Some of the 23's were rated for 500hp, but I never saw one with 600!
JFTR, properly rigged and tuned, he should exceed 70 mph. Something's not set up right.
I rigged a 23' with twin 2.4L 220hp Mercs back in '86 that could achieve 71 mph.
It made 63 mph out of the box, then jack-plates, some lower mods and prop tuning got it to 71. And they were both RH rotation!
That guy's got 35% more hp!
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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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