Steam gun boats (SGBs) were small Royal Navy vessels built from 1940 to 1942 for Coastal Forces during World War II.

The Admiralty wanted Denny to produce a design that was suitable for pre-fabrication construction to enable large numbers to be built.

[3] The seven vessels constructed were built by Yarrow, Hawthorn Leslie, J. Samuel White and William Denny and Brothers, entering service by the middle of 1942.

Over time the addition of 18 mm (0.7 in) protective plate over the sides of the boiler and engine rooms, together with the extra armament and crew, increased the displacement to 260 tons and their service speed was consequentially reduced to 30 knots.

[citation needed] Veritable battleships of the coastal forces, the steam gun boats were heavily armed and could maintain high speed in a seaway.

SGB 7 was sunk in this action; as a consequence the Admiralty noted their vulnerability and refitted them with additional armour over their engine and boiler rooms.

[7] SGB5 (Grey Owl) was damaged in a fight with German armed trawlers off Berneval while escorting landing craft in the Dieppe Raid August 1942.

[citation needed] 1st SGB Flotilla was later based at HMS Aggressive, Newhaven, Sussex on the south coast of England.

[9] Although the experimental powerplant proved very successful, it was too complex and supporting technology too immature for wider service at that time, and SGB9 was placed in reserve at the end of the trials in 1957.