Originally Posted by 603doug
Tried my 13.25x17 prop today, 2 divers and gear got up on plane faster and seem to have midrange speed where before it was all or nothing. I still have to check out rpms only manage 4500.
Max rpms will vary with load. It will also be affected by how much water is in the hull, where the load is situated, how much fuel, etc.
You want to prop for max-rpm-within-mfgr's-spec at a "normal" load, with the engine trimmed up. In my case, I propped for full load of fuel, with 2 people on board, and just accepted the rpm drop with dive gear (and more divers.) If I expect to have a lot of divers and gear on board, I swap to a much lower pitched prop.
General rule is about 200 rpm per inch of pitch, all other things being the same (though it's kind of a black art, and never goes quite the way it's supposed to.) Other general rule is about 100 rpm per quarter inch of diameter.
Figure out a baseline for testing (as far as load goes) and work it up from there; you can't tell anything if you're testing under constantly changing conditions.