Visitors entering the boat at the petty officers’ quarters have a chance to look into fuel ballast tank 7 port. The pressure hull is to the right, and a thinner outer skin to the left. The space between them is actually fuel ballast tank 7 port. This boat is not of a double-hulled construction, as the outer skin of the ballast tanks does not wrap entirely around the underside of the keel. A fuel ballast tank is a ballast tank that is filled with fuel at the beginning of a patrol, and fills with water as fuel is consumed from it. Once all fuel has been consumed from it, the tank is blown out and then used as a normal water ballast tank when diving. The small pipe visible inside this ballast tank is the discharge line from the auxiliary drain pump, which is found in the forward torpedo room. (Caption courtesy of Matt Hamacher)