Submarine hulls are rather \'thin\' with respect to their diameters - at least in structural terms. When a thin walled cylinder is subjected to an external pressure, the failure mode of the hull will be by longitudinal buckling of the skin. To prevent the buckling, circumferential stiffeners are added. This way you get the stiffness and resistance to buckling of a very thick hull (as thick as the ribs) but without the weight.

Structurally, it doesn\'t matter too much if the stiffeners are inside or outside of the pressure hull. If they are inside the stiffeners are protected from exposure to seawater and corrosion. If they are outside the pressure hull (but inside the streamlined outer hull - this assumes a fully double hull design) corrosion must be prevented.

The space between the inner and outer hulls is used for ballast tanks (and fuel tanks). These enclosed spaces need dividers, so it is a natural place to put some external stiffener ribs. But to prevent corrosion, I suspect that every nook and cranny must be accessible (in drydock) for inspection, grinding, and painting - I don\'t know if WWII subs came with \'lifetime\' anti-corrosion painting for the internal spaces in the seawater exposed ballast tanks - I suspect not, but info would be appreciated.

The ballast tanks were pretty thick, extending many inches or even well over a foot beyond the pressure hull (sorry for the English units - just like a backwards yank you know). This would allow a lot of structural depth to the ribs. Accomodating ribs sticking a foot into the hull in the interior would be very hard.

A rib inside the sub might be even more inaccessible, under pipes and equipment, and exposed to a variety of bilge water, fuel, dirt, and even battery acid that might make it better to put the ribs out in the seawater. Also, internal ribs would interfere with the already cramped interior.

Most pictures of half completed subs I\'ve seen show a lot of external ribbing. Internal ribbing was probably used as well, in addition to using the watertight compartment bulkheads and other internal structures to add stiffness to the hull in as light a way as possible. All in all, I suspect that putting most of the ribs external to the pressure hull (but inside the pressure hull) was the best and normal way to do it.