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.

"." .@. 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.

"WildestDream" wrote in message
...
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.

I'll double check the book, but it seems to me like your range is about what
mine should be too. It just sounds like I need to head to the dealer and
get the gauge looked at, then go with the RPM from there.

Thanks.

j.

or if u have an old automotive analizer ie: dwell, volt, amps & Tack, u
could just hook it up and let somebody drive it and see if it jives with
your boat tack. I have 2 old one's havent used them in awhile. No u can't
have one sorry.

"jaybird" wrote in message
. ..

"WildestDream" wrote in message
...
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.

I'll double check the book, but it seems to me like your range is about
what
mine should be too. It just sounds like I need to head to the dealer and
get the gauge looked at, then go with the RPM from there.

Thanks.

j.

"WildestDream" wrote in message
t...
or if u have an old automotive analizer ie: dwell, volt, amps & Tack, u
could just hook it up and let somebody drive it and see if it jives with
your boat tack. I have 2 old one's havent used them in awhile. No u can't
have one sorry.

awwwww... and here I was thinking that you'd be generous. Hehe....

I'm going to try out a 23 pitch prop and see what that does for me. I have
a Bravo I Mercruiser that came with a 21 pitch, but have seen the Bajas with
the same engine and a 23 pitch. I'm just going for the extra speed.

j.