Raised my motor, but questions...

[erebus]

Boats now on her mooring in Cape Cod Bay, so after work this evening in the dead flat calm I took her on another sea trial with a known prop.
Took off the unknown mystery prop and threw on a Michigan Wheel Vortex 13.25X17P we had kicking around the shop.
Top end RPMS were about ~5600 at 29kts.
The original mystery prop topped out at ~5000.

So from this I can infer that the mystery prop is probably a 19" or a 20"?
200 rpm's per inch of pitch, right? so if the 17p runs 5600 and the mystery runs 5000 then its got somewhere around 3 inches more pitch?
I'm guessing its a 19p.
So, the merc prop calculator with the input from today's tests tells me I should run a 19p prop on my boat/motor combo (actually it said an 18.38").
It recommends the 4 blade spitfire 12.5"x19"p. Or a 13x19 black max.
4 blade gives more stern lift right?
Going to stick with aluminum for now, as sometimes I need to do a little dredging to make it back to my mooring if I spend too long at the beach.
No point ruining a nice stainless. At least not yet.