The third-batch submarines were armed with seven 21-inch (533 mm) torpedo tubes. A half-dozen of these were in the bow and there was one external tube in the stern. They carried six reload torpedoes for the bow tubes for a total of thirteen torpedoes. Twelve mines could be carried in lieu of the internally stowed torpedoes. They were also armed with a 3-inch (76 mm) deck gun. The third-batch S-class boats were fitted with either a Type 129AR or 138 ASDIC system and a Type 291 or 291W early-warning radar.