Steam Gun Boat 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 boat s ( " 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 . Design . 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 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 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 . Service . 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 estury 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 " 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 " . . . " . " Grey Owl " was damaged in a fight with German armed trawlers off Berneval while escorting landing craft in the Dieppe Raid August 1942 . Boats . Nine vessels below were all ordered on 8 November 1940 . These boats formed the 1st SGB Flotilla which was initially formed at Portsmouth , but later based at HMS " Aggressive " , Newhaven , Sussex on the south coast of England . SGB 5 was damaged in the Dieppe raid after meeting a German convoy of R boat s . In 1944 the six survivors were all converted to fast minesweeper s 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 turbine s 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 .