Rigsy:
Don and I had a discussion about this a few years ago. His view was that the back pressure generated by the exhaust being underwater (at a depth within reason) was trivial. The engineer in me had to run the numbers. I forget exactly how the calculations came out, but as usual he was correct. In fact, some boats are designed this way.
Of course, if you're on the edge of a problem, I suppose you could see a performance effect, but my guess is that what you're seeing by moving the crew around has more to do with changing the hydrodynamics of the boat than back pressure.
Bill
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