"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