The USS Dolphin (AGSS 555) was unique in a number of ways. Commissioned in 1968, it was the last diesel-electric boat built for the U.S. Navy. Unlike contemporary nuclear-powered boats that were designed for speed and efficiency when submerged with their streamlined hulls, Dolphin was built to withstand the enormous pressures found at extreme depths. At the time of her commissioning she was said to be able to operate at greater depths than any known submarine.

“Has more sonar per ton than any other submarine in the world.”

It was built for deep diving research, to include oceanographic bottom surveys and studying underwater acoustics. According to the ship’s “welcome aboard” pamphlet when commissioned, Dolphin had “as much sonar equipment as the biggest Fleet Ballistic Missile submarines,” and “has more sonar per ton than any other submarine in the world.”

It could fire torpedoes using an externally mounted torpedo tube used for weapons testing, not combat. She was later fitted with a single bow tube, but that tube was eventually removed.

“We go so deep, where the pressures are so great,” the brochure said, “it is best to keep any irregularities in the hull, such as hatches, to a minimum.”

Unlike other U.S. subs, which featured weapons loading hatches, Dolphin had only one hatch. The welcome aboard brochure apologized to visitors about the inconvenience. “We go so deep, where the pressures are so great,” the brochure said, “it is best to keep any irregularities in the hull, such as hatches, to a minimum.”

The diesel electric propulsion system featured the latest in silver-zinc battery technology for submerged running. She had no snorkel to operate her diesels when running at periscope depth, and had to keep her single hatch open when her diesels were running.

If a significant casualty was not corrected in the appropriate time, the SSMS would blow main ballast tanks, move forward at full speed and set the stern planes to quickly rise to the surface.

Built at the Portsmouth Naval Shipyard in Kittery, Maine, Dolphin was equipped with a state-of-the-art Submarine Safety Monitoring System (SSMS) that monitored and logged key functions and parameters, such as pressures, temperatures and engine RPMs. It sounded alarms if something was outside of tolerances and could actually take control of the boat in extreme situations. If a significant casualty was not corrected in the appropriate time, the SSMS would blow main ballast tanks, move forward at full speed and set the stern planes to quickly rise to the surface.

At 1,000 tons, Dolphin was smaller than any of the Navy’s attack boats, but had a 12-ton internal payload, which was more than the other research vehicles available to the Navy at the time – Trieste, Aluminaut, Alvin, Deep Quest, Deep Star and Star 1 – combined.  What’s more, she could submerge for long periods, so her crew of three officers, 15 crewmembers, and four research scientists could remain at depth and on station for days instead of hours.

She could submerge for long periods  so her crew of three officers, 15 crewmembers, and four research scientists could remain at depth and on station for days instead of hours.

The 152-foot Dolphin pressure hull is a constant diameter cylinder, closed at its ends with hemispherical heads. She has operated as deep as 3,000 feet.

In a 1967 United Press International report, David Bradley wrote, “Technological advances that have gone into her construction are highly classified. If sea trials bear them out, they will become standard equipment on subs in the American fleet in the next 50 years.”

Heroic actions of the crew were instrumental in saving the ship.

In a 2002 incident, Dolphin was cruising on the surface off the coast of San Diego, Calif. recharging batteries when a flooding situation shorted electrical panels and started fires. Heroic actions of the crew were instrumental in saving the ship, but the crew had to be evacuated and the submarine support vessel Kellie Chouest towed back Dolphin back to San Diego the following day. She was repaired and returned to service in 2005 for one year before the Navy decided to decommission her.

Today, Dolphin is on public display at the San Diego Maritime Museum.