The World War Two SDV called Sleeping Beauty

Flag The Sleeping Beauty was built for Special Operations Executive (SOE) in 1942 as a means of attacking enemy shipping in port, in a similar manner to the two-man Italian 'maiale' (pig) human torpedoes which had been used against the British. Although there was a direct copy of the maiale called the Chariot Mk.I, the Sleeping Beauty (aka Motorised Submersible Canoe) was an entirely British concept. It combined the best features of metal canoes such as sea-keeping and compactness, with the engineering required for underwater travel. Its most distinctive feature was that it was a single-man craft with the pilot sitting towards the back in an open cockpit. When the craft submerged he had to wear breathing apparatus. Less well appreciated is that unlike the short-ranged maiale, the Sleeping Beauty was designed to operate over relatively long ranges using its electric motor to travel on the surface as well as submerged, and with paddles and/or a sail as a fall-back if the batteries ran out.

Sleeping Beauty MSC Motorised Submersible Canoe

The Sleeping Beauty has never attracted the attention it deserves, possibly because:

a) the secret was too well kept for too long

b) it was not used on any successful ship attack missions.

The design was however hugely influential and it was the main SDV for many years after the war, basically until all the remaining units were worn out. It was also part of the famous but ill-fated Australian Z-Force raid on Singapore called Operation RIMAU (aka Operation HORNBILL). Even less well known is that in the immediate postwar years it was tested by US Navy Underwater Demolition Teams (UDT) and is considered one of the direct forefathers of the modern US Navy SEALs Swimmer Delivery Vehicles (SDV, a term only coined in the 1960s).
Sleeping Beauty MSC Motorised Submersible Canoe
Wartime shot of Sleeping Beauty (Underwater Heritage Trust)

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Underwater Heritage Trust
I cannot write about the Sleeping Beauty without mentioning the incredible work carried out by the Underwater Heritage Trust. The Trust (website) started out recording the secret history of the British Chariot Mk.I and Mk.II submersibles and corresponding Italian craft - the Trust's links to the famous Italian Decima-MAS frogman and their postwar descendent unit the COMSUBIN ('Incursori') are strong. This led to a project to preserve and display surviving examples including a Chariot Mk.II and an Italian SSB 'maiale'. These craft are extremely rare but no Chariot Mk.I survived so they went down the truly exceptional road of building a functioning replica Chariot Mk.I which, along with a real Mk.II is on display at Eden Camp (website).

Their more recent project has been to build a faithful replica of the Sleeping Beauty. This took many years of hard work and has resulted in an excellent copy. Of course replicas inevitably lack the depth of the real thing, but the Trust's efforts are still incredibly important for preserving the memory of these craft and the heroes from several nations who used them, some losing their life in the process.
Sleeping Beauty MSC Motorised Submersible Canoe
The Underwater Heritage Trust's Sleeping Beauty on display at the Natural History Museum, London. Photos Jonathan Jackson of National History Photo Unit.
Sleeping Beauty MSC Motorised Submersible Canoe
Sleeping Beauty MSC Motorised Submersible Canoe
Robert Hobson (r) and Mike Jones (l) of the Underwater Heritage Trust stand with the faithful replica Sleeping Beauty at a recent event.

Length: 12 ft 8 in (3.86 m) (versions vary slightly)
Beam: 2 ft 3 in (0.69 m)
Displacement: 600 pounds (270 kg)
Speed: 4.4kts max, 3.5kts cruise
Endurance: 40 nautical miles
Maximum Operating depth: 50ft (15m)
Personnel: 1 crew. It was possible to carry a second an on the deck and a two-an version was built

Images from Underwater Heritage Trust
Sleeping Beauty MSC Motorised Submersible Canoe Sleeping Beauty MSC Motorised Submersible Canoe