My rpm on a 5.8L 14.5x19 prop is about 4600 to 4800 right in range with the
spec's in the manual, I get about 32 mph wot. my manual says a 7.5 liter
ford should get 4400-4800 rpm at wot. I would check your rpm gauge with a
reliable gauge. a 7.5 should idle @ 700-800. That may tell if your gauge is
anywhere close @ low rpms. every inch of pitch increase is a decrease of 200
rpm & vice versa, putting a 23 pitch prop on will decrease your rpm 200 rpm,
unless it cavitates or ventilates, than the rpms will go up and the speed
will go down, I don't know what the theoretical speed increase would be. btw
I have a 26' cruiser @ 5200 lbs. dry, alittle under powered I know. but
gets 4 adults up easy. You may need a tuneup or a new gauge 3200 sounds real
low.


"jaybird" wrote in message
...
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.