HMS X1: The Submarine Built to Be a Battleship

Laid Down at Chatham in Secrecy

HMS X1 was built at HM Dockyard, Chatham, laid down on 2 November 1921 and completed on 23 September 1925, commissioning that December as, at the time, the largest submarine in the world. She was conceived as a submersible commerce raider, a radical concept for a submarine, designed to overpower the escorts of enemy shipping using heavy gun armament rather than relying purely on torpedoes.

The idea was not a British invention, and treating it as a piece of Admiralty eccentricity does the designers a disservice. It came out of German drawing offices. In 1916 the Imperial German Navy ordered the U-139 class, very large boats with two 150mm deck guns and a proper rangefinder in an armoured conning tower, built to fight on the surface as well as beneath it. After 1918 those boats were divided among the Allies and taken apart by their new owners, and the plans of a further, uncompleted class came to the Admiralty as a war prize. X1 is what the Royal Navy did with them.

The tactical logic is worth setting out, because "submarine with big guns" makes it sound sillier than it was. The lesson of 1917 was that convoy defeated the U-boat, and a convoy in those years was screened by a thin handful of sloops, trawlers and elderly destroyers. A submarine that could surface at fifteen thousand yards and sink the escort in a gun action would not need to attack a single merchant ship. She would simply remove the roof, and every other submarine in the area could then do the work.

Put that way it is a genuinely elegant answer to the problem, and it has the further merit of being unarguably legal. Warships are warships and may be sunk on sight. It is a design that sidesteps the whole moral and legal argument about submarines and merchant shipping by never going near a merchant ship.

Her construction was shrouded in deliberate secrecy, not least because a treaty signed at Washington in 1922 had sought to bind submarines to the rules of prize warfare when dealing with merchant vessels, making her intended role a truly awkward one to acknowledge publicly.

That legal position deserves setting out carefully, because it is routinely garbled. The Washington conference of 1921 and 1922 produced several separate agreements. The famous one, the Washington Naval Treaty, is about capital ships and aircraft carriers and says nothing whatever about submarines and trade.

Submarines were dealt with in a different instrument signed on the same day, 6 February 1922, the Treaty relating to the Use of Submarines and Noxious Gases in Warfare. It did not ban attacks on merchant ships. It required submarines to obey the existing rules of prize warfare: stop the vessel, search her, and put her crew somewhere safe before sinking her, with a ship's boats in the open sea explicitly not counting as safety. It went so far as to say that anyone breaching those rules should be treated as a pirate.

And it never came into force at all. France declined to ratify it, precisely because of the submarine provisions, so the whole thing lapsed. Britain and the United States had both ratified, and the Royal Navy behaved as though it were binding, which is a different matter from it actually being law.

The chronology is also worth pinning down. X1 was laid down on 2 November 1921, three months before that treaty was signed and while the conference was still sitting. The awkwardness arrived after the keel did. What Britain had by 1925 was the largest submarine in the world, built to a concept the country had just spent a conference denouncing, at a moment when nobody wanted photographs of her in the newspapers.

A Submarine Built Like a Cruiser

X1 carried four QF 5.2-inch guns mounted in twin unarmoured turrets with a range of 16,000 yards, an extraordinary armament for any submarine, alongside six 21-inch bow torpedo tubes with one reload torpedo apiece. Operating her main armament required a crew of 58 for the guns alone, a remarkably enormous commitment of manpower for what was still, fundamentally, meant to function as a stealthy vessel.

Half the Ship's Company Served the Guns

The manpower figure is worth putting in proportion. X1 carried a company of about 111, eleven officers and a hundred ratings, so those 58 men were rather more than half of everybody aboard. More than half the crew of a submarine existed to work weapons that could only be used while she was on the surface and therefore at her most vulnerable.

They were not improvised weapons either. She carried a fire-control tower with a nine-foot telescopic rangefinder, which is a light cruiser's gunnery arrangement transplanted into a submarine, and roughly 400 rounds stowed in waterproof magazines. The intention was six rounds a minute from each gun. In practice the ammunition hoists could not keep up, which is the sort of failure that sounds minor and is not: a gun that cannot be fed is a gun that has stopped firing while the destroyer coming at you has not.

Her torpedo battery had a similar problem at a slower pace. Six bow tubes with a single reload each is a heavy first salvo, but reloading in that hull took something like twenty-four minutes, which in a surface action is an eternity and underwater is longer still.

The hull itself was the most advanced thing about her. The pressure hull ran to nineteen feet and more in diameter, an inch thick, divided into ten watertight compartments, and it let her go deeper than any British submarine of the day. How much deeper is genuinely disputed. Some records give her a test depth of 350 feet and others put her nearer 500, and the sources consulted do not agree, so both figures are given here rather than a false precision.

Defeated by Her Own Engines

X1's career was dogged from the start by persistent diesel engine troubles. Her starboard camshaft driveshaft fractured in January 1928, and the port shaft failed in the same way that April, while her ammunition hoists proved troublesome enough to prevent the rate of fire her design had promised.

There is a particular irony buried in her machinery. Her two main shafts were driven by eight-cylinder Admiralty diesels of 1,500 horsepower apiece, which is British enough. But the two auxiliary sets that charged her batteries, 1,200 horsepower each, were MAN diesels lifted out of the surrendered German submarine U-126.

So a British submarine drawn from captured German plans went to sea running on captured German engines, and it was the engines that finished her. Both main and auxiliary plant gave trouble from the beginning. Full-power trials in March 1926 produced 7,135 horsepower against a theoretical 8,000, and her very first deployment, out to Gibraltar, ended with her needing engine repairs when she got there.

The camshaft failures are the telling detail. Two shafts, the same component, the same fracture, three months apart, one of them after a full refit at Malta. That is not bad luck or rough handling. That is a design being asked for more than the metallurgy of the 1920s could deliver in a space fixed by the diameter of a pressure hull, and there is no way to fix it except by building a different submarine.

Commanding officers who served aboard her during her Mediterranean deployment reported that she suffered from overcrowding with auxiliary machinery, cramped accommodation and poor ventilation, and in practice she spent more of her short career under repair than she ever spent at sea, a remarkably disappointing outcome for such an ambitious design.

A Brief, Troubled Life

HMS X1 was placed in reserve after 1930 and finally scrapped at Pembroke Dock on 12 December 1936, bringing to a close the story of an experimental submarine that had promised to be a genuine game-changer and instead became a cautionary tale about the limits of trying to combine a battleship's firepower with a submarine's stealth.

She was not, however, a cautionary tale anybody heeded immediately. France built the same idea larger. Surcouf, launched in 1929, carried two 8-inch guns and a floatplane in a hangar, and the London Naval Treaty of 1930 made an exception for her that is almost comic in its specificity: a general limit of 2,000 tons and 5.1-inch guns, an allowance of three boats each up to 2,800 tons with guns up to 6.1 inches, and then a further clause permitting France one unit already launched of 2,880 tons with 8-inch guns. X1 exceeded the general limit on both counts and lived inside the three-boat exception. The treaty had to be written round the two of them.

Surcouf was lost in the Caribbean in February 1942, most probably run down at night by a merchant ship that never established what it had hit, though the loss has never been definitively explained. Between them the two boats settled the argument. Nobody built a gun submarine again.

What the Royal Navy did instead is the real epilogue, and it was sensible. It went back to building moderate patrol submarines and made them very good indeed. HMS Parthian and HMS Regent were the large ocean-going boats that actually fought the next war in the Mediterranean. HMS Porpoise took the specialised-role idea and applied it to minelaying, where it worked. HMS Swordfish was the first of the small S class, which became the most numerous submarine class Britain ever built.

Every one of those was a fraction of X1's size and every one of them was more use. That is the lesson, and it cost eleven years and a great deal of money to learn: a submarine wins by not being seen, and everything bolted to the outside of her that helps her fight on the surface is weight, drag and complication spent on the one situation she should be trying hardest to avoid.