Welfreighter The Welfreighter was a Second World War British midget submarine developed by the Special Operations Executive ( SOE ) for the purpose of landing and supplying agents behind enemy lines . It only saw action once and was not particularly successful . Design . After the success of the X class midget submarines , an attempt was made by SOE ' s technical division , Inter Services Research Bureau ( ISRB ) under the command of Lt . Colonel John Dolphin to design a submersible craft for covert missions . These would include the landing and supplying agents behind enemy lines , intelligence gathering work off hostile coastlines , and delivering explosive depth charge s to enemy shipping routes . This design became the Welfreighter . It was intended that the Welfreighter could travel surfaced by night towards an enemy - held coastline , submerging as and when necessary to avoid detection . The special agents would then be disembarked and go ashore along with their equipment , stored in the special containers . The Welfreighter would then sail out to sea and submerge itself to wait until the next night . At a pre - arranged time , or upon receiving a sound signal from the landing party ( made by a mechanical device ) it would surface again and pick up the agents , before heading out to sea either to rendezvous with a larger surface vessel or return to base under its own power . The initial concept was a true miniature submarine which could from a distance be mistaken for a conventional motor boat and which could hold two agents along with its crew of two men , and which could also carry up to one ton of supplies in sealed containers . The designed range was to be up to on the surface , with a range of at speeds of up to . While submerged it was to be capable of diving to a depth of and travelling up to underwater . Design work began towards the end of 1942 with the building of a 1 / 4 scale model which was used for tests in an experimental tank at Vickers ' plant at St . Albans . By February 1943 the tests had produced several alterations to the original design including modifications to the hull form to make it stable under tow at speeds of - . Prototypes . Following the model tests , permission was granted for the construction of a full size prototype at the SOE establishment known as " The Frythe " near Welwyn Garden City . The first prototype had little resemblance to the later versions . When launched at Staines in May 1943 it permitted further tests but showed flaws in that it was not stable under tow at speeds over and lacked freeboard . A second prototype was begun in June and was tested throughout the autumn and winter . A third prototype was begun in September 1943 . Its surface range - - exceeded the specification , but at the expense of reducing the storage capacity by . Its speed was less than the specification , at maximum while surfaced , with a mere submerged . It did however perform satisfactorily when pressure tested to a depth of . Production . In early 1944 a specification for a final Mk III version was proposed by SOE , which eventually entered series production . The range was now specified as , with an additional using disposable tanks ( which however meant a loss of the diving capability ) . A further could be added if part of the storage capacity was used . The accommodation had also been enlarged ; as well as the two crewmen , 4 passengers could be carried on short voyages , or 3 for longer trips . The cargo capacity had also been enhanced to enable between 1 . 5 and 2 tons to be carried . Underwater endurance had also been augmented , the craft now being capable of supporting 6 persons for up to 40 hours submerged . Finally , surface speed had been increased to cruising speed , or maximum . Even the electric propulsion was upgraded : it could deliver 2 - 3 knots . By now it was apparent that it was unlikely that the Welfreighter would be needed in Europe , therefore consideration was given to its use in the Far East . Due to the urgency of production it was not possible to create a dedicated factory . As had happened with the Welman submarine the year before , production was contracted out to specialised engineering firms already engaged in production of war material . In the spring of 1944 the Letchworth company of Shelvoke and Drewry Ltd . was awarded a contract to produce Welfreighters . By early September an order for up to 34 Welfreighters was placed for delivery as soon as possible after October 1 . The company had no experience of ship construction , and was from the sea . Security surrounding the production of the Welfreighter was tight , and few non - vetted employees knew the truth until after the end of the war . Completed craft were transported at night under canvas covers to Station IX at Welwyn , where they were fitted out , the compass and periscope and secret equipment fitted . The craft were then balanced , trimmed , and pressure tested in the establishment ' s water tank . They were then transported onwards to Fishguard in Wales , for sea trials . Appearance . The Welfreighter outwardly resembled a conventional motor boat . While surfaced it was propelled by a Gardner 4LW diesel omnibus engine driving a 4 - bladed propeller , while underwater propulsion was provided by two electric motors , driving a pair of small propellers . Two " masts " on the craft ' s foredeck housed a Barr and Stroude periscope and a magnetic compass . A dummy mast and sail could be attached to these masts to help disguise the craft as a fishing smack . The foredeck was raised up to give some headroom inside the craft , and was fitted with small square viewports . To the rear of the main structure was a raised deck , beneath which were housed 6 high pressure air cylinders . To the rear of this area was a cargo well , fitted at the stern with a drop - down tailgate . This could house seven cylindrical drums of cargo . Carefully loaded so as to give a slight positive buoyancy , these drums could hold equipment for use by agents or saboteurs and could be floated out through the tailgate and towed ashore by agents , either swimming or using an inflatable boat . Each Welfreighter was manned by a Sub Lieutenant RNVR as Commander , and a Naval Engine Room Artificer ( ERA ) or Army mechanic ( REME ) . Up to four special agents could travel as passengers , although two would be more usual . These would commonly be Army personnel , who would be expected to assist with handling the craft while on the surface . Service . Europe . Before the Welfreighter became properly operational the secret war in Europe was nearly over . In late 1944 plans were discussed to deploy the craft to the Adriatic , to assist operations in the Balkans , where SOE was involved in supporting partisan s in northern Greece , Albania , and Yugoslavia . It was also suggested that Welfreighters could be used to plant mines in the entrances to German ports , but this was dropped when it was demonstrated that a suitable charge would have to weigh over , which would be too much for the craft ' s buoyancy and trim . Trained Welfreighter crews were ready for operations through the autumn of 1944 , but neither Combined Operations HQ or SOE ever required anything of them . Far East . Two Welfreighters were shipped to Fremantle , Australia , in late 1944 , to become part of the clandestine Services Reconnaissance Department ( commonly known as Z Special Unit ) . Extensive trials were conducted , including towing trials and exercises using high powered motor launches and Motor Torpedo Boat s .