UC-47 was a German Type UC II minelaying submarine - 420 tons surfaced, 502 tons submerged, 51.85 metres long. She was ordered on 20 November 1915, laid down on 1 February 1916, launched on 30 August of that year, and commissioned into the German Imperial Navy on 13 October 1916. Two six-cylinder four-stroke diesels of 300 PS each pushed her at a surface speed of 11.7 knots; two 460 PS electric motors gave her 6.7 knots submerged. She could travel 7,280 nautical miles surfaced at 7 knots, or 54 nautical miles submerged at 4. Her armament was a mix of weapons made for different jobs: six 100 cm mine tubes carrying eighteen UC 200 contact mines, three 50 cm torpedo tubes (one stern, two bow) and seven torpedoes, and one 8.8 cm Uk L/30 deck gun. Her dive time was forty-eight seconds. Twenty-six men worked her, ate in her, slept in her, and finally died in her.

Through her thirteen war patrols UC-47 was credited with sinking fifty-eight Allied vessels. Her preferred method was the silent one: cross a British coastal lane on the surface at night, dive, lay her mines in a pattern across the shipping channel, and slip away. Days or weeks later, freighters and trawlers would hit them. The crews aboard those vessels often never knew which U-boat had killed them - the mines did the work after the submarine had moved on, sometimes hundreds of miles. UC-47's torpedo work, by contrast, was direct and personal: she had to surface or come to periscope depth, identify a target, line up the firing solution and watch the wake of the torpedo run in. The men aboard the freighters she sank were sailors of British, Allied and neutral ships - merchant marine, fishermen, naval auxiliaries - who had families ashore the same as the men under the German water who killed them.

By the autumn of 1917 the Royal Navy was running a fleet of small fast patrol boats - the P-class - explicitly designed to hunt and kill coastal U-boats. They were light, sharp-bowed, and armed with deck guns and depth charges. On 18 November 1917, off Flamborough Head, HMS P-57 spotted UC-47 on the surface. Birnie ordered the patrol boat onto an interception course at full speed. P-57 struck UC-47 amidships - a ramming attack was a standard ASW technique of the time, and a very effective one against a thin-skinned submarine pressure hull. As UC-47 began to dive, P-57 followed with depth charges, the heavy steel cans rolling off the stern and detonating below the surface. The submarine never surfaced again. All twenty-six of her crew were killed. Birnie was decorated for the action.

UC-47's wreck has been located and identified, lying at approximately 54.05 degrees north, 0.38 degrees east, off Flamborough Head. Modern multibeam sonar surveys have produced detailed images of the hull on the seabed - and helped document, as one news article phrased it, the horrors of submarine warfare in WWI. She is a designated war grave. The men inside her are still there. The mines she laid and the torpedoes she carried may also be, though some will have corroded into the surrounding silt. The Royal Navy that killed her now patrols the same waters in modern frigates. The merchant marine that she killed still has its memorials in coastal ports along the Yorkshire and Lincolnshire coast. War at sea leaves no permanent trace on the surface. Below it, the evidence stays.