Aquamatic Propellers

First off, I'm no expert but I have learned a few things as a first time Aquamatic owner behind my 1972 houseboat.

A couple things you need to know before you buy a prop for your Aquamatic outdrive.

Aquamatics came with counter rotating props but this can be easily changed - odds are though you'll take a counter rotating prop.

You'll need to find out whether you need a short hub or long hub propeller. This is the short hub dimension.

If your dimensions match the picture, you'll need a short hub propeller. If your dimensions are different, you take the long hub propeller but you can fit either a short hub or long hub propeller with the proper mounting hardware.

Aquamatic Prop Mounting Hardware

After you decide which hub size you need, you'll find a Solas propeller here for considerably less than a OEM prop. It's where I get mine...but they don't seem to carry the short hub model anymore.
iBoats Aquamatic Propellers

If your boat is considered a speedy model, a 17 or higher pitch may be in order; heavy boats will likely take a 13 or 15 pitch.