"." .@. wrote in message
...
On Tue, 26 Aug 2003 18:06:18 GMT, "jaybird" wrote:

I have a quick question about the equation of:

prop pitch (21) vs.
speed (55mph) vs.
max RPM at WOT (about 3200 but that might be a gauge problem) (on a 23
ft.
boat with a 7.4L I/O):

I'm thinking that 3200 RPM is too low for WOT on that engine. If I
change
to a 23 pitch prop I know that I'll get an increase in speed, but about
how
much should I expect, and will the RPM go up?

Thanks,

j.


Ok, first things first.

1) Fix the tach. Without an accurate tach you are wasting your time,
energy, and money.

That's the first thing on the list to be done... I think the needle may be
sticking.


2) Find out what redline on your engine is. 3200 RPM sounds too low,
but who knows?

3) A 23 pitch prop may or may not increase speed. If you are really
only turning 3200 RPM, I can guarantee that you will lose speed.

That's what I thought too, but a 7.4L engine should be turning faster than
that.


4) You aren't going to get something for nothing. You must trade RPM
for MPH and vice versa. Going from 21" to 23" will decrease your RPM,
which may or may not increase your MPH, depending on available excess
HP.

Ok, I think that was the answer I was looking for.


5) Expect best performance and speed at WOT where the RPM is in the
manufacturer's published maximum operating RPM range.

I don't know what your final drive ratio is, but 23" sounds likely to
be the wrong prop for your boat. Maybe not.

I'll just take it down to the dealer and see what they say then. I didn't
know if it was something I could do myself or not.

Thanks.

j.