The Welfreighter outwardly resembled a conventional 37 feet (11 m) motor boat. While surfaced it was propelled by a Gardner 4LW 44 hp (33 kW) diesel omnibus engine driving a 4 bladed propeller, while underwater propulsion was provided by two 2 hp (1.5 kW) 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 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.