The S class was a medium sized patrol submarine, designed with the proposed international restriction of 600 tons displacement in mind. Intended for use in north European or Mediterranean waters, they were highly manoeuvrable and fast diving with a heavy armament of torpedoes. This allowed large salvoes to be fired from long range in an effort to counter the improved A/S techniques.

This proved to be a very successful design that was put back into production during the war, 62 boats of this class being built. Some boats were partially welded and the later ones fully welded construction, improving the strength of the pressure hull and consequently increasing their diving depth.

The specifications as given above is for the first group.

Further specifications on the later groups; 2nd Group;
Same as the first group except;
Displacement: 670 / 960 BRT
Complement: 4 officers + 35 men.
Length: 208.75 feet
Sunfish had more powerful diesel engines 1900 HP instead of 1550 HP. Therefore her surface speed was 15 knots.

3th Group;
Same as the first group except;
Displacement: 715 / 990 BRT
Complement: 6 officers + 42 men.
Length: 217 feet
Armament:
1 3" gun
3 MG AA
7 21" torpedo tubes (6 bow, 1 external stern)
13 torpedoes
Speed: 14.75 / 9 knots
Power: 1900 / 1300 HP

4th Group;
Same as the first group except;
Displacement: 715 / 990 BRT
Complement: 6 officers + 42 men.
Length: 217 feet
Armament:
1 3" gun
3 MG AA
6 21" torpedo tubes (6 bow)
12 torpedoes
Speed: 14.75 / 9 knots
Power: 1900 / 1300 HP
The fourth group was actually the same as the third group but without the external stern torpedo tube. Although some early units of this group were fitted with the external stern tube.

Some ships in the third or fourth group carried a 4" gun instead of a 3" gun.

Submarines of this class that have detailed patrol info added are shown in bold in the list below.