On many sailboats, the prop diameter is limited by the hull
clearance or the aperture, so the answer to the question of what is the right size is simple: the biggest diameter that will fit. Once you have the prop diameter, the engine
HP, and the gear
reduction ratio, you can calculate the pitch
using, e.g., the methods describe in Dave Gerr's book.
But sometimes there is a choice of marine
gears with different reduction ratios. For example, the Yanmar
YM series engines can be ordered with 2.21:1, 2.62:1, or 3.22:1 gears. What is the best choice of reduction gear
when the prop diameter is limited? I'd like to repower
3YM20. My prop diameter is limited to 14 inches. I can go with the 2.21:1 reduction and a 14x11 two blader, or 2.62:1 and 14x14. The efficiency of the 2.21 is a little bit better (calculated using the Bp-delta method in Gerr's book), but the difference is very small (maybe 2-3%). Michigan Wheel
recommends going with the 2.21:1 ratio and lower pitch
. However, a reputable local prop shop warned me that cavitation will get worse with the 2.21:1 gear and recommended 2.62:1 and the higher pitch.