Inside the pressure hull are spaces for the crew, machinery, and necessary supplies. The outer hull forms the skin upon which the submarine floats when on the surface. The superstructure forms the bow, the stern and the main deck of the submarine. The superstructure is free-flooding and draining through a series of limber holes in the superstructure. Pressure hull is divided into compartments separated by pressure bulkheads with watertight doors.
Frames strengthen the hull.
S. project number was Pu89. The Vetehinen was intended to be a prototype of a rapidly built 500-ton boat. Germany did not, however, built boats of this size and selected a 750-ton boat that was under construction in Spain. These submarines were direct ancestors to the Type VII in German Fleet. The boats had riveted construction.