Hey Bill, Definately plan to photo tach the engine in a few weeks when I go back in the water. The prop has been changed around some with the prev owner and we think it came from ABC with a 22x22 but wanted to see if there are any other 31 Duffy owners on here with the same power. When Im finished I want it to hit 3400 or 3450 on the pins with the hand held tach
On a guess and assuming the boat has a 2:0 gear the wheel should be anyone of the following:
22 X 22
23 X 21
Maybe a #4 cup.
If the boat is real light with no F/B & less that 200 gallons of fuel
Maybe a 23 X 22 #4 cup.
You need to photo tach. Yanmar panels are usually off by about 100 RPMs to high. Also always photo tach your boat with all fresh fuel filters. A dirty Racor or primary can cost you 100-200 RPMs.