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.

"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

On Sun, 26 Feb 2006 20:43:43 GMT, "Injam"
wrote:

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

Given your conditions of identical boats, props and gear ratios, that
is correct. The only other variable is "slip" and that should be
about the same unless the bigger engine is significantly heavier.

If you want to go faster with the larger engine you will either need
to run at higher RPMs or switch to a prop with more pitch, more blade
cuppage, or both.

There is an excellent book available on propellers if you want to
learn mo

The Propeller Handbook: The Complete Reference for Choosing,
Installing, and Understanding Boat Propellers by Dave Gerr.

Available at www.amazon.com

"Wayne.B" wrote in message
...

There is an excellent book available on propellers if you want to
learn mo

The Propeller Handbook: The Complete Reference for Choosing,
Installing, and Understanding Boat Propellers by Dave Gerr.

Available at www.amazon.com



Aw,,, Wayne had to spoil it. We were having fun confusing the banannas out
of him and ourselves.

RCE