Fairmile B motor launch The Fairmile B motor launch was a type of motor launch built by Fairmile Marine during the Second World War for the Royal Navy for coastal operations . Design . While the Type A had been designed entirely by Fairmile , the Type B design had come from Bill Holt of the Admiralty based on the lines of a destroyer hull and the detailed design and production was taken on by Fairmile . Like all their designs it was based on total prefabrication so individual components could be contracted out to small factories for production and these arranged as kits that would be delivered to various boatyards for assembly and fitting out . Altogether approximately 650 boats were built between 1940 and 1945 . Like the A Type , the B Type were initially intended as submarine chaser s , so the boats were fitted with ASDIC ( sonar ) as standard . Their main armament initially reflected their anti - submarine focus , with 12 depth charge s , a single Hotchkiss 3 - pounder gun aft , and one set of twin 0 . 303 - in machine guns . The specifications given are for the original 1940 British version . As the war moved on , the vessels were adapted to other roles and the armament was modified and upgraded such as the replacement of the 3 pounder with one or more 20 mm Oerlikon cannon . Some boats were configured as motor torpedo boats . Service . The first Fairmile B motor launch was completed in September 1940 , with a further 38 from the first two production batches entering service before the end of the year . All boats were essentially the same , although they could be adapted to serve in several roles by the expedient of having pre - drilled rails on their decks spaced to allow the fitting of various types of armaments . Although their armament initially reflected their main anti - submarine mission , nine of them were fitted with 21 inch torpedo tube s taken from ex - US " Town " - class destroyers ; they formed the 2nd ML Flotilla tasked with anti - invasion duty , until the threat had passed . During the Siege of Malta , they were successfully employed as minesweeper s , as the larger specialist craft were too vulnerable to air attack . A number served in the St Nazaire Raid as assault transports , however their light construction meant that they suffered heavily ; twelve B motor launches were lost in the action , out of sixteen deployed . Many were later converted to rescue motor launches with small sickbays aft of the funnel , and several more were converted to use as War Office ambulance launches with larger sickbays . Canada built eighty boats . These were built in thirteen different boatyards to slightly different specifications and used as escort vessels . Eight of these ( ML392 - 399 ) were built by Le Blanc for the Royal Navy . These eight boats were transferred under Lend - Lease to the US Navy , because US coastal protection had been depleted by transferring ships to the Royal Navy for convoy work . The US Navy used them as submarine chasers ( SC1466 - 1473 ) until their forces could be built up . There are two surviving examples on the west coast of Canada serving as party charter vessels . New Zealand built twelve boats . These were used in New Zealand waters and around the Solomon Islands , and included HMNZS " Maori " and " Kahu " . At least six boats ( ML380 - 383 , 829 and 846 ) were built by South Africa and commissioned during November 1942 . These were sent as the 49th Fairmile Flotilla ( SANF ) to Burma and deployed along the Arakan coast . The boats saw much action in support of ground forces and disrupting Japanese supply lines . Post war they were often taken on as pleasure boats and a number of Fairmile Bs are on the National Register of Historic Vessels Four currently survive in the UK , two of which are in excellent condition . One is RML497 . Many others of the type are known to survive around the world , some still in commercial service as tourboats .