Originally Posted by Mollers
Hi, welcome. there are 3 or 4 core questions which pop-up very regularly on this site, two of which you are asking.
How could he ask 2 questions in one (albeit long) sentence?
Prop selection is generally done to the emgine mfgr's suggested max rpm, on a boat with a "normal" load (normal is in quotes because everyone's idea of normal is different. For one person, that may be 4 large individuals with dive gear and full fuel, for another it may be 1 110lb person and half a tank.)
This can be difficult (not impossible, just difficult) if the boat has no tachometer. If that is the case, beg or borrow someone's auto tune-up kit, which ideally will have an inductive tachometer.
Selecting the prop comes down to running the thing as fast as it'll go (throttle wide open, and trim up until the prop ventilates, and nudge it back down again), and seeing where the rpm's fall. If it's not in the upper end of the mfgr's suggested rpm range, a prop switch is in order. General rule is 1" pitch equals about 200 rpm; 1/4" diameter equals about 100 rpm. Larger pitch values and larger diameters mean less rpm. Switching from aluminum to stainless should lose about 200 to 250 rpm assuming everything else is the same (it usually isn't.)
4-stroke vs. 2-stroke? I think I'll stay out of that one.