The Welfreighter concept.
It was obvious that the insertion and recovery of agents, arms and intelligence from behind enemy lines during WW2 presented a major risk to all concerned. Before the days of helicopters and reliable parachuting techniques such insertions could only be conducted in safety by fixed wing aircraft to covert landing zones, or by sea. With the proven success of the X craft midget submarines a fresh attempt was made by SOE's technical division, ISRB, to create a submersible craft specifically designed for covert missions: insertion and resupply of agents behind enemy lines; carry out intelligence gathering work off hostile coastlines; or deliver explosive depth charges to enemy shipping channels. It was this design that became the Welfreighter.
The initial concept was remarkable. A true mini submarine, but one that from a distance could be mistaken for a coastal motor boat, that could accommodate up to two agents alongside the crew of two, and which could also transport up to one ton of stores strapped in sealed containers on its deck. A self-sufficient vessel that having been towed by ship or parent submarine to the scene of operations, could operate independently for up to a week. The designed range was to be up to 600 miles on the surface, with a radius of action of 200 miles or more, at speeds of up to 8 knots. Once submerged it should be capable of operating to a depth of 130ft, while running on electric motors. Underwater range should be over 40 miles, or with 4 personnel aboard, submerged endurance should be up to 10 hours. It is hardly surprising that the existence of this craft was classified for so long.
Design work commenced in late 1942 with the construction of a 1/4 scale model which was used for tests in an experimental tank at Vickers in St Albans. By February 1943 the testing programme had produced several alterations to the original design. Modifications produced a form that was stable under tow at between 10 and 15 knots. Authorisation was granted for the construction of a full sized prototype in the workshops at the Frythe.
This, the first prototype Welfreighter, had an appearance nothing like the the later versions. It has been described as resembling the front end of a submerged London bus. Nevertheless when launched for trials at Staines in May 1943, it did permit tests which confirmed some of the design criteria. The main failures were that it was not stable under tow at more than 7 knots, and it lacked freeboard.
In June 1943 work was commenced on an operational prototype. This second craft (which is the first proper Welfreighter - and I shall call No 1 ) ) was tested through the Autumn and winter of 1943. But many problems were encountered, which kept the design team at the Frythe occupied. The craft was very sensitive: balance and trim threw up many design challenges.
In September 1943, as the work at the Frythe developed, the construction of a second operational prototype craft was authorized with a budget of £ 3000.00 This Mk II prototype craft does not appear to have met the design brief either. While the surface range, at 1000 miles, actually the exceeded the specification, this seems to have been at the expense of stores capacity - some 240lbs less than required - at 2000 lbs.. The endurance under water is stated as 10 hours for 4 people, but the submerged range is not given, although a reference to 15 miles does exist. The craft was incapable of a performance anywhere close to the required speed range. The potential on the surface was only 6 knots maximum (5 knots cruising) while somewhat less than 2 knots was possible while submerged. However the diving performance was on target. It could submerge totally within 60 seconds descending to an operational depth of 50 ft. The craft also seems to have performed successfully when pressure tested to 130 ft maximum.
The development work on the Welfreighter continued through the winter of 1943 at the Frythe. Prolonged diving tests lasting many hours were conducted on the prototype submerged in the great tank in the hanger there. More advanced tests were conducted in the Queen Mary reservoir at Staines.( Station IX A) From the Autumn of 1943 Station IX established a testing base at Fishguard, ( at first known as Station IXc) using the harbour facilities at Goodwick. This was the pre war Great Western Railway pier, from which services to Ireland were suspended during WW2.
One problem for the ISRB team was that, in the end, the craft would need to be manned by Officers and crews drawn from the Royal Navy. The Admiralty had their own standards for submersible craft, and their own design / development organisation. (This had already successfully produced the Chariot and the X craft.) The Royal Navy could draw on decades of research and hard won experience. The design team of ISRB, although highly skilled engineers, were in the main not Naval people. Because they lacked Naval experience and training ISRB engineers were regarded by the Admiralty as incapable of designing anything worthwhile for use at sea.! This led to great friction and problems during trials.
The delays in the Welfreighter testing programme led to questions at high level. SOE wanted to have this idea Operational. Plans that required such a craft, were under consideration for the Adriatic. However the Admiralty continued to insist that they would decline to accept any craft that failed to meet exacting naval standards. By late spring 1944 it appears matters came to something of an unsatisfactory compromise.
The prototype Welfreighters required very careful handling at sea. To remain on the surface ballast tanks had to be kept pressurised. While on the surface, under way, the exhaust from the diesel engine was diverted to maintain constant air pressure within the side buoyancy tanks. Once these side tanks were flooded the craft would submerge. Ballasting he craft and its cargo to acheive "flooded trim" which was necessary to operate submerged was achieved by adjusting the contents of a compensating tank. Fore and aft Trim of the hull underwater was adjusted by pumping a little of the contents of the compensating tank into a trim tank. To regain the surface the craft depended on 6 high pressure compressed air cylinders, located in the rear well deck. This compressed air was needed to pressurise and empty the flooded tanks. First the side tanks had to be pressurised to displace the water Once the craft reached the surface an electric blower would be used to displace the contents of the forward main ballast tank. Only then could the main engine could be used, and the craft proceed on the surface.
The Royal Navy held the opinion that this sequence would not be satisfactory under operational conditions. Concern was also expressed about the conditions for the crew. The reserves of air for the crew while submerged were far below Naval standards. In the end a system for CO2 absorbtion, using protosorb crystals, was developed by ISRB engineers for the Welfreighter. This mechanism went some way to resolving the oxygen issue. Repeated attempts were made by ISRB engineers to provide solutions that would satisfy and address other Admiralty requirements. Many months of delay resulted and several engineering issues were never resolved to Royal Navy satisfaction. In the end pressure from SOE forced the project forward, and in June 1944 the Admiralty authorised the firm of Oppermans to begin construction of an initial 6 Welfreighters for delivery by 1st October 1944.
In early 1944 a specification for a Mk III version had been proposed by SOE. It is thought that this is the final version: the one that entered series production. However it was only in June 1944 that SOE were permitted to develop these ideas. (Due to this sequence of events it seems reasonable to conclude that the order with Oppermans would have been for a much revised version of the Mk II .)
The range of the Mk III Welfreighter was now specified as 1000 miles with full diving capability, with an additional 1000 miles using jettison tanks (losing diving capability). A further 600 miles was possible if part of the stores capacity was used: Total 2600 miles. The accommodation had also been enlarged. In addition to the two permanent crew, 4 passengers/agents could be carried on short runs, reducing to 3 for longer distances. The Freight capability had also been enhanced to enable between 1½ and 2 tons to be transported. For the Mk III craft the underwater endurance had been increased. It could now support 6 persons for up to 40 hours submerged. Finally the speed on the surface had been improved to give a 5½ knot cruising speed - 7 knot maximum. Even the electric propulsion was upgraded: it could deliver 2-3 knots. While under tow by a parent craft or submarine -theoretically at least -speeds of up to 18 knots were claimed.( This towing speed is thought very unlikely - X craft while submerged were never towed faster than 10- 12 knots....it proved impossible to control the trim at higher speeds.)The diving performance, however, remained the same as the Mk II. Capability and endurance had been improved and the specification was now for a craft that had even greater potential.
Later that summer the Admiralty reconsidered the contract awarded to Oppermans concluding that they would be unable to meet the timescale. As a result the Oppermans contract was withdrawn: they were instead to produce just one prototype.( The Welfreighter designated WF2 in this history is presumed to be this version.)
It was clear by now that the war in Europe was going in the Allies favour. It had become unlikely that the Welfreighter would be needed in Europe. Therefore consideration was given to its use in the Far East, in the war against Japan.
By early September a fresh proposal led to the placing of an order for up to 34 Welfreighters with Shelvoke and Drewry Ltd in Letchworth, for delivery as soon as possible after 1st October. ( It is presumed that these would have incorporated the latest specification from ISRB, and would therefore have been the Mk III version.)
Within SOE the operational potential for which use of the Welfreighter could be considered had also undergone substantial revision. No longer simply a craft for brief covert missions along hostile coastlines, the Mk III was judged to possess the endurance capability to perform almost completely independent from support craft on missions of substantial duration.