From World War II’s human torpedoes, to today’s Swimmer Delivery Vehicles (SDVs), concepts for transporting crew, weapons and equipment covertly into areas of operation are being reinvented by navies. 

By Anita Hawser 

One of the major sea victories in World War II was the Italian Navy’s attack on two British battleships, HMS Queen Elizabeth and HMS Valiant, and the tanker Sagona, in Alexandria Harbour in 1941. The Navy crept into the harbour late at night undetected astride three or four 1.5 tonne torpedoes, which had two detachable warheads clamped on their nose. These so-called “human torpedoes” —  Siluro a Lenta Corsa (Low Speed Torpedo – SLC) — got within 50 to 60 yards of their targets before submerging to carry out the critical final stages of their attack: attaching the warheads to the ships in the harbour.

Following the success of the 1941 attacks, more SLCs were manufactured, but by 1943 Italy had surrendered and did not get to use them in operations. However, the element of surprise that these human torpedoes brought to the battlespace left a lasting impression on the UK, which decided to develop its own version, the Chariot manned torpedo, which was towed by a submarine. The Chariot later gave way to "midget" submarines like the X-Craft, which had a greater range and were used by the British Navy against German warships and in preparatory operations for the D-Day landings. 

Germany also developed various versions of a manned torpedo; the Neger, intended for one man; and the more sophisticated Marder. In 1944 these torpedos carried out successful attacks against allied vessels. At the end of the war, the focus shifted from using manned torpedos to attack ships to reconnaissance. Elements of the human torpedo — particularly their shape — are still found today in Swimmer Delivery Vehicles or SDVs, which are used to insert and extract Special Forces and their equipment close to shore so they can carry out covert operations. 

Today’s SDVs are more about helping Special Forces and combat divers overcome the limitations of their  equipment and being exposed to water for long periods of time before they even reach their destination. “After five hours hard swimming, you’re not much of a soldier,” said Anders Magnerfelt, managing director of JFD Sweden, speaking at Underwater Defence Technology (UDT) in Stockholm in May. Thanks to lithium-ion batteries, today’s SDVs have much longer endurance than WWII’s human torpedos. Some SDVs even have heating systems to keep divers warm in colder waters.

JPD’s SEAL Carrier, which operates in three modes — surface, semi-submerged and submerged — can carry up to six divers and travel at speeds in excess of 30 knots, with a range of more than 150 nautical miles on the surface and more than 15 Nm under water. “SDVs will continue to develop their speed, endurance and performance,” says Magnerfelt. He also anticipates that we’ll see more ‘hybrid’ craft, which can operate on the surface and under water.  The advantage of hybrid craft, compared to traditional SDVs that can only operate under water, says Magnerfelt, is that they can use their high surface speed to get out of tricky situations. "This capacity is especially useful in shallow water operations where the manoeuvre space is limited," he says. 

Other advances in SDVs are centred round keeping forces drier and more comfortable as they transit to their mission. In the mid-1990s, the US developed a dry Advanced SEAL Delivery System (ASDS) or midget submarine, which would have transported SEALs in a dry environment. The ASDS was launched from Los Angeles and Ohio-class submarines, but the programme was eventually abandoned in 2009 due to various technical, cost and reliability issues. 

The Shallow Water Combat Submersible, which will transport a crew of six or more, is now being developed as a replacement for the Navy SEALs Mk8 SDVs. The US also plans to build a long-range Dry Combat Submersible to carry a crew of six over longer distances deeper into enemy territory. In September 2018, the UK requested to buy three SEAL SDV MK 11 Shallow Water Combat Submersibles from the US at a total estimated cost of $90 million to replace its SDVs. Both the UK and US navies use Dry Deck Shelters to launch SDVs from specially modified submarines. 

Magnerfelt says JFD will develop constructs around craft that can go underwater and have a dry chamber to ensure that Special Forces are well and truly mission ready by the time they reach their destination. “The diver has limitations,” he explains. “Spending six to 10 hours in cold water breathing different gases is a tough job. If the divers can stay out of the cold water they’ll be able to do their job much better.” JFD has expertise in pressure hulls and in building fast craft and advanced diver systems. “With a pressurised hull, you can increase the endurance so divers can stay in an area for longer,” says Magnerfelt. “It also means you can use the platform for mission support, as a command and control platform, or as an ISR platform.”

At UDT in Sweden in May, Ben Kinnaman, CEO of manned and unmanned navigation specialist Greensea Systems, talked about a precision navigation system for Diver Propulsion Devices (DPDs) or SDVs, which could turn them into optionally manned or fully autonomous vehicles able to conduct a wider range of missions (mine countermeasures, ISR, payload delivery, search and rescue) than just crew and equipment transportation. “We’re turning the DPD into an unmanned underwater vehicle,” said Kinnaman.

Greenseas technology forms the basis of the RNAV2 Precision Navigation and Automation System, which has been integrated into US-based STIDD’s DPDs. The navigation system enables the vehicle to transition between manned, semi-autonomous and fully autonomous operations. The RNAV2 can also be dismounted from the DPD and used by swimming divers in hand-held mode. “Putting an advanced navigation system into a DPD makes for a smarter vehicle,” says Kinnaman, adding that its focus is on assisting the diver by allowing them to revert to autopilot mode and programme in way points, mission planning and predetermined tasks.

The 'human torpedo' has come almost full circle. From its beginnings in World War II, today's SDVs are not only an essential mode of transport for crew and equipment, but could enable forces to operate under the water for longer periods and perform a much wider range of missions. 

"After five hours hard swimming, you’re not much of a soldier.”

A US Navy Dry Deck Shelter for storing Swimmer Delivery Vehicles on submarines (US Navy photo)