24 ft pontoon boat, recently purchased used and in nice shape.

Force 75hp OB, came with the boat when new as per original owner; I trust
him (older fellow), the boat was obviously well-maintained and not abused.
Runs fine. Plenty of power for our needs.

RPM's top out at about 4500-4600. I believe the book says 4750-5250. The
motor goes into pretty bad cavitation after a couple of minutes of running
at anything over 2800-3000 RPM or so, particularly in choppy water. Running
across wakes, forget it---runs like a car with a slipping clutch, I have to
back it down to 1500 or so to get the "grab" back.

I was in the water yesterday and took a look at the prop. It's spotless, and
there are no sharp edges, damage or other blemishes on the lower unit. The
part number looked like a Mercury number. We had no pen or paper to write
the number down, but it was pretty close to 48-73133A40-13P. Possibly 73113
instead of 73133, can't remember.

I don't know props all that well, so I really couldn't tell by looking at it
whether or not it's cupped, but the Mercury part numbers I can find seem to
indicate that it is. That is, the numbers I could find show all the numbers
in the 73xxx range are cupped props.

I ask because last week I talked to a friend who was an experienced Mercury
mechanic. He was pretty sure a cupped prop would solve my cavitation problem
(if the existing prop isn't cupped), and that I'd need to change the pitch
downward if I went to a cupped unit. He said the cupped prop effectively
adds 1/2 a size to the pitch.

Bottom line---now I don't know where to go with this prop size business, and
could use some advice. Since I can't find the part number listed, I don't
even know for sure which prop I have on the boat now.

BTW, the motor is mounted as low as it will go on the transom (it's in the
top-most mounting holes, and the motor bracket is sitting right on top of
the transom. It's a 75ELPT model, which I'm assuming means "Extra Long Power
Trim". I have to run the trim all the way down, or else the cavitation
problem gets even worse.

Any help is appreciated. I will even entertain flames if it gets me the info
I need!!

Thanks, guys.

Dean