Replenishment at sea (RAS) or underway replenishment (UNREP) is a method of transferring fuel, munitions, and stores from one ship to another while under way. Most ships can receive replenishment on either side. Aircraft carriers of the U.S. Navy, however, always receive replenishment on the starboard side of the carrier. The design of an aircraft carrier, with its island/navigation bridge to starboard, does not permit replenishment to the carrier's port side. (learn more: en.wikipedia.org/wiki/Underway_replenishment)