HMS Seahorse was a first-batch S-class submarine (often called the Swordfish class) built for the Royal Navy during the 1930s.

While returning to port after her first and uneventful patrol, Seahorse was erroneously attacked with depth charges by a British aircraft.

On 18 November, Seahorse spotted two German ships, very probably the destroyers Z21 Wilhelm Heidkamp and Z19 Hermann Künne, but failed to maneuver into an attack position.

[3] For surface running, the boats were powered by two 775-brake-horsepower (578 kW) diesel engines, each driving one propeller shaft.

[2] Ordered on 13 March 1931, HMS Seahorse was laid down on 14 September 1931 in Chatham Royal Dockyard and launched on 15 November 1932.

After her uneventful patrol, Seahorse, returning to Dundee, was erroneously attacked with depth charges by a British aircraft at 20:07 (UTC) in position 56°40′N 01°04′W / 56.667°N 1.067°W / 56.667; -1.067.

Seahorse dived again and hit the 220 feet (67.1 m) deep bottom heavily, causing damage to her ASDIC dome.

The next day, Seahorse sighted the surfaced German U-boat U-36 which was attacking the Danish merchant ship N.J. Ohlsen.

On 18 November, she sighted two ships, very probably the German destroyers Z21 Wilhelm Heidkamp and Z19 Hermann Künne, but Seahorse could not maneuver into an attack position.

[5] On 13 December, Seahorse departed Blyth to patrol the British east coast, but returned to port two days later after, having been recalled.