The Evolution of US Combat Diving. Part 3, the SDV.

Exploring the evolution of combat diving.

This is Part 3 of 7 taken from my Submarine Medical Officer Thesis “A Review of the Combat Diving Evolution of US Naval Special Warfare with a Focus on the Necessity of Disruptive Technologies and Innovative Scientists.” Stay tuned for part 4 next week.

The Swimmer Delivery Vehicle

Another diving related invention that drastically expanded the capabilities of the US Navy combat diver was the Swimmer Delivery Vehicle – later renamed the SEAL Delivery Vehicle (SDV). An insightful report from 1952 stated, “When it is necessary to operate near an enemy-held shore in as complete secrecy as possible, the approach to the objective must be made underwater, and the final leg must be made by swimming or in a small submersible. Moreover, since many coasts throughout the world have shallow gradients, scuba-equipped combat swimmers would run out of gas and/or be overly fatigued without the benefit of a small powered submersible.”1

Although the closed-circuit re-breather greatly enhanced the underwater stealth of the UDTs, the SDV profoundly expanded their nautical range and payload carrying capacity. Until the advent of the SDV, the divers’ range was limited by the dive time of the specific UBA utilized, and the amount of gear infiltrated was dependent upon the individual SEAL’s carrying capacity; but with the SDV, this dynamic changed.

The Early SDVs. The SDV, although not officially used in combat by the UDTs until 1972, has a lineage that stretches back to WWII and even earlier.2 When Dr. Lambertsen rescued his LARUs from retirement for the UDTs in 1947, he also recovered two “Sleeping Beauty” submersibles (Fig. 8) that were used by the OSS MU in WWII. The OSS had borrowed this technology from their British counterparts – the Special Operations Executive (SOE), another highly secretive WWII intelligence and special operations unit. The “Sleeping Beauties” were motorized submersible canoes that were twelve feet long, weighed 600 pounds, and were capable of delivering combat divers to an underwater objective.3 After the successful USS Grouper diving operation in 1948, Dr. Lambertsen and LCDR Fane performed additional testing from a different submarine – the USS Quillback – but this time they brought a Sleeping Beauty. With Dr. Lambertsen piloting the mini-submersible, he intercepted the Quillback then attached a bow line from his Sleeping Beauty to a buoy towed by the submarine. Then he piloted the early SDV down to a cradle on the deck of the Quillback to be recovered by a waiting crew of divers. Essentially, Dr. Lambertsen spearheaded the first “SDV” training evolution.4  

The UDTs continued experimenting with the Sleeping Beauties and various other inventions including a fully enclosed British mini-submarine – an idea that was initially greeted with great enthusiasm, but was eventually scrapped to maintain independence from submariner authority. Over the next few decades, the UDTs continued testing new equipment and capabilities with the hope of eventually transporting combat divers underwater further and faster than swimming alone.6

In the 1950s, the UDTs initiated efforts to develop their own “wet” submersibles. After discarding a few undesirable early prototypes, the UDTs built the MK VI SDV in 1964 with the help of Naval Coastal Systems Center in Panama City. The MK VI evolved and led to the development of a much more stable and reliable MK VII, which was the first SDV to be utilized operationally.

In June 1972, the Navy initiated a highly secretive prisoner of war (POW) rescue mission (OPERATION THUNDERHEAD) off the coast of Vietnam utilizing two MK VII SDVs.7,8 The USS Grayback, a unique submarine modified for SDV operations, snuck under the US fleet undetected and inserted a four man UDT/SEAL SDV team on an island just off the coast of Vietnam. They were to infiltrate the island and watch for a prearranged signal from a boat that was to be carrying the escaped POWs. However, unknown to the men in the SDVs, the POWs were unable to communicate their failed escape attempt. During the remainder of the bungled operation, and after a series of miscommunications and missteps, a SEAL Lieutenant was killed and another SEAL was seriously injured,9 two MK VII SDVs were destroyed, and the Grayback was attacked by a US Navy ship. Despite the operational failure, the mission popularized the idea of transporting combat divers faster and further while remaining undetected underwater, but it also illuminated the need for an improved version of the MK VII SDV.10

The MK VIII and MK IX SDV. The Navy began development of the MK VIII SDV (Fig. 9) at a secret test facility in California. The MK VIII, nicknamed “the bus” and “the Eight-boat,” was larger than previous SDVs – carrying 6 divers – and included more advanced controls than previous models. The “Eight Boat” served as an underwater taxi, delivering SEALs to their underwater objectives over extended distances with a much larger payload than previously possible. The MK IX SDV (Fig. 10) was built at the same time with a different mission in mind, and remained in service until 1991. The MK IX was a flatter, sleeker, more maneuverable SDV that carried two divers in the prone position and was primarily used for reconnaissance and mine warfare. It was later equipped with two torpedoes that could be used to attack ships from a distance with the SDV still submerged.11,12,13 Having learned to operate SDVs effectively with earlier models, the UDT/SEALs “finally had a unique capability for operating clandestinely under the seas over relatively long distances.”14

The Impact of the SDV. Of note, not only did the SDV add a valuable capability to the NSW arsenal, but it also contributed to the complete reorganization of US Naval Special Warfare (NSW). In 1983, at a meeting in Washington D.C., NSW leadership decided to decommission the UDTs and create two SDV Teams because as one SEAL stated, “Operating SDVs is probably the hardest thing we do in Naval Special Warfare. The care, shepherding, and husbandry required by the SDVs are beyond the normal training of a SEAL. Keeping them moving, and the SEALs within them breathing, is a very technical and demanding specialty.”17 But this reorganization was ultimately little more than a renaming and a shuffling of personnel and equipment. The UDTs were essentially “absorbed” by the SEAL teams and the SDV SEALs worked in somewhat of a subspecialty of the SEAL Teams.18 While all SEALs are trained combat divers, the SDV teams are the combat diving experts. “We do concentrate our diving expertise in the SDV teams. Those are the people who spend the most time, the most hours; I should say days or weeks per year underwater. The rest of the SEALs are relatively short duration underwater swimmers.”19

Problems with reliability plagued the SDVs initially, but by the 1990s, as LCDR Tom Hawkins, the first commander of SDV Team Two stated, “They got extremely good at what they did.”20 Another SEAL said, “I feel the SDV is the future of naval special warfare…The ability to do things very few people can do is very satisfying to me. I can do what they can do. They can’t do what I can do.”21 But the SDV needed one final technological piece – the Dry Deck Shelter – before the growing SEAL combat diving arsenal and infrastructure would be complete.

Continued in Part 4…


  1. Joiner, Naval Forces: A Look Back, 2-14-15.
  2. Kelly, Brave Men, 157.
  3. O’Donnell, First SEALs, 134,235-238.
  4. Butler, “Closed-circuit Diving,” 3-20.
  5. OSS MU Operator Piloting a Sleeping Beauty, picture, accessed December 19, 2015,
  6. Kelly, Brave Men, 162-165.
  7. Ibid., 151-157, 163-164
  8. Kelly, Never Fight Fair, 205-214.
  9. The SEALs were killed/injured when casting from a helicopter at night. The SEALs jumped out of the helicopter much higher than planned due to the inability to see the water well.
  10. Ibid.
  11. Joiner, Naval Forces: A Look Back, 2-15
  12. Joiner, Naval Forces: The Rest, 85
  13. Kelly, Brave Men, 163-164
  14. Kelly, Brave Men, 165.
  15. MK VIII SDV, picture, accessed December 19, 2015, 2013/03/NovSEAL-053.jpg.
  16. MK IX SDV, picture, accessed December 19, 2015,
  17. Dockery, Complete History, 573
  18. Ibid., 573, 559
  19. Joiner, Naval Forces: A Look Back, 2-71
  20. Kelly, Brave Men, 165-166.
  21. Ibid., 173.

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