Steam gun boat (SGB) was a Royal Navy term for a class of small naval vessels used during the Second World War. The class consisted of nine gun boats, powered by steam, and built from 1940 to 1942 for the Coastal Forces of the Royal Navy.
They were developed in parallel with the Fairmile D motor torpedo boats ("dog boats"), specifically as a response to the need to hunt down German E-boats and also as a response to the scarcity of suitable diesel engines. While sixty were planned only an initial batch of nine were ordered on 8 November 1940, of which seven were completed.
The steam gun boats were conceived to answer the seeming need for a craft which was large enough to put to sea in rough weather and which could operate both as a 'super-gunboat' and a torpedo carrier, combining the functions of the Motor Gun Boat (MGB) and Motor Torpedo Boat (MTB) in the same fashion as did the German E-boats. The Admiralty wanted Denny to produce a design that was suitable for pre-fabrication construction to enable large numbers to be built.
They were the largest of the Coastal Forces vessels, and were the only ones to be built of steel (to meet the fast production requirement - all other Coastal Forces craft were of wood). They resembled a miniature destroyer, and were perhaps the most graceful of all the craft produced during the Second World War. However their comparatively large silhouette was a drawback, making them too easy a target for the faster German craft.
They were 145 ft 8 in (44.40 m) long and had a displacement of 172 tons (202 tons fully fuelled). They were powered by two 4,000 hp steam turbines using special flash boilers. These boilers proved to be particularly vulnerable to attack and - once the vessel had broken down - required a major effort to repair. Steam had the advantage of quietness but demanded a large hull. Large wooden hulls were not feasible for mass production so steel was used. This meant hulls and machinery were beyond the scope of the small yards engaged in the rapid expansion of the coastal forces, and the SGB thus competed for berths in yards already hard put to produce urgently required convoy escorts. Also they competed in the demand for mild steel and steam power plants against the more urgently demanded destroyers; accordingly the planned 51 further vessels were never ordered, while the two units ordered from Thornycroft were never begun[clarification needed] due to enemy action. The seven vessels actually completed were built by Yarrow, Hawthorn Leslie, J. Samuel White and William Denny and Brothers, entering service by the middle of 1942.
Fuel consumption was heavy with the added disadvantage that, while a petrol boat could start from cold and get away immediately, the SGB had to remain in steam. 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.
Veritable battleships of the coastal forces, the steam gun boats were heavily armed and could maintain high speed in a seaway. In action E-boat commanders respected the SGBs almost as much as destroyers.
A lack of steel and turbines meant that the 52 boats initially planned was reduced to an order of nine boats which received the designations SGB 1 to 9. Numbers 1 and 2 were cancelled when their hulls were badly damaged by an air raid on the Southampton area. The 1st SGB Flotilla was formed at Portsmouth by mid-June 1942, under the command of Lieutenant-Commander Peter Scott, son of the Antarctic explorer Captain Robert Scott, later a noted ornithologist, conservationist and broadcaster. Their first fleet action took place in the Baie de Seine (the estuary of the Seine River) shortly after midnight on 19 June, when two vessels - SGB 7 and 8, under the joint command of Lieutenant J. D. Ritchie, in company with the Hunt-class destroyer Albrighton encountered several E-boats escorting two German merchantmen. SGB 7 was sunk in this action; as a consequence the Admiralty noted their vulnerability and refitted them with the additional armour over their engine and boiler rooms. At the same time the six survivors were renamed after wildlife in the form "SGB Grey....".
The nine vessels laid down, listed below, were all ordered on 8 November 1940.
|Yarrow, Scotstoun||24 January 1941||29 August 1941||21 February 1942||For sale 20 August 1949|
|Yarrow, Scotstoun||24 January 1941||25 September 1941||15 March 1942||For sale October 1947|
|Hawthorn Leslie, Hebburn||17 April 1941||27 August 1941||1 April 1942||Sold to British Iron & Steel Corporation and scrapped 15 December 1949|
|Hawthorn Leslie, Hebburn||28 March 1941||17 November 1941||30 April 1942||Sold 13 October 1947. Houseboat in 1949|
|SGB7||Denny, Dunbarton||3 February 1941||25 September 1941||11 March 1942||Sunk by gunfire from German surface vessels in the Seine Estuary 19 June 1942|
|Denny, Dunbarton||3 February 1941||3 November 1941||17 April 1942||Sold 3 February 1948|
|J. Samuel White, Cowes||23 January 1941||14 February 1942||4 July 1942||Sold about 1957. Converted to houseboat, currently moored at Hoo St Werburgh.|
SGB 5 was damaged in the Dieppe raid after meeting a German convoy of R boats. In 1944 the six survivors were all converted to fast minesweepers and all (except SGB9/Grey Goose) were sold off in the years after the war.
SGB9 remained in service as a propulsion trials vessel from 1952 to 1956, her steam engines replaced by Vospers with a pair of experimental Rolls-Royce RM60 marine gas turbines and becoming the first vessel to rely solely on gas turbines for propulsion. The highly advanced turbines featured intercooled compressors and recuperators to boost turbine power output and reduce fuel consumption. Over the original steam machinery, the gas turbine powerplant provided 35% more power while weighing 50% less and using 25% less space. 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. With the experimental engines removed SGB9 was sold off in 1958, becoming a mercantile repair hulk. Sold in 1984, the hulk was converted to a houseboat and renamed Anserava. She is currently moored on the River Medway near Hoo St Werburgh in Kent, England.