Oilers (AO)

Photograph of T2 tanker converted to Navy service

Mare Island #5595-43. Via

Oilers are ships designed to supply fuel oil to other ships and forward bases.  In the U.S. Navy classification system, no distinction was made between oilers and tankers, except that those oilers that were capable of refueling a ship while under way were eventually redesignated as AORs. Informally, the term "oiler" was used for ships capable of replenishment at sea and "tanker" was used for ships that merely carried oil from port to port.

The U.S. Navy considered fuel ships the ideal platforms for testing new machinery, since they were fairly large ships that were heavily operated, but were not as essential as warships and would be less of a risk if the machinery did not work out well. Thus, Maumee was the first surface ship in the Navy built with diesel engines, while collier Jupiter was the first Navy ship with turboelectric drive. Jupiter would later be converted to the Navy's first aircraft carrier and renamed Langley.

The officers of Maumee speculated about the possibility of refueling ships under way, which had never been attempted before except in the calmest conditions. The crew obtained schematics for destroyers and sketched out schemes for refueling at sea, but it was not until the U.S. Fleet began deploying its destroyers to Europe after the U.S. intervention in the First World War that Maumee began experimenting with underway replenishment. The approach used was broadside refueling, where the ship being refueled came alongside the Maumee. However, at this early stage in the development of the technique, the destroyer was towed by the oiler, rather than keeping station under its own power.

Photograph of refueling in rough seas
National Archives. Via Wildenberg (1996)

Both the Americans and the Japanese practiced underway refueling between the wars and had a number of oilers equipped for this task. It was found that over-the-stern fueling, where the ship being refueled was towed by the oiler, was impractical for larger warships. It was not possible to build a tanker with reasonable characteristics that could refuel a battleship in this manner. Enough power was required to run the oil heaters (required to warm the oil so it would not be too viscous to pump), the oil pumps, and the tension engine (which kept the oil hose at the correct tension to avoid either parting or dropping into the cold ocean water) that the remaining power was sufficient to maintain a speed no better than six knots, which was unacceptable. Broadside refueling of carriers under way was first attempted by the U.S. Navy in 1939, was found to be practical in spite of the seamanship required, and became the standard practice.

The U.S. Navy' contingency plan for war in the Pacific, ORANGE, envisioned an enormous fleet train that included 100 or more oilers. However, funding for naval construction became very tight in the late 1920s, and it was not until 1933, when prospects for funding began to improve, that specifications for new fleet oilers were developed. The most difficult requirement was high speed, at least 16 knots, to allow oilers to keep up with a general rate of advance of 10 knots while slowing periodically to refuel ships. There were also requirements to have a range of 6000 nautical miles (11,000 km) on the ship's own fuel bunkers, a capacity of at least 10,000 tons of fuel oil plus excess capacity for commensurate lubricating oil, the capability to ship gasoline in place of fuel oil, a heavy defensive battery, and deck space for crated aircraft and landing craft. These requirements were out of the question for the bulk of civilian tankers, even with subsidies, so the Navy made a distinction between fleet auxiliaries capabile of keeping up with the main battle force and auxiliaries acceptable for naval service. The latter were expected to be converted merchantmen, while the former would either be built for fleet service or would be subsidized with the features needed for fleet service, such as more powerful machinery for higher speed.

During Fleet Problem XVIII in 1937, there was considerable difficulty maintaining a speed of advance of 14 knots by Base Force. This led to a recommendation that all fleet auxiliaries have a minimum sustained speed of 18 knots.

When funding became available for modern Navy auxiliaries in 1937, the Navy argued that it would be far more cost effective to subsidize civilian tankers with appropriate speed and other characteristics than to build its own tankers. This proposal was implemented with the Cimarron and several of its sisters, which were taken over by the Navy after being built for Standard Oil with subsidies for more powerful engines and better hull forms. This became the pattern for virtually all the oilers used by the Navy in the Pacific War.

The Japanese Navy also heavily subsidized civilian tanker construction, concluding as the Americans had that this would be more cost effective than continuing the construction of Navy tankers. Under the stimulus of the subsidies, Japanese tanker tonnage more than tripled from 1934 to 1940, reaching 364,000 tons. The higher speed of the new ships mean that carrying capacity more than quadrupled, reaching 4 million tons per year by 1940. Of the 49 oceangoing tankers Japan possessed in 1940, 33 were capable of 16 knots or better. A number of these ships were requisitioned just before war broke out as the Kyokuto Maru class, and total Navy tanker requisitions by 7 December 1941 amounted to 270,000 tons — a huge fraction of total available tanker tonnage.

When war did break out, neither navy had enough oilers.  However, the Americans eventually built large numbers of these ships, while the Japanese lacked the industrial base to make good their deficiencies. The U.S. Navy economized its fleet oilers by employing them close to the combat zone, chartering commercial tankers to bring oil to forward bases where the fleet oilers refueled.

An important innovation in American oilers was the retrofitting of spar decks (raised platforms) over the main deck. These were originally installed to raise the cargo winches and the refueling crews above the level of turbulent waves in heavy seas, but they quickly began serving as general cargo decks, allowing the oilers to carry spare hose and fittings as well as drums of lubricating oil and other general stores for the ships being refueled. By the autumn of 1944, delivery of oxygen and acetylene tanks, provisions and general stores, and ammunition had become a regular part of the oilers' role in the fleet train, and all oilers were instructed to leave port with 150 drums of lubricating oil. At about the same time, oilers began using the Elwood or wire-span method, which allowed ships to refuel at a greater distance from each other (60 to 180 feet or 20 to 60 meters). This allowed refueling at greater speeds and in rougher weather.

British planning had always assumed that there would be refueling bases close to any theater of action, and British proficiency at underway refueling was poor even as late as 1945. While an American carrier could refuel in two hours, a British carrier took all day.

Japanese oilers

1TL class

1TM class

1TS class

2AT class

2TL class

2TM class

3TA class

3TE class

3TL class



Kyokuto Maru class

Ondo class

Shiretoko class

U.S. oilers

Chicopee class
Chiwawa class

Cimarron class

Escambia class

Kanawha class
Kennebec class

Mattaponi class

Patoka class

Suamico class

T2-SE-A1 class
T3-S class
T3-S2 class


Hastings (2007)
Wildenberg (1996)

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