I replaced the 'optional' fixed 3-blade 15" x 9 degree with a feathering MaxProp 15" x 9 degree, as recommended by MaxProp, and am very happy with the performance. I believe the 'standard' on a C36II was a 2-blade 15" x 10 degree, which some hated. I can motor at about 7 kts, but, like Leslie, chose a more sedate 6-ish at about 2,000 rpm. It's the sweet spot between time, fuel economy, and shaking up the crew.
As I'm sure you've heard before here, everything is a compromise. In this case between cost, motoring speed, and sailing efficiency. Folding props are great for sailing, particularly in lighter air, but generally suck on the motoring front and cost a bomb. I chose a feathering prop (and paid for it) because it hit the sweet spot for lighter air sailing and motoring efficiency. Fixed props are better for motoring (except in reverse where I think my feathering prop wins), but is like dragging a gallon bucket behind your boat. None of the efficiency lost matters if you have enough wind (over 15?) because you have enough power to burn and you're limited by hull-speed. It's in the lower wind situation where folding props come into their own. Google "feathering propeller test" and you'll have an afternoon of reading pleasure.
