From memory (and hence probably wrong) the RN "standard" was 5-inch diameter and the USN 7-inch (???).
This comes from a CAFO re. co-operation between the navies in the Atlantic where (eventually) all ships would have 5"/7" adaptors such that either navy could connect to the others' hoses. This was very early in 1942.
( The difference between 5 and 7-inch hoses is much more than it might seem as the area depends on the square of the radius, and relative friction losses also reduce with increase in size. )
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The RN did use oiling at sea regularly before, using the "stirrup" method (larger ships using their "boat cranes" to support the hose towards the receiving ship).
They also used astern oiling, but pre-war the flexible copper hoses were prone to breaking. After the capture of a German supply ship the RN found their flexible rubber floating hoses. These were adapted for RN use by Dunlop, who also manufactured them in the USA for the USN.
By early 1944 ALL North Atlantic convoys had at least one (and usually two) oilers who mainly travelled as ordinary merchant ships, but who could oil escorts on demand (mods include a raised platform on the quarterdeck, a raised platform running fore-and-aft, and extra derricks). Most could also transfer stocks of depth charges to escorts who had run low.
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