UC-71 was 50.35 metres long, a beam of 5.22 metres, a draught of 3.64 metres. She displaced 427 tonnes on the surface and 508 tonnes submerged - small, by the standards of any later war, but well-suited to the shallow North Sea and the approaches to British coastal ports. Two six-cylinder four-stroke diesel engines, each producing 300 horsepower, drove her on the surface. Two electric motors of 620 horsepower drove her submerged. Her dive time was forty-eight seconds. She could go down to fifty metres. On the surface she made twelve knots; submerged, seven point four. The numbers that mattered most, though, were the ones that defined her purpose. Six 100-centimetre mine tubes. Eighteen UC 200 mines. Three 50-centimetre torpedo tubes - two on the bow, one on the stern - and seven torpedoes. A single 8.8 cm Uk L/30 deck gun for finishing off merchant ships on the surface. A complement of twenty-six men in a steel cigar small enough that the whole crew could see each other from one end of the boat to the other.

The Type UC II submarines were designed for one job above all others: to creep into the approaches of an enemy port, dispense a string of contact mines from those six vertical tubes in the forward casing, and slip away before anyone realised what had passed through. The mines did the rest. A merchant ship, days later, would touch a horn and tear open her hull. Sixty-three ships, sunk by torpedo or by mine - that is the credited tally for UC-71's nineteen patrols, and it is worth pausing on the human meaning of that number. Each of those sinkings represented a crew, sometimes drowned in the dark, sometimes pulled from the water by passing trawlers, often civilian merchant seamen carrying coal or wheat or steel for the British war effort. The submariners who laid the mines never saw the results. Their war was an abstraction of charts and chronometers. The killing they did was done by patience, by mathematics, and by the unforgiving chemistry of saltwater contact fuses.

The Armistice of 11 November 1918 required Germany to surrender her U-boat fleet. The boats were to sail to British ports under German skeleton crews and be handed over to the Royal Navy. Most went. Some did not. UC-71, on her surrender voyage in February 1919, sank in the North Sea. The official record for decades was ambiguous - lost at sea, cause unclear. What modern research and crew testimony have established is that the German crew opened her valves and let her go down rather than deliver her to the British. They were, by the standards of their own service, doing the honourable thing: a captain's last duty to his ship. The legal status was murkier. The Allies had demanded the boats intact. The crew, like the crews of Ludwig von Reuter's interned High Seas Fleet at Scapa Flow a few months later in June 1919, decided that ownership had not yet transferred and that they could still choose. UC-71 sank in the same North Sea she had hunted across for two years.

The wreck of UC-71 lies in the North Sea between Heligoland and the Frisian coast - the same approximate water she patrolled, sometimes through the very minefields HMS Ariel and her sisters had laid. Modern divers have located the hull. The boat is intact enough that the design details are still recognisable: the six vertical mine tubes forward, the deck gun mount, the conning tower. Around her, the seabed is busy with twentieth-century debris - other wrecks, abandoned ordnance, fishing gear. The North Sea is a generous keeper of what people have given it. The Type UC II boats are not famous in the way that the great fleet U-boats of the Second World War are famous. They were small, they worked at night, they did their killing by proxy. But sixty-three ships is sixty-three ships, and UC-71's quiet career in a quiet boat, ending in a quiet scuttle two miles deep in a winter sea, is as accurate a portrait of the U-boat war as any of the more famous stories.