In 1940, the Admiralty issued a set of requirements for a new gunboat design that would incorporate features from both motor torpedo boats and gunboats. The primary task of this new vessel would be to counter the German S-boat threat and to protect the British coastline in areas where larger vessels, such as destroyers, couldn’t operate.
With WW2 already in full swing, the need for this kind of vessel was considered high and an emphasis was put on ease of mass-production, in order to manufacture as many ships as quickly as possible.
The Denny company was tasked with designing a vessel around the Admiralty's specifications. They armed the vessel, similar in design to the contemporary Fairmiles, with cannons, machine guns and torpedo tubes, decided to construct it from steel so as to make it fit for mass-production, but soon ran into a serious problem.
The conventional power supply of the time came from either petrol or diesel engines, neither of which was readily available in wartime Britain. Thus, the decision was made to outfit the new vessel with two steam turbines, producing 4,000 horsepower each.
The Admiralty approved the design and ordered an initial nine vessels to be produced, whilst a total of sixty were planned. However, these plans were soon dropped and out of the nine vessels that were laid down, seven actually saw completion.
Nonetheless, the seven ships that did come out of the shipyards proved themselves in combat, despite some endurance and efficiency problems presented by the steam propulsion system. Thanks to their heavy armament, they quickly gained respect from German S-boat captains, who often tried to avoid encountering an SGB.
Only a single SGB was lost in combat during WW2, with the majority serving into the Cold War before being decommissioned from service.