Tench-class submarine

Tench-class submarines were a type of submarine built for the United States Navy (USN) between 1944 and 1951. They were an improvement over the Gato and Balao classes, only about 35 to 40 tons larger, but more strongly built and with a slightly improved internal layout. One of the ballast tanks was converted to carry fuel, increasing range from 11,000 nautical miles (20,000 km; 13,000 mi) to 16,000 nautical miles (30,000 km; 18,000 mi). This improvement was also made on some boats of the previous two classes. Further improvements were made beginning with SS-435, which are sometimes referred to as the Corsair class. Initial plans called for 80 to be built, but 51 were cancelled in 1944 and 1945 when it became apparent that they would not be needed to defeat Japan. The remaining 29 were commissioned between October 1944 (Tench) and February 1951 (Grenadier). The last submarine of the Tench class, as well as the last submarine which served during World War II, remaining in service with the U.S. Navy was USS Tigrone (AGSS-419) which was decommissioned on 27 June 1975.

One Tench-class submarine is reportedly still in active service, USS Cutlass (hull number SS-473), which was transferred to the Republic of China Navy in 1973. ROCS Hai Shih (S 791) is expected to remain in service until 2026, 82 years after she was first launched in 1944.

Three Tench-class boats survive as museum ships; USS Torsk (SS-423) and USS Requin (SS-481) in the United States, and USS Thornback (SS-418), renamed TCG Uluçalireis (S 338), in Turkey.

The as-built diesel-electric propulsion layout was the same as the last few Balao class, with four Fairbanks-Morse or General Motors Cleveland Division two-stroke diesel engines supplying two low-speed double-armature direct-drive electric motors to drive two shafts. All except Corsair received the Fairbanks-Morse 38D 8-1/8 engine with 10 cylinders; Corsair had GM 16-278A engines. The direct-drive electric motors were much quieter than the reduction gear arrangement of previous classes, and they made the drive train much more reliable due to the fact that the gearing was an element prone to shock damage from depth charges. Two 126-cell Sargo-type lead-acid batteries provided submerged power to the electric motors.

A design weakness of earlier classes solved by the Tench re-design were the ballast tank vent riser pipes that passed through the interior of the boat in the forward and after torpedo rooms. These pipes allowed #1 and #7 Main Ballast Tanks (MBT) (located in the single hull sections of the boat) to vent air during diving, which allowed water to flood into them from below. The tops of these tanks formed the walking deck in the interior of both rooms and thus the normal location of the vent valves (the top of the tank) could not be used. The riser pipes allowed the tanks to vent, but when the tanks were full these pipes contained water at full submergence pressure inside the torpedo rooms. If these pipes ruptured during depth charge attack, catastrophic flooding would occur. Solving this problem initially proved quite difficult, but ultimately required the complete rearrangement of the ballast tanks. #1 MBT was moved to a location forward of the end of the pressure hull, thus allowing it to vent directly into the superstructure like the rest of the MBT's. This move eliminated the riser pipes completely. #7 MBT, after stability and buoyancy calculations were run, was found to be redundant and was converted to a variable fuel oil/ballast tank, increasing the class's surfaced range. These changes forced a rearrangement of the associated piping runs and locations of many of the other tanks. Being almost entirely internal, these changes resulted in a boat that was visually almost indistinguishable from the earlier Balao class, with the exception of a sharper angle (or knuckle) at the lower corner of the bow (only visible when the boat was drydocked). Another difference was the elimination of small bulges around the motor room that previously accommodated the reduction gears.

A significant benefit of the tank rearrangement was that these boats could carry two additional torpedoes for the lower bow tubes in the forward torpedo room, giving a total of 26 torpedoes carried. Later, this would be increased again with two additional torpedoes for the bottom stern tubes in the aft torpedo room, for a total of 28 torpedoes carried. Submarine crews had been asking for more torpedoes, but this could not be accommodated in the previous classes, as the vent risers restricted the available space.

At the beginning of World War II the standard torpedo for US fleet submarines was the 21-inch, Mark 14 torpedo. As the war progressed, the Navy introduced the electric wakeless Mark 18 Torpedo and the Mark 23 torpedo, a simplified high-speed-only version of the Mark 14. Additionally, a small 19" swim-out acoustic homing Mark 27 torpedo supplemented the armament in fleet boats for defense against escorts. Near the end of the war, the offensive Mark 28 torpedo acoustic homing torpedo was introduced. Well after the war the Mark 37 Torpedo was introduced.

Many targets in the Pacific War were sampans or otherwise not worth a torpedo, so the deck gun was an important weapon. Due to war experience, most Tench class were armed with a 5-inch (127 mm)/25 caliber gun, and some boats had two of these. Additional anti-aircraft guns included single 40mm Bofors and twin 20mm Oerlikon mounts, usually one of each.