Moose is right: leave the tunnel if you're going to run a surface-piercing drive. You'll be able to run a flatter/straighter angle of thrust and still surface your propeller, thus more fully utilizing your power.
But it won't back up for squat. No surface piercing drive does.
Propeller slip while reversing on a surface piercing drive averages between 65-80% if I remember correctly. Although, it's theoretically possible to reduce that somewhat, but at the cost of forward thrust efficiency while surfaced.
In other words, you could have better reverse capability if you changed to a non-surfacing propeller, and actually ran the propeller close to the surface without ventilating.
It would be interesting to watch a 23' running about 500hp in front of an ASD6, though...
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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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