USSR: Project 621
The siege of Sevastopoli in the autumn of 1941 marked a dramatic turning point in the Russian countryside: due to the collapse of the Soviet defensive system, more than 100 thousand soldiers were trapped in the stronghold, surrounded by Nazi troops. For some time we proceeded to supply the besieged by sea, but the heavy losses suffered by the surface vessel, pushed the command of the Black Sea Fleet, already in April 1942, to employ submarines for the transport of what was necessary to the besieged, and for the evacuation of the sick, wounded, and women and children trapped by the lightning advancing Nazi. The largest submarines in service in the Soviet navy, class L (XIII Series), could only carry 95 tonnes, the smallest ones, of course, even less. Despite this, before the fall of Sevastopoli on 3 July 1942, after more than eight months of siege, the Soviet navy was able to transport more than 4000 tons of supplies and evacuate about 1500 people.
Based on this experience, the construction of real transport submarines was evaluated. Already in July 1942, the designers began to work on Project 607, a boat capable of carrying 300 tons of cargo, and also equipped with two folding cranes to facilitate loading and unloading operations; for the following April, the projects were ready, and we could move on to the construction phase, but the improvement of the military situation, killed on 607. The concept of transport submarine, however, continued to keep the bench between Soviet admirals and designers even after the end of the war. According to the memories of Admiral William H Standley, American ambassador to Moscow during the period, Stalin once asked him:"Why not build transport submarines? They could bring us supplies without any problems from the fleet or Nazi planes.
In 1948, in 1948, Central Design Office 18, now Rubin), developed Project 621, a kind of submersible LST, to conduct amphibian raids behind enemy lines. It was, for its time, an authentic giant, 147 metres long and with a surface displacement of 5845 tons, equipped with two vehicle bridges and capable of carrying a complete infantry battalion: 750 men, 3 jeeps, 10 T34 wagons, 12 trucks, 12 85 mm cannons and 3 La-5 fighters. While men and vehicles would land from a bow ramp, planes had to be catapulted from the deck deck. It is not quite clear how they would have been recovered, and, in my opinion at least, the boat appears to be too light for that type of cargo, which, in a quick calculation, should account for around 25% of its displacement.
Much more than these projects are known, except that they have apparently never been further developed.