2.9 mpg for a 23' is VERY good. I don't think you'll get more than 8-10% improvement with a fourstroke, at best.
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Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury
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