The Alarm-class torpedo gunboat was the penultimate class of torpedo gunboat built for the Royal Navy. The class was contemporary with the early torpedo boat destroyers, which were faster and thus better suited to pursue of enemy torpedo boats. By the First World War, ships of the class had either been sold, converted to submarine depot ships or minesweepers, or reduced to harbour service. Three of the 11 completed torpedo gunboats of the Alarm class were lost during the war while serving in the minesweeping role.
The Alarm class was designed by Sir William White in 1889 as an enlarged version of his previous . They had a length overall of , a beam of and a displacement of . They mounted two sets of vertical triple-expansion steam engines, two locomotive boilers, and twin screws. This layout produced , giving them a speed of with forced draught. They carried between 100 and 160 tons of coal and were crewed by 91 sailors and officers.
While officially classed with the Alarm class, Speedy was actually a separate design. The Naval Defence Act of 1889 authorised the purchase of an Alarm-class torpedo gunboat built to a design by John I. Thornycroft & Company and built in their yard at Chiswick. Speedy was a three-funnelled vessel (compared to the two-funnelled Admiralty design), but the key difference was the use of water-tube boilers instead of locomotive-type boilers; she produced at least and could make . The use of water-tube boilers was a key feature of the new torpedo boat destroyers that would make torpedo gunboats (including the Alarm class) obsolete.
At build the class was fitted with two QF /45-pounder guns, four 3-pounder guns and one Gardner machine gun. Five torpedo tubes were fitted in the first five vessels, but this was changed to three 18-inch (450 mm) torpedo tubes in the rest of the class. They were arranged as a pair of revolving deck mounts, a pair of fixed deck mounts (deleted in the later vessels) and a single bow-mounted tube; three reloads were provided.