I ran a 1991 Mercury XR4 2.4L on the back of my old '72 Seafari. Originally 175hp, after rebuild it produced about 210 hp. I had a Bob's nose cone on a lower unit from a 225 Merc, low water-pickup, and ran the motor fairly high on the transom, turning a one-of-a-kind custom 3 blade prop from Power Tech, 15x16.5P
My maximum speed (with everything just right) was 53 mph at 6050 rpms. But everything had to be perfect to get that speed: minimum weight and minimum windage, air temp below 50*, low humidity, running into an 8-10 mph wind, moderate chop to get more hull out of the water, etcetera.
Most of the time I couldn't get over about 47-48 even with the higher hp. And I normally never ran over my 25-26 mph cruising speed because of fuel consumption with a 29 gallon tank.
So I repowered with a 90 Optimax, and went from less than 2 mpg, to nearly 7 mpg average. I loved it.
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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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