and hammers.

-W

"Gene Kearns" wrote in message

Perhaps I did a poor job of making my point. Your rule of thumb
works
when one compares apples and apples....

Selecting HP for given hull (outboard)
 

On Sun, 05 Oct 2003 20:51:16 -0400, Gene Kearns
wrote:
I'm a bit mystified as to why I went from 150HP and a 13.75X17 prop to
140HP and a 13.75X19 prop and still run the same speed at top end!!
Not that this isn't quantifiable.... but I sure lack all of the
data...

==========================
Let's do the numbers:

SQRT 150 = 12.247

SQRT 140 = 11.832

The difference equals 0.415 which when divided by 11.832 equals about
3%. For a WOT of 30 knots, this means that you should have
theoretically gotten about 1 knot extra at the high end if everything
else matched up perfectly. The implications are that you need to
double the horsepower to go 40% faster, and that assumes no increase
in weight or drag which hardly ever happens. (SQRT 2 = 1.414)