H47 had been laid down on the Clyde at William Beardmore and Company in Dalmuir on 20 November 1917, in the closing months of the First World War. She was one of the last batch of British H-class submarines — a coastal patrol design originally derived from the American Holland 602, adapted to Royal Navy specifications and built in British yards as the U-boat campaign of 1916 made every torpedo-tube the Admiralty could acquire urgent. By the time H47 was commissioned, on 25 February 1919, the war was over. She spent the 1920s in the diminished peacetime navy: training, exercises, patrols, the slow professional rhythm of submarine work between wars. At 423 long tons surfaced and 510 submerged, 171 feet long with a complement of twenty-two, she was a small boat by even contemporary standards.

The collision area lies off the Pembrokeshire coast near St Govan's Head, in St Bride's Bay — open water used for submarine training because it offered depth, room, and proximity to the Royal Navy's facilities at Milford Haven. On the morning of 9 July, the larger L-class submarine L12 was exercising in periscope depth attacks. H47 was running on the surface. The post-incident inquiry found that L12 had surfaced at speed without spotting H47 closely enough, and that the smaller boat — sitting low on the water, with her conning tower the only substantial radar return decades before radar — had no time to manoeuvre. L12 struck H47 abreast the engine room. The collision tore open H47's pressure hull below the waterline. She sank in less than thirty seconds.

Submarines of this generation had no escape hatches that could be operated from inside under flooding conditions. There was no time to dog watertight doors, no time to climb the ladder to the conning tower hatch, no time for anything except for the boat to disappear. Three men reached the surface — almost certainly because they were on the bridge or the upper hull when the collision happened — and one of those three died of his injuries before he could be brought aboard L12. The twenty-one who went down with H47 were British submariners doing their job on a calm summer morning. Their names appear on the National Submariners' War Memorial at Embankment Gardens in London, with the names of the rest of the silent service's interwar dead. The bodies were not recovered. H47 was not salvaged. She is still on the bottom in about 50 metres of water.

Submarine accidents drove submarine safety. Each disaster — H47 in 1929, M2 in 1932, Thetis in 1939 — forced the Royal Navy to redesign hatches, develop escape apparatus, change exercise procedures. After H47, surface-to-surface signalling protocols between exercising submarines were tightened: boats no longer relied on visual lookout alone in shared exercise areas, and dive intentions had to be confirmed by radio. The lessons did not stop the sinkings, but they shortened the list of subsequent ones. Every advance in submarine survivability through the rest of the twentieth century — DSEA breathing sets, Steinke hoods, deep-submergence rescue vehicles — was paid for in part by the men of boats like H47.

There is no monument on shore for H47. The waters where she lies are unmarked at the surface. Fishing boats and ferries pass over the wreck regularly without knowing it is there. The Royal Navy Submarine Museum at Gosport keeps a record of the loss, and the names of the lost are read each year at remembrance services for the silent service. The story of H47 is the kind of story submariners tell each other — not as folklore but as caution. A clear morning. Calm water. Routine exercise. Twenty-one men below. The next watch never came on.