Swimmer Delivery Vehicles (SDVs) are midget submersibles designed to transport frogmen from a combat swimmer unit or naval Special Forces

Special forces

Special forces, or special operations forces are terms used to describe elite military tactical teams trained to perform high-risk dangerous missions that conventional units cannot perform...

 underwater, over long distances. SDVs carry a pilot, co-pilot, and combat swimmer team and their equipment, to and from maritime mission objectives on land or at sea. The Pilot and co-pilot are often a part of the swimmer team.

For long-range missions, SDVs can carry their own onboard compressed air supply to extend the range of a swimmer's own air tank or rebreather equipment.

There are two kinds. The "wet" type, where combat swimmers ride on the outside, exposed to the water. Examples of wet SDVs include the American "SEAL Delivery Vehicle

SEAL Delivery Vehicle

The SEAL Delivery Vehicle or is a manned submersible and a type of Swimmer Delivery Vehicle used to deliver United States Navy SEALs and their equipment for special operations missions....

".

There is also the "dry" type, where swimmers ride in a compartment inside, not exposed to the water. Examples include the American "Advanced SEAL Delivery System

Advanced SEAL Delivery System

Advanced SEAL Delivery System was a midget submarine operated by the United States Navy and United States Special Operations Command that is designed to provide stealthy submerged transportation for special operations forces from the decks of nuclear submarines for primary use as an insertion...

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SDVs are used in maritime missions such as infiltrating combat swimmers into an enemy port or planting limpet mines on the hull of target vessels. They are also used to land a combat swimmer team covertly on a hostile shore in order to conduct missions on land. After completing their mission, the team may return to the SDV to exfiltrate back to the mother-ship.

For extended missions on land, a team can be re-supplied by contact with other SDVs.

The origins of the SDV stems from the Motorised Submersible Canoe

Sleeping Beauty (canoe)

The Motorised Submersible Canoe , nicknamed Sleeping Beauty, was built by British Special Operations Executive during World War II as an underwater vehicle for a single frogman to perform clandestine reconnaissance or attacks against enemy vessels.-Design:The MSC was designed by Major Hugh Quentin...

created during WWII.