² = Armament variation, see armament table below
ª = Armament variation, see armament table below

1:600 Group 1 "S" class. The small structure abaft the conning tower is a medium frequency direction finder - MF/DF. © Andrew Arthur

1:600 Group 2 "S" class. The hull was longer, and the casing extended further forward and slightly more aft than the Group 1. The dive planes were changed, 3" gun detached from the conning tower for a better sky arc and the conning tower re-profiled compared to the Group 1.© Andrew Arthur

1:600 Group 3 "S" class, with 4" gun and no stern torpedo tube. In this group, the conning tower was re-profiled, a larger bridge being added, and the casing flared and extended far forward. In this particualr batch, the 4" gun was mounted in a housing, and given a rotating half-shield. © Andrew Arthur

1:600 Group 3 "S" class, with 4" gun and stern torpedo tube. The latter meant that the casing was extended right aft. The 4" gun was mounted seperate from the conning tower, in a circular cage.© Andrew Arthur

Since the First World War, all submarine building had been placed on large, long ranged vessels for Far-Eastern service. It was with the "S" class that the emphasis fell on smaller, handier vessels more suited to home & Mediterannean deployment, and rapid production. It was also decided that all future construction would use internal oil tanks to prevent tell-tale leaks in case of tank breach.
This class was designed to replace the "H" class, but slightly larger, faster but lightly enough armed to be small and handy. In service, they were most satisfactory - they could crash dive twice as fast as a "U-boat" and were built in greater numbers than any other submarine for the RN - ever. Also, production extended for 15 years, and all throughout the war.
The original two, the Swordfish and Sturgeon had a large casing for the 3" gun - on a retractable mounting, but this was complex and ungainly and was soon removed, the gun being bolted to the casing and surrounded by a cage to prevent the crew being swept overboard in high seas. The other pair were so completed, but this group had an extra tall conning tower, so designed to clear the original 3" gun.
The first group were also different in that they had single electric motors on each shaft, all others had the standard twin coupled units on each shaft. Sunfish had extra-powerful diesels for a 15 knot surface run. All group 1 and 2 production was finished by 1938.
With the war's outbreak, the class was put back into production, as the Group 3, with various modifications and imprvements. Up to Scotsman, welding was only partly used, but after that, they were wholly welded. Apart from the first 5 ( which were later modified to this standard ), they had a new casing with a long, flared bow, and the deck was level all the way aft, boxing in an external stern torpedo tube ( later removed in some units. ) The conning tower was modified, a 20mm Cannon added on a small nest on the rear end and AW type 293 RDF fitted. Cast iron replaced some steel components and Aluminium was used sparingly owing to war shortages of these materials.
From Scotsman onward, Far Eastern deployment was used, and they were modified to suit these tasks. Firstly, much more fuel was carried, mainly in ballast spaces, to extend operational radius. The stern tube was removed, and the 3£ gun was replaced by a 4" gun : surrounded by a large casing - the gun was used much more in this theatre, and this allowed much more shells to be carried for the gun. Sirdar recorded the longest "S" class patrol at 49 days.
Due to the wartime situation, equipment and appearance varied largely between units, and as succesive re-fits were carried out, Snorkels, RDF, new Asdics etc. added, they became quite singluar in appearance from their brothers & sisters.