The SDV MK 6 was the U.S. Navy’s experimental swimmer delivery vehicle. After testing and development on this craft, production began on the SDV MK 7. It was a gutted PR-77 rebuilt from the inside out with a range of experimental systems. Its round fiberglass hull had a superior guidance system but proved difficult to stabilize and too small for crew and cargo. The hull was extended 4 ft to carry 4 men, and a completely new tail configuration was added to make it much more like a submarine. The SCUBA worn by its occupants for deep demolition work, however, left a tell-tale bubble trail. After the roll out of the SDV MK 7, the SDV MK 6 was used as a test-bed.