Underway replenishment

Replenishment at sea (RAS) (North Atlantic Treaty Organisation/Commonwealth of Nations) or Underway replenishment (UNREP) (US Navy) is a method of transferring fuel, munitions, and stores from one ship to another while under way.

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 however 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 refuelling and restocking at sea had been made as far back as 1870, when HMS Captain of the Channel Squadron was resupplied with coal at a rate of five tons per hour. However, the speed was far too slow to be generally 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 think tank 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 RN between 1888-90 alone.

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.

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