Underway replenishment (UNREP; U.S. Navy) or replenishment at sea (RAS; North Atlantic Treaty Organization/Commonwealth of Nations) is a method of transferring fuel, munitions, and stores from one ship to another while under way. First developed in the early 20th century, it was used extensively by the United States Navy as a logistics support technique in the Pacific theatre of World War II, permitting U.S. carrier task forces to remain at sea indefinitely.
History
Concept
Prior to underway replenishment, coaling stations were the only way to refuel ships far from home. The Royal Navy had an unparalleled global logistics network of coaling stations and the world's largest collier fleet. This capability allowed the Navy to project naval power around the world and far from home ports. This had two disadvantages: the infrastructure was vulnerable to disruption or attack, and its use introduced a predictable pattern to naval operations that an enemy could exploit. Early attempts at refueling and restocking at sea had been made as early as 1870, when HMS Captain of the Channel Squadron was resupplied with coal at a rate of five tons per hour. The speed was too slow to be practicable, and calm weather was required to keep the neighbouring ships together. Lieutenant Robert Lowry was the first to suggest the use of large-scale underway replenishment techniques in an 1883 paper to the Royal United Services Institute. He argued that a successful system would provide a minimum rate of 20 tons per hour while the ships maintain a speed of five knots. His proposal was for transfer to be effected through watertight coal carriers suspended from a cable between the two ships. Although his concept was rejected by the Admiralty, the advantages of such a system were made apparent to strategists on both sides of the Atlantic. Over 20 submissions were made to the Royal Navy between 1888 and 1890 alone.
First trials
The main technical problem was ensuring a constant distance between the two ships throughout the process. According to a report from The Times, a French collier had been able to provision two warships with 200 tons of coal at a speed of six knots using a Temperley transporter in 1898. The United States Navy also became interested in the potential of underway replenishment. Lacking a similar collier fleet and network of coaling stations, and embarking on a large naval expansion, the Navy began conducting experiments in 1899 with a system devised by Spencer Miller and the Lidgerwood Manufacturing Company of New York. His device kept a cable suspended between the two ships taut, with a quick-release hook that could travel up and down the line with the use of a winch. The first test of the device involved the collier Marcellus and battleship Massachusetts. The Royal Navy embarked on more extensive trials in 1901, and achieved a rate of 19 tons per hour. To meet the requirement for a rate of at least 40 tons per hour, Miller implemented a series of improvements, such as improving the maintenance of tension of the cable, allowing heavier loads to be supported.
Miller also collaborated with the British Temperley Company, producing an enhanced version, known as the Temperley-Miller system. Royal Navy trials with this new system in 1902 achieved an unprecedented average rate of 47 tons per hour and a peak rate of 60 tons per hour. The Thames Ironworks and Shipbuilding Company also patented its "Express equipment", which delivered supplies to the broadside of the ship, instead of from the aft. The company offered the system to the Admiralty, claiming that it had achieved a rate of 150 tons per hour, but the offer was declined. A Royal Navy engineer, Metcalf, suggested an alternative system in 1903, where two cables were used, and the cable tension was maintained with the use of a steam ram. Trials were held in 1903, which demonstrated an optimal operating speed of 10 knots with a transfer rate of 54 tph. Although it was a superior system and met with a formal endorsement from the Admiralty there is little evidence that such equipment was employed by any navy. In May 1905, the U.S. Navy tested an improved Miller-Lidgerwood rig using the Marcellus and the battleship Illinois near Cape Henry. These coaling tests achieved 35 tph while steaming at seven knots, which fell short of expectations. None of these coal systems approached the transfer rates required to make RAS practicable, considering that a battleship required over 2000 tons and a small destroyer required 200. As a result, it could take 60 hours or more to refuel a battleship, with both vessels steaming at five knots, during which time both were vulnerable to attack.
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