The vast majority of the British public, and even military historians, have never heard of them. COPPists — a combination of naval navigators and submariners with SBS (Special Boat Service) swimmers — played a key role in the planning and execution of Operation Overlord, the D-Day invasion of Europe. Admiral Ramsay, the architect of the naval element, said: ‘On their operations depended to a very great extent the final success of Operation Overlord.