Built during World War II, Liberty cargo ships were versatile and could be modified as training ships (Z3-EC2-SC1), tankers (Z-ET 1-S-C3), freshwater distilling ships (two modified Z-ET1-S-C3 tankers designated as AW-1 and AW-2 by the Navy), colliers (EC2-S-AW1), boxed aircraft carriers (Z-EC2-S-C5), tank carriers (Z-EC2-S-C2), repair and maintenance ships, animal transports, troop transports, Army aircraft repair ships, and hospital ships. Many were converted after the war to serve as surveillance radar ships supporting the Distant Early Warning (DEW) Line or as missile range instrumentation ships with a variety of high-tech antennae.1 One Liberty was even converted to a floating nuclear power plant (the former SS Charles H. Cugle renamed the USS Sturgis [MH-1A]).2 Perhaps the most unusual modification was a little-known proposal by the U.S. Navy to convert a Liberty ship into an experimental minesweeper equipped with aircraft engines for propulsion and buoyancy material to clear underwater mine fields after the war.
The Liberty selected for this experiment started out as the SS John L. Sullivan. She was built in Richmond, California, at the Permanente Metals Corporation and launched on 25 May 1943. The John L. Sullivan survived the war and was chartered by the military during the Korean War, after which she was obtained by the Navy and became the YAG-37. The YAG designation stood for “miscellaneous auxiliary service craft” and was used along with several other Liberty ships in different experimental projects.
Experimental Minesweeper Concept
During World War II, new pressure mines were dropped in Japanese waters by U.S. aircraft and were exceptionally hard to sweep at the end of hostilities. Unlike moored magnetic or acoustic mines, a pressure mine could only be detonated by a change in the pressure of the water surrounding it. Normally, this was caused by the passage of a vessel of substantial size over the mine. Although these mines were designed to neutralize themselves and become non-explosive after a short period of time, the only way to be certain that the mines were harmless was to run large ships over them. The German Navy used a similar concept during the war with “sperrbrecher” (or “barrage-breaker”) minesweepers, when 200 converted merchant ships were modified with reinforced hulls, loaded with buoyancy materials, and equipped with sweeping gear to sail ahead of Nazi warships to clear underwater mines.
In late 1945, several sacrificial U.S. vessels were tasked to develop a pressure wave over the mines that had the same characteristics of targeted vessels. The Navy used a troop ship (USS Marathon [APA-200]), a Victory cargo ship (SS Pratt Victory), and a Liberty cargo ship (SS Joseph Holt) for the “Guinea Pig Squadron” to sweep the seaways off Japan. A crew of 22 volunteers wore tank helmets to protect their heads while standing on mattresses or wooden gratings. The ship’s ceilings were padded with mattresses for added protection. The engines and boilers were operated by remote control from the main deck and flying bridge to avoid crew members having to work down below in the ship’s hull.3
Despite sweeping Japanese waters for over 300 missions, the Marathon was the only ship of the three to detonate a mine. It did so with no self-inflected damage.4 The Navy finally abandoned all minesweeping efforts in May 1946 with some 13,000 mines still undetected. However, 87 U.S. merchant ships sank between the Japanese surrender in September 1945 and 1950, in most cases by striking mines.
The Navy began investigating how minesweepers could be built to work in both shallow waters and the high seas. A minesweeper filled with buoyant material to potentially keep the vessel from sinking was an interesting proposition. The concept of having a vessel’s propulsion and steering components positioned above the weather deck, thus limiting potential damage to the rudder or propeller shaft, was also appealing. The Navy believed that using aircraft engines for vessel propulsion would avoid the risk of entangling propellers and rudders from underwater obstructions and could be used on other vessels such as tugboats, fuel lighters, transports, and amphibious assault craft. Commander William M. Nicolson with the U.S. Navy’s Bureau of Ships stated:
The job was to see what you could do experimentally to prove this. We took an old Liberty ship and filled the hull with Styrofoam so it wouldn’t sink. We got four spare engines, surplus jet [turboprop] engines off a Constellation airplane, mounted those engines on forty-millimeter rotating platforms, and drove the ship with these. We had four of them and could drive the ship at about eight knots. We named her DUMBO. They used up a lot of fuel. We took it down to Panama City and ran it through the minefields for pressure signatures. And it worked. It actually worked well enough that we got it into a shipbuilding program to actually build some, instead of this experimental platform. We were trying to find some way of sweeping pressure mines in particular because nobody had figured that out. I had a project to develop a ship, an unsinkable floating platform that, when towed through a minefield, would create the pressure signature of a ship. If you blew up the mine, you wouldn't sink the ship, but you would get rid of the mine.
This was not a new idea. Small flat bottom watercraft called “swamp buggies” had used deck engines for propulsion ever since Alexander Graham Bell invented the first airboat in 1905.5 The idea of using aircraft engines for deck propulsion was not implemented until 1950, when the British installed four Rolls Royce Derwent V jet engines—each developing a thrust of 3,500 pounds at 14,500 revolutions a minute—on a 1,600-ton paddle wheeler named the Lucy Ashton.6 Tests were performed to measure the effects of drag and friction on a large vessel. The Lucy Ashton’s stream-driven paddles were removed, meaning it had no underwater propeller or shaft; however, unlike the YAG-37, the aircraft engines were not steerable. In fact, the Lucy Ashton could not even go in reverse since the jet engines used at the time had no reverse thrusters. Furthermore, the noise level from the four jet engines was ear-piercing.
Project Conversion Work
The Navy selected the Rawls Brothers shipyard in Jacksonville, Florida, in 1957 to perform the conversion work on the YAG-37. The shipyard was located on the St. Johns River near Jacksonville and had made modifications to several naval vessels during World War II.
The four turboprop aircraft engines used in the conversion were Pratt & Whitney T34s taken off a Navy Lockheed R7V-2 Super Constellation transport aircraft. The T34 engine was initially used on the nose of a Boeing B-17G Flying Fortress as a testbed and later installed on several other aircraft, such as the Boeing YC-97J Stratofreighter (later becoming Aero Spacelines Super Guppy used by NASA) and the Douglas C-133 Cargomaster.
The turboprops used the jet propellent fuel “JP-4,” a 50/50 percent blend of kerosene and gasoline. Two 10-foot diameter fuel tanks were installed on the weather deck ahead of the ship’s superstructure instead of the cargo holds to minimize the chance of fire since JP-4 was prone to a buildup of static electricity when moved through extended metal pipes. Air starter pods designed by the Air Research Corporation were used to start the engines. Hamilton Standard produced the 15-foot square tipped three-bladed variable-pitch propellers used on the T34 engines. These giant propellers were also used on the R7V-2 Super Constellation that had the T34 turboprop engines.
Several hull changes were needed in the conversion of the YAG-37, starting with removing the ship’s entire propulsion system—including the propeller and shaft, reciprocating engine, boilers, condensers, and steam return lines. The ship’s rudder was kept in place in the event the vessel needed to be towed. The ship’s bilge tanks for ballasting were also retained since the YAG-37 now rode much higher in the water with her propulsion system removed. A flat metal plate covered the top of the smokestack. The YAG-37’s original three 120-volt generators were replaced by a General Motors 3268A diesel engine powering a 120-kilowatt Allis-Chambers generator, which operated the rotating mounts for the four turboprop engines and supplied power for lighting fixtures and additional power features.
The engines were positioned both fore and aft on the ship’s World War II 40-mm gun mounts and could be rotated 360 degrees individually, in pairs, or all together. Three portable air compressors and an accumulator tank controlled the rudder and anchor. The Kidde fire extinguisher company designed and installed a CO² fire-suppression system. The ship’s cargo holds were partially filled with buoyancy material such as empty 55-gallon drums and Styrofoam ping-pong balls. The Navy used empty drums during sink at-sea live-fire training exercises, which held buoyancy and formed crumple zones where the drums deformation assisted in preventing explosive shock waves from transferring throughout the vessel’s structure. In comparison, the buoyant material used by the German “sperrbrecher” World War II minesweeper SS Neckar consisted of three layers, beginning with 5,600-tons of a shock absorbent bottom layer of sand, followed with 10,286 empty iron barrels in the middle layer, and 32,286 empty wooden barrels as the top layer.7
Testing revealed several problems, starting with the ship’s speed. The four turboprops could only be operated at a half-power setting of 3,000 horsepower (hp) for each engine because of safety and maintenance considerations, which restricted the YAG-37’s maximum speed to eight knots. A Liberty ship normally ran at 11 knots using the original reciprocating steam engine and underwater propeller operating at 2,500 hp. Heavy seas were also not the best operating environment for sensitive aircraft engines mounted on a ship’s deck. Any misalignment of the turboprops could also transmit strong hull vibrations.
Additional performance issues emerged when running the turboprop engines on hot muggy days as opposed to operating on cold and less humid days. The project test was undertaken in Florida during summer, but winter operations involving icing conditions would be another problem entirely—especially since turboprop engines could produce sea spray, which would freeze on contact and thicken the ship’s superstructure. The engines themselves would need a deicing system when operating in frigid weather. Excessive noise levels were also a concern, as was the case with the Lucy Ashton since the noise on the YAG-37 when operating the four turboprop engines was quite loud. However, this was not necessarily a dealbreaker. The Navy’s aircraft carriers were also very loud during flight operations.
During the research, The project test revealed that the YAG-37 became quite maneuverable going forward and backward . The YAG-37 and could feasibly move sideways and perform spinning motions. It could also maneuver next to a pier without tugboat assistance.
Unfortunately, the experiment of using turboprop engines for a ship’s propulsion was not a total success when the project ended in 1957. However, the Navy did indicate “possible future usefulness of this mode of power for ships up to 13,000 tons where speed was not essential.”8 The theory of using a converted merchant ship to sweep minefields for pressure signatures did hold some merit as Nicolson explained:
[The YAG-37 project] actually worked well enough that we got it into a shipbuilding program to actually build some, instead of this experimental platform. But it fell out, the pressure and juggling money back and forth, and they never built one. The last phase of that was to blow mines off under it in the Chesapeake Bay. We proved that it would work, but it was expensive in terms of fuel and the priority system dropped it out the bottom.
The YAG-37 retained the onboard buoyancy materials after the aircraft engines were removed when the ship was transferred to the Underwater Explosion Research Division at the Norfolk Naval Shipyard, where it was used to measure the effects of underwater explosions of different sizes, distances, and directions on the ship’s hull. In late 1958, the vessel was scrapped at Wilmington, North Carolina. The YAG-37 project was one of many innovative roles that Liberty ships played in assisting the Navy in developing different forms of seaborne propulsion and minesweeping capabilities.
1. John Henshaw, Liberty’s Provenance: The Evolution of the Liberty Ship from its Sunderland Origins (Annapolis, MD: U.S. Naval Institute Press, 2019), 101–04.
2. Eric Pearson, “The First Floating Nuclear Power Plant,” Steamship Historical Society of America PowerShips 80, no. 312 (Winter 2020): 48–52.
3. “Navy is Proud of Minesweepers,” Tampa Times, 30 December 1945.
4. Hector Donohue, Mines, Mining and Countermeasures (Bondi Junction, N.S.W.: Australian Defense Industries Ltd., 1994), 62; “Guinea Pig Ships, Crews Test Mines,” Fort Worth Star-Telegram, 2 January 1946, 4.
5. Alexander Graham Bell, Aerial Locomotion, With a few Notes of Progress in the Construction of an Aërodrome (Washington, D.C.: Judd & Detweiler, Inc., 1907), 26.
6. “Air Props-For Ships,” U.S. Navy All Hands, December 1957, 55.
7. Lawrence Paterson, Hitler’s Forgotten Flotilla: Kriegsmarine Special Forces (Annapolis, MD: U.S. Naval Institute Press, 2017), 30.
8. “Plane Engines Push Navy Ship,” Daily Press (Newport News, Virginia), 30 August 1957.