Hi Jon.
Sorry I didn't answer earlier, but the power has been out for a couple of days.
Here's an old sketch of the leaf type hatch spring arrangement I'll be using. This is just a concept sketch and naturally the actual mechanism will be somewhat different.
As I said, the pressure applied by the spring pushes down on the hatch hinge pin, making it less likely to leak while near the surface. I don't have the spring tension adjustment shown here, but basically it's just a bolt that pushes down on the spring where the spring is mounted on the hull.
Let me know if you get this sketch. Thanks.
Frank D.