Less than 20, and you'll have to mod the boat to do it. You'd have to build a loop to close the circuit where the other batt is supposed to be. You'll also be out of balance which is a pretty big issue. I say less than 20 because:

A - it may not even plane off with one pack, and
3 - running that slow you'll also be running wet (lots of hull in the water). You really need to air this boat out to get into the high 30's.

good luck is your supervee shorter than it should be? (25)