I had a heavy 18' taskforce dory with 85 premix yamaha, came with 19" prop. It used to stall as you engaged gear when cold, it struggled to accellerate until it got to 1500rpm then was ok but wouldn't go above 5100rpm. Changed to 17" and all my problems gone, WOT 5400 (limited 5500) no change in top speed (not measureable anyway) but MUCH better accelleration and it didn't stall.
Incidentally the Yamaha dealer I was getting bits from used to race these engines, they used to remove the limiter and run them at 6500 rpm and didn't have any problems.
I got a propellor repairer to exchange my props for new pitched ones, cost £50 each but got the equivalent of brand new for old into the bargain! (and they were tatty)
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