Michael
OK here is the short method.
1.) read the name plate for hp and RPM off the engine. serial # would be good
2.) read the name plate on gear reduction for gear reduction 2.X or ?
3.) Open your Maintenance log book write the above down, Then go visit
https://www.miwheel.com/inboard-propellers/prop-it-now/
here is a second site, but it is off in cal my prop. so please use caution on it's usage. and gives pitches not sold.
http://www.vicprop.com/displacement_size.php
Assume any where between 7.5 and 7.25kts for a 42 ? = the max motor speed you will do (Not what you would wish for, but normally 1 to 1.5 kt below a great beam reach sailing)
punch calculate (= done) well there is some fudging you can do, (I like the 2 blade sailor type prop; so again, a little more adjustment) but you are in the ball park. There is another page of details you could know, but the above will get you close (the guy you buy the prop from may have a more detailed cal sheet and ask other questions.
In general you go 1.5 deg more pitch for every 1" in OD reduction if not to far away from the best fit, but from what Bud noted the std was a 20" OD prop. for clearance. There are many notes on this aspect as well, but the less clearance you have, the more noise and flow irregularities you will have. For the full keel try not to have less than 10% the blade radi. = 1" clearance on a 20" prop = ok+/-. Both of the above, are in general and may vary with prop manufacture and are NOT set in stone.
PLEASE NOTE THE ABOVE IS FOR A CLEAN PROP so if you want it to work well (KEEP IT CLEAN!!!!!!!!!!!)
Norm
hope this helps
