HMS Thunderbolt (N25): The Submarine Sunk Twice

Birkenhead as HMS Thetis-Built

This submarine was built by Cammell Laird at Birkenhead and launched on 29 June 1938 under the name HMS Thetis, a Group 1 T-class boat of 1,090 tons surfaced displacement. Her story is inseparable from one of the worst peacetime disasters in British submarine history.

The number of people aboard is the first thing to understand about it, because it explains a great deal of what followed. A T-class submarine was designed for a company of about fifty-nine. On 1 June 1939 there were a hundred and three people in her.

That was not carelessness. It was an acceptance dive, the trial at which a builder formally hands a submarine over, so besides her crew she carried Cammell Laird managers, fitters and electricians, Admiralty overseers, technical observers, a Mersey pilot and a caterer. Nearly twice the design complement, most of them civilians, most of them with no submarine training of any kind, in a hull with escape equipment for the crew she was meant to have.

The Thetis Disaster

On 1 June 1939, during dive trials in Liverpool Bay, Thetis carried 103 people aboard, her normal complement of 59 supplemented by shipyard workers and technical observers. A test cock on torpedo tube number five had been blocked by enamel paint, and confusing bow cap indicators led her torpedo officer, Lieutenant Frederick Woods, to open the tube's inner door while its outer hatch remained open to the sea.

Water flooded in and the submarine's bow sank to the seabed, 150 feet down. Only four men escaped through the aft escape chamber: Woods himself, Captain Harry Oram, Leading Stoker Walter Arnold and Fitter Frank Shaw.

The remaining 99 men, 51 crew and 48 shipyard and naval personnel, died of carbon dioxide poisoning in the trapped forward section, a catastrophic loss that shook the Royal Navy and the submarine-building industry alike.

They Could See Her the Whole Time

There is a fact about the geometry of this that the summary leaves out, and it is the thing that makes the Thetis disaster different from every other submarine loss. She was two hundred and seventy-five feet long and she sank in a hundred and fifty feet of water.

Her bow went into the bottom and her stern stayed up. A long stretch of it stood out of the sea, in daylight, off the North Wales coast, visible to everybody. Ships came and lay alongside. Aircraft flew over. Newspapers photographed it. And a hundred and three men were inside a submarine that anybody could reach out and touch.

The Liverpool salvage vessel Vigilant got wires round the protruding stern and tried to haul her further out of the water so that a hole could be cut in the hull with oxyacetylene gear. It was the right idea and it very nearly worked. The strain was too great, the hawser parted, and Thetis slid backwards under the surface while the men who had been trying to save her watched.

What killed the men inside was not water. Most of the boat was dry. In a sealed compartment the oxygen lasts longer than people expect, but the carbon dioxide the occupants breathe out accumulates, and past a certain concentration it causes headache, then confusion, then unconsciousness. With a hundred and three people in a space meant for fifty-nine it went roughly twice as fast as it should have. They had somewhere between one and two days, and they knew it.

Four men got out through the after escape chamber before it stopped working, and they are worth naming every time the story is told: Woods, Oram, Arnold and Shaw. They saved themselves. Nobody outside got anybody out of Thetis at all.

The disaster led directly to a vital safety improvement, a device that became known as the Thetis clip, which allowed a torpedo tube's inner door to be tested safely before being fully opened, a change that has likely saved lives ever since.

It is worth explaining what the clip does, because it is one of the neatest safety fixes in engineering. A torpedo tube has two doors: a bow cap at the sea end and a rear door inside the boat. Opening the rear door while the bow cap is open floods the submarine, so there is a small test cock which is opened first to see whether water comes out. In Thetis the test cock was blocked with enamel paint from the building, so it produced nothing and the tube looked dry when it was full of the Irish Sea.

The Thetis clip holds the rear door open by a fraction of an inch and no further. If the tube is flooded, water comes through the gap and the door can be slammed shut against a trickle rather than a jet. It is a piece of shaped steel that costs almost nothing, and it works, and it exists because ninety-nine men died in Liverpool Bay.

The Royal Navy learned three separate lessons from three separate disasters and it took all of them. Thetis taught that men trapped in a boat cannot wait to be rescued and must be able to get themselves out. HMS Truculent, run down in the Thames in 1950, taught that getting out is only half the problem when the tide is running. HMS Sidon, at Portland in 1955, taught what a torpedo can do to the submarine carrying it.

Salvage and Rebirth

Thetis was salvaged by the Liverpool and Glasgow Salvage Association, though the recovery claimed a further life when a diver died of decompression sickness on 23 August 1939.

She was intentionally grounded at Traeth Bychan on Anglesey on 3 September 1939, the very day Britain declared war on Germany, before being fully repaired and recommissioned in 1940 under a new name, HMS Thunderbolt, commanded by Lieutenant Commander Cecil Crouch.

The renaming is usually explained as superstition and there is more to it than that. A submarine has to be crewed by men who will take her deep without thinking about it, and no sailor in Britain in 1940 did not know what had happened to Thetis. Whether or not anybody at the Admiralty believed the name was unlucky, they certainly understood that it would be in the mind of every man who joined her.

It is also worth remembering what her crew were stepping into. The boat had been on the bottom for four months with ninety-nine bodies in her. She was raised, opened, cleared, stripped and rebuilt, and the men who did that work were mostly from the same yard and the same town as the men who had died in her. Then a new crew took her to sea and fought her for three years.

A Distinguished War

As Thunderbolt, she went on to fight with genuine distinction, sinking the Italian submarine Capitano Tarantini in the Bay of Biscay on 15 December 1940.

Six months after being recommissioned she sank another submarine, which is worth pausing on. Capitano Tarantini was on her way into Bordeaux, where the Italians ran a submarine base alongside the Germans for operations into the Atlantic, and she was caught on the surface at the mouth of the Gironde within sight of home.

For a boat that had spent the previous summer on the bottom of Liverpool Bay full of dead men, that is a considerable turnaround in about eleven months.

From late 1942 she was converted to carry manned torpedoes known as Chariots, and took part in Operation Principal against shipping at Cagliari in December 1942, and in January 1943 her Chariot teams successfully mined and sank the Italian light cruiser Ulpio Traiano and the freighter Viminale at Palermo, remarkable special operations that demanded extraordinary courage from the men who rode those torpedoes into enemy harbours.

A Chariot needs describing rather than named. It was a torpedo about twenty-two feet long with an electric motor, ridden by two men sitting astride it in rubber diving suits and breathing sets, travelling at about three knots a few feet below the surface. The nose was a detachable warhead of several hundred pounds of explosive.

The crews had to navigate into a defended harbour in the dark, get under the target, clamp the warhead to her bilge keel with magnets, set a clock and ride out again. The breathing sets used pure oxygen, which becomes toxic below about thirty feet, so they could not simply go deep to avoid trouble. Cold was the constant enemy; the suits leaked and men lost the use of their hands.

Thunderbolt's job was to carry them there. The Chariots were stowed in pressure-tight cylinders welded to her casing, which meant she had to surface close inshore off an enemy port, put men over the side into the water, and then wait, or go and come back. The Palermo raid worked: her teams mined and sank the brand new light cruiser Ulpio Traiano, which had never been to sea, and the liner Viminale.

That is a cruiser destroyed in her own harbour by two men on a torpedo, which is about as good a return on an evening as the Royal Navy ever had. Most of the Chariot crews on those operations were captured.

Lost a Second Time

HMS Thunderbolt was finally sunk on 14 March 1943 off Sicily by the Italian corvette Cicogna, which detected her and delivered a fatal depth-charge attack. She went down in some 4,430 feet of water with the loss of all hands, a submarine that had already survived one of the worst disasters in the service's history only to be lost a second time in action.

The Cicogna attack is unusually well documented from the Italian side, which is why the date and the ship are certain when so many submarine losses are not. She held the contact and worked at it, and Thunderbolt went down in very deep water off the north coast of Sicily.

That depth is the reason there is nothing further to say. She lies far beyond the reach of anything that could examine her, and she is a war grave. The difference between her two sinkings is stark and worth stating plainly: the first happened in a hundred and fifty feet of water in sight of shore, and it is the one everybody remembers. The second happened in nearly a mile of water with an enemy overhead, and nobody has ever seen her since.

Memorials to her crew, and to those lost in the original Thetis disaster, stand today in Holyhead and Birkenhead, a lasting reminder of a story that touched an extraordinary number of families not once but twice.

The location of those memorials is not accidental. The Thetis dead were overwhelmingly Merseyside men. Fifty-one were her crew, and the other forty-eight were shipyard workers, fitters and staff, which means the loss fell on a small number of streets in Birkenhead all at once, in the way a pit disaster falls on a mining village.

Then it happened again. The men who died in her the second time were a different crew entirely, in a different sea, in a war, and they are commemorated alongside the first, because it was the same hull and because nobody could reasonably separate the two stories.

One hull, two crews, two disasters and one town. No other submarine in the history of the Royal Navy can be described that way, and the men who rebuilt her and then took her to sea knew exactly what they were stepping into.