There were two distinct subgroups. The first were boats of 842 tons, comprising those ordered under the 1939 War Emergency, 1940 and 1941 Programmes (except "Sea Devil" and "Scotsman"), plus "Sturdy" and "Stygian" of the 1942 Programme; these carried an external stern torpedo tube in addition to the six bow tubes. The second subgroup were boats of 814 tons, comprising "Sea Devil" and "Scotsman" of the 1941 Programme, plus those ordered under the 1942 and 1943 Programmes (except "Sturdy" and "Stygian"); these carried no external torpedo tube, but had a thicker welded pressure hull providing for an operational depth limit of - compared with the limit in the first subgroup.