It's very obvious to me that there are hordes of very safe submarines that don't have horizontal supports between the the support rings. The outer skin and the rings should be your first line of defense against pressure. Even though the horizontal sections can be the same size as the rings, they will not be as strong as the rings, since they are not curved. I was just thinking they could give you that much more safety.
One additional advantage I see of using horizontal supports is in the event of a collision with something at depth. If a vertical crease is put in the hull I don't want it to continue creasing in between the main support rings using the pressure on the whole main hull as leverage against that crease.
In the last couple of days I've been doing some hull section stress test on a program called Cosmos from Solidworks. Here is some pics and animated views of those tests I put on You Tube.
Remember Amateurs built the Ark.
Professionals built the Titanic, and I'm the guy that plans to build a submarine in a hay field.
Brent Hartwig