Background - Got a 21' pontoon with 40 HP 2-stroke, wasn't expecting a
speed demon but was hoping I could squeeze a little more out of it.
Only hitting 13 and change (GPS measured speed).
Someone suggested that the prop was too big since it was only turning
5k rpms. So asked around, seemed like a 9 was the consensus. Check it
out yesterday, it has a 10 on it.

So I'm all excited thinking this is going to help me out... plug the
numbers in the prop calculator (
http://www.boatpropellers.com/propcalc.html ), maybe 1/2 mph increase?
Assuming the engine will spin up to 5500 with a 9. Sound right?

I wasn't sure about 'prop slip' so I reversed it out by what I knew
from current numbers. Came out to 45% and from what I read that's
possible with non-planing hulls. Guess I'm also wondering if there may
be more or less prop slip with the 9 vs. the 10.