Hello,

I'm trying to understand propellers.

If you have two identical boats, but the outboard horsepower ratings are
different. One boat has a 200 hp and the other a 250 hp. Both motors have
the same Gear Ratio. If the propellers are identical and the RPM of the
outboard is the same on each boat. Say 6000 RPM. Because of the weight
advantage will the 200-hp boat be faster than the 250 hp? I'm thinking less
weight less slippage otherwise they would be going the same speed.



Sincerely,

Injam

Why? If the propellers are turning at the same RPM and they have the same
pitch, why would the 250 hp be faster?


"Harry Krause" wrote in message
. ..
Injam wrote:
Hello,

I'm trying to understand propellers.

If you have two identical boats, but the outboard horsepower ratings are
different. One boat has a 200 hp and the other a 250 hp. Both motors have
the same Gear Ratio. If the propellers are identical and the RPM of the
outboard is the same on each boat. Say 6000 RPM. Because of the weight
advantage will the 200-hp boat be faster than the 250 hp? I'm thinking
less weight less slippage otherwise they would be going the same speed.



Sincerely,

Injam




If the 200 hp engine is properly propped, the 250 hp engine will most
likely be underpropped with the same prop. The weight diff between a 200
hp and a 250 hp is not usually significant, certainly not enough t0 canel
out a 25% increase in hp.

Now, to your question...the 250 still might be faster, even if it is
underpropped.

Q. What's a propeller?
A. A machine for propelling an aircraft or boat, consisting of a
power-driven shaft with radiating blades that are placed so as to thrust air
or water in a desired direction when spinning.

Now why would the 250 hp be faster? It would seen to me that in order to go
faster the propeller would have turn faster, but that are not.


"Shortwave Sportfishing" wrote in message
...
On Sun, 26 Feb 2006 19:40:19 GMT, "Injam"
wrote:

Hello,

I'm trying to understand propellers.

What's a propeller?

If you have two identical boats, but the outboard horsepower ratings are
different. One boat has a 200 hp and the other a 250 hp. Both motors have
the same Gear Ratio. If the propellers are identical and the RPM of the
outboard is the same on each boat. Say 6000 RPM. Because of the weight
advantage will the 200-hp boat be faster than the 250 hp? I'm thinking
less
weight less slippage otherwise they would be going the same speed.

Identical boats, identical props, identical RPMs, it going to be the
more horsepower that wins the day.

It would seem to me that in order to go faster the propeller would have to
spin faster.

"Shortwave Sportfishing" wrote in message
...
On Sun, 26 Feb 2006 14:47:57 -0500, Harry Krause
wrote:

Injam wrote:
Hello,

I'm trying to understand propellers.

If you have two identical boats, but the outboard horsepower ratings
are
different. One boat has a 200 hp and the other a 250 hp. Both motors
have
the same Gear Ratio. If the propellers are identical and the RPM of the
outboard is the same on each boat. Say 6000 RPM. Because of the weight
advantage will the 200-hp boat be faster than the 250 hp? I'm thinking
less
weight less slippage otherwise they would be going the same speed.

If the 200 hp engine is properly propped, the 250 hp engine will most
likely be underpropped with the same prop.

Not necessarily - they could have exactly the same prop with no
noticeable decrease in efficiency. If I remember correctly, there are
250 Merc Optimax's at the marina with the same props I have on my
E-TECS.

The weight diff between a 200 hp and a 250 hp is not usually significant,
certainly not enough to canel out a 25% increase in hp.

Now, to your question...the 250 still might be faster, even if it is
underpropped.

Usually, the power band is towards the upper end of the RPM band, thus
more horse power, more speed.

Then again, I could be wrong - just seems to make sense to me.

Think of it this way.

If I'm screwing a wood screw into a piece of wood and my friend Charley is
doing the same thing. The wood is the same and the pitch on the screws are
the same. Screws are identical, but Charley is a lot stronger than I am. We
turn the screws at the same RPM. Will Charley screw his screw in before me?

"Injam" wrote in message
ink.net...
It would seem to me that in order to go faster the propeller would have to
spin faster.

"Shortwave Sportfishing" wrote in message
...
On Sun, 26 Feb 2006 14:47:57 -0500, Harry Krause
wrote:

Injam wrote:
Hello,

I'm trying to understand propellers.

If you have two identical boats, but the outboard horsepower ratings
are
different. One boat has a 200 hp and the other a 250 hp. Both motors
have
the same Gear Ratio. If the propellers are identical and the RPM of the
outboard is the same on each boat. Say 6000 RPM. Because of the weight
advantage will the 200-hp boat be faster than the 250 hp? I'm thinking
less
weight less slippage otherwise they would be going the same speed.

If the 200 hp engine is properly propped, the 250 hp engine will most
likely be underpropped with the same prop.

Not necessarily - they could have exactly the same prop with no
noticeable decrease in efficiency. If I remember correctly, there are
250 Merc Optimax's at the marina with the same props I have on my
E-TECS.

The weight diff between a 200 hp and a 250 hp is not usually significant,
certainly not enough to canel out a 25% increase in hp.

Now, to your question...the 250 still might be faster, even if it is
underpropped.

Usually, the power band is towards the upper end of the RPM band, thus
more horse power, more speed.

Then again, I could be wrong - just seems to make sense to me.

"Shortwave Sportfishing" wrote in message
...

Identical boats, identical props, identical RPMs, it going to be the
more horsepower that wins the day.


He's asked a good question. Assuming equal prop slippage, (or for the sake
of exercise, assume none), I think both engines would theoretically run the
boats at the same speed at a given RPM. The fact that the 250 hp engine
being a bit heavier might contribute to a bit more slippage in the real
world and may, indeed, be slower than the 200 hp.

Not to be confusing here... what I am saying is that if you treat the
engine and prop as a lossless, slipless gear train, then both engines should
produce the same speed on the same boat with the same props.

I think.

RCE

"Shortwave Sportfishing" wrote in message
...
On Sun, 26 Feb 2006 14:47:57 -0500, Harry Krause
wrote:

Injam wrote:
Hello,

I'm trying to understand propellers.

If you have two identical boats, but the outboard horsepower ratings
are
different. One boat has a 200 hp and the other a 250 hp. Both motors
have
the same Gear Ratio. If the propellers are identical and the RPM of the
outboard is the same on each boat. Say 6000 RPM. Because of the weight
advantage will the 200-hp boat be faster than the 250 hp? I'm thinking
less
weight less slippage otherwise they would be going the same speed.

If the 200 hp engine is properly propped, the 250 hp engine will most
likely be underpropped with the same prop.

Not necessarily - they could have exactly the same prop with no
noticeable decrease in efficiency. If I remember correctly, there are
250 Merc Optimax's at the marina with the same props I have on my
E-TECS.

The weight diff between a 200 hp and a 250 hp is not usually significant,
certainly not enough to canel out a 25% increase in hp.

Now, to your question...the 250 still might be faster, even if it is
underpropped.

Usually, the power band is towards the upper end of the RPM band, thus
more horse power, more speed.

Then again, I could be wrong - just seems to make sense to me.

I hear what he's saying .... he's treating it like a fixed gear train and
both engines produce enough power to turn the prop at 6000 RPM. The speed
would likely be the same, with a possibly a slight edge to the smaller
engine.

I agree with Harry's point. It might be possible to increase the pitch of
the prop on the larger engine to take advantage of the extra 50 horses.
That boat would now be faster at 6000 RPM due to a lower gear ratio.

RCE

"Injam" wrote in message
ink.net...

It would seem to me that in order to go faster the propeller would have to
spin faster.


Or the same speed with a higher pitched prop. In your example, 6000 RPM is
about max for most outboards I know of. If the engine has enough extra
power to swing a higher pitched prop at 6000 RPM, it will be faster, for
sure.

RCE

wrote in message
oups.com...

Injam wrote:
Think of it this way.

If I'm screwing a wood screw into a piece of wood and my friend Charley
is
doing the same thing. The wood is the same and the pitch on the screws
are
the same. Screws are identical, but Charley is a lot stronger than I am.
We
turn the screws at the same RPM. Will Charley screw his screw in before
me?


Yes, because Charley is able to apply greater downward pressure on the
screw as he turns it and thereby increase the efficiency, even if the
torque and speed is identical. But that doesn't apply to your boat
example.


I'd have to chew on that one for a while, Chuck. There's not much efficiency
loss (i.e. slippage) when turning a screw into wood. Charley just won't be
as tired at the end of the day.



The higher HP engine could produce more torque, but the amount of work
being done to turn the prop 6000 rpm is not a variable in your example.
If the prop is turning 6000 rpm and all the other factors are the same
the two boats would travel the same distance per rotation of the prop.


Yep.

RCE

So you are saying that the heavier motor would not slip as much as the
lighter one?


wrote in message
oups.com...

Injam wrote:
Think of it this way.

If I'm screwing a wood screw into a piece of wood and my friend Charley
is
doing the same thing. The wood is the same and the pitch on the screws
are
the same. Screws are identical, but Charley is a lot stronger than I am.
We
turn the screws at the same RPM. Will Charley screw his screw in before
me?


Yes, because Charley is able to apply greater downward pressure on the
screw as he turns it and thereby increase the efficiency, even if the
torque and speed is identical. But that doesn't apply to your boat
example.

The higher HP engine could produce more torque, but the amount of work
being done to turn the prop 6000 rpm is not a variable in your example.
If the prop is turning 6000 rpm and all the other factors are the same
the two boats would travel the same distance per rotation of the prop.