The third and by far the most numerous group of S-class submarines consisted of 50 boats. They were the largest and most heavily armed of the S class and required more men to crew. They were one knot faster on the surface, but two knots slower when submerged. Most of the group were built at the yards of either Scotts, of Greenock or Cammell Laird & Co Limited, of Birkenhead, with a handful being built at Chatham, or by Vickers Armstrongs Ltd, of Barrow-in-Furness. Construction was carried out throughout the war, particularly between 1941 and 1945. Equipped with a greater fuel capacity than their predecessors, they operated much further afield, in the Mediterranean and in the Pacific.