Hi Jonathan Eno
When flying an airplane, and when turning, you have to pull the stick just a little, as well as turning, in order to keep present altitude. I believe it's the same 'problem' (effect?) that we have here.
I also believe that you're right about the roll-issue. Does your sub list when you turn, by any chance? If it does, then maybe we're dealing with the hull's profile being different seen horizontally, than the vertical profile, and when the sub then roles, the whole hull acts as a dive plane? To test this, tie two beams of wood or similar, making your sub more like a three-hulled boad temporary, and thus harder to list side to side.
Alternative: Try to counter-act the roll/list using your dive-planes. Do observe though, that when turning at high(er) speed, the water flow rate over the outboard dive planes are somewhat higher than the water flow rate over the inward dive planes. Ofcause this is real small forces at work, but still.... Try to offset the diveplanes so that the dive plane on the turning circles outside gives less angle than the inside one does, AND work in oppersite direction. (Donno if your sub can do this?) If not, let'em work together, still counter-acting the incorrect responce of the sub. (Just like the guy in the plane would do..)
My last comment is that maybe your turns cause "turbulense" in the water, messing up the buoyancy along the hull's back, causing it to surface/dive? Again, use the dive planes to counter-act.
I guess the fault would be corrected if the sub had a higher mass, making it less sensitive, if none of the above helps... Hope you find the cause.... interesting one, this one.
Yours Sincerely, Robert Holsting, Denmark
1/81 SSBN Ohio Class scratch builder, more at www.robse.dk
"Never be afraid to try something new; remember that it was amateurs who build Noah's Ark, and professionals who build the Titanic"