In 1952 a then-classified report, “Underwater Swimmers,” prepared for the Office of Naval Research, Panel on Underwater Swimmers, Committee on Amphibious Operations of the National Research Council, described the basic task of swimmer delivery vehicles (now called SEAL Delivery Vehicles, or SDVs). These prescient words are just as relevant today as they were more than 60 years ago:

“Whenever it is necessary to operate near an enemy held shore in as complete secrecy as possible, the approach to the objective must be made under water. The first part of the approach can be made in a fleet-type submarine, but these 1500-ton vessels cannot operate submerged in water shallower than 60 feet, and depths less than 150 feet are considered hazardous. The final submerged approach must be made by swimming or in a small submersible. On many coasts throughout the world, depths less than 60 feet extend out several miles from shore. In these areas even men equipped with SCUBA would not have enough breathing gas to swim the distance and return. Moreover, they would be seriously fatigued when they reached their objective after their swim of several hours. To supplement their swimming, they must have a small, powered submersible.”

SDVs are employed by U.S. Navy SEAL operators. They are small free-flooding submersibles that can transport up to six combat-equipped SEALs. Free-flooding means that the SEALs are surrounded by sea water during the entire mission, where they breathe compressed air from the SDV’s internal life-support system or from tactical underwater breathing apparatus.
Until formation of the Naval Special Warfare SEAL Delivery Vehicle Teams in 1983, they were known as Swimmer Delivery Vehicles in Navy doctrinal publications. SDVs have and continue to play an unseen and literally unknown yet vitally significant role in maritime special operations.

Establishment of the U.S. combatant submersible capability began during WWII by the Office of Strategic Services Maritime Unit (OSS MU). A “submersible canoe” was developed by the British during World War II and called the “Sleeping Beauty.” It was employed by OSS MU during extensive training and exercises, but was never actually deployed for combat operations. The same capability was adopted by the post-war Underwater Demolition Teams (UDTs) in 1947. The one-man submersible displayed little functional military potential; however, it substantiated and characterized the need for improved and expanded UDT capabilities.

Sleeping Beauty ashore
Sleeping Beauty ashore
Sleeping Beauty under way.
Sleeping Beauty under way.

Modern-day SDVs are powered by lithium-ion batteries and boast state-of-the-art navigation, communication, life-support systems, and weapons-delivery system capabilities. They can also be transported, launched, and recovered from surface ships. Typical missions include reconnaissance and surveillance, ship and harbor attacks, infiltration and exfiltration into denied or politically sensitive areas, and mine reconnaissance and hydrographic surveys.

The Navy SEAL Museum currently displays three eviscerated SDV models that span the contemporary developmental period: Mk 8, Mod 0, Mk 9, Mod 0, and Mk 7, Mod 6.

SDV Mark VII (Mk 7)

Mark VII Mod O SDV

Mk 7 SDV was the first production SDV accepted by the U.S. Navy and the first truly reliable submersible designated for deployment and combat use by the UDT and SEAL Teams. There were six different models or configurations of this SDV, where each involved often significant design and engineering changes for upgrades in safety and capability. The Mk 7, Mod 6 SDV is displayed in the Navy SEAL Museum’s collection. This SDV began experimental service in 1967, and conducted its first mission off the coast of North Vietnam in June 1972.

Depending on passenger and cargo (weapons and ordnance) configurations, this SDV could transport the SEAL pilot and up to three additional SEALs; two divers sitting back-to-back in forward and aft compartments – separated by the SDV’s main ballast tank (see photos). The acoustic and magnetic signature of the SDV was minimized by the use of non-ferrous materials and a fiberglass hull. Sliding canopies over the forward and aft compartments provided easy access for personnel and payloads, while facilitating battery and component removal during maintenance. Its instruments and other electric components were housed in water-tight containers; pressure proofed up to the depth of several atmospheres. This SDV had an integrated compressed air life-support and ballast systems. It was propelled by an electric motor, powered by rechargeable silver-zinc batteries, and employed a single screw configuration.  Later SDV models were equipped with a Doppler navigation system, forward-looking sonar system, and a submarine rendezvous and docking system that assisted in recovery to the host submarine. The basic design features of the Mk 7 SDV can be found in the Mk 8, Mk 9, and other combatant submersible configurations.

Mark VII SDV

SDV Mark VIII (Mk 8)

The Mk 8 SDV on display at the Navy SEAL Museum was designed for personnel and weapons delivery. It began replacing the Mk 7 SDVs in the early 1980s. The Mk 8, Mod 0 could carry six fully equipped divers or two or three divers and weapons, which included the Mk 4 and Mk 5 limpet mines and the Mk 36 charge, which was also a limpet. The SDV’s pilot and navigator sat side-by-side during transit (see photos).

The Mark VIII, Mod 0 SDV was propelled by electric motors. It was powered by various kinds of rechargeable batteries and employed a single screw configuration.  This SDV was equipped with a Doppler Inertial Navigation System (DINS), forward-looking obstacle avoidance sonar (OAS) system, and a submarine rendezvous and docking system (RDS) that assisted in recovery to the host submarine. Through a long-term modernization program, the SDV was upgraded to a Mod 1 configuration, which remains in service with continually improving capabilities.

MK 8 SDVs were used to accomplish classified mine reconnaissance and demolition missions during OPERATION Desert Storm. During OPERATION Iraqi Freedom they were used to secure off-shore gas and oil terminals. SDV operations have been and continue to remain substantially classified.

Mark IX SDV

Mark 9, Mod 0 SDV (Mk 9)

The Mark 9, Mod 0 SDV (Mk 9) on display at The National Navy SEAL Museum was designed primarily for hydrographic reconnaissance and weapons delivery. It could carry two fully equipped SEALs and a family of underwater weapons. The Mk 9 SDV employed a low-profile design, because it was intended for ship attacks in harbors and anchorages and near-shore, shallow-water reconnaissance missions. The pilot and navigator operated the SDV while riding side-by-side in a prone position (see photos). This SDV was propelled by electric motors, powered by rechargeable silver-zinc batteries, and employed a twin-screw configuration to provide increased maneuverability in shallow water. Like its companion Mk 8, Mod 0 SDV, it was equipped with a Doppler Inertial Navigation System (DINS), side-looking and forward-looking obstacle avoidance sonar (OAS) systems, and a submarine rendezvous and docking system (RDS) that assisted in recovery to the host submarine.

A large cargo compartment was located behind the pilot and navigator’s feet that could carry satchel demolition charges and firing devices, the Mk 4 and Mk 5 limpet mines, and the Mk 36 charge, which was also a limpet. The SDV could also employ the Standoff Weapon System (SWA), which was a modified U.S. Navy Mk 37 submarine torpedo; a system designed specifically for the Mk 9. The integrated SDV-SWA could be transported by the submarine Dry Deck Shelter (see photos below). The SWA completed formal operational evaluation; however, it was ever operationally deployed, because the MK 9’s were divested from the active inventory in 1982 to avoid manpower and budget redundancies. Also, because the Mk 8 SDV could accomplish all of the Mk 9 SDV missions – except the SWA, and this was only because of DDS internal diameter limitations.

SDV Submarine Deck Shelters (coming soon)

See SDVs in action; including operations from the DDS: www.dvidshub.net