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.

[1] 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.

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.

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.

Following the model tests, permission was granted for the construction of a full size prototype at the SOE establishment known as The Frythe in Welwyn.

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 7 knots (13 km/h) and lacked freeboard.

Its surface range - 1,000 miles (1,600 km) - exceeded the specification, but at the expense of reducing the storage capacity by 240 pounds (110 kg).

In early 1944 a specification for a final Mk III version was proposed by SOE, which eventually entered series production.

Each Welfreighter was manned by a Sub Lieutenant RNVR as Commander, and a Naval Engine Room Artificer (ERA) or Army mechanic (REME).

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 partisans 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 224 pounds (102 kg), which would be too much for the craft's buoyancy and trim.

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).