Harry Hastings, born in 1908 and a Freeman of the City of London as well as the Company of Watermen & Lightermen, was thirty-two years old when, on Tuesday 28 May 1940, he was requested by the Admiralty to take a ship downriver to Southend for an undisclosed purpose. At the time, Hastings was employed as a lighterman by Clemence Nolan. His father, an elderly publican of The Gloucester Arms at Kingston, owned Tigris I along with several pleasure steamers and motor boats for public hire. These vessels had already been inspected, categorized, and numbered by the Admiralty, designated as either hospital ships or fire floats if required.

Tigris I was a former First World War submarine chaser converted by Tough Brothers at Teddington into a passenger boat. She operated as a summer pleasure cruiser, carrying up to 350 tourists between Richmond and Hampton Court. At the time of the Admiralty’s request, she had recently been fitted with a new engine that had not yet been fully run in.

Hastings’ father asked his son, along with fellow lighterman Bill Clark, aged thirty-three, and Harry’s brother Warren Hastings, three years younger, if they would be willing to take Tigris I to the Admiralty’s intended destination. Despite the secrecy surrounding the mission, they speculated that the vessel was needed to evacuate people—possibly children—from London. Bill Clark later recalled being offered "a day's work and the fare back home" to join the trip. Due to a shortage of work at Nolans, both Bill and Harry were granted the week off.

At 7:00 a.m. on Monday morning, Tigris I departed for Gravesend, accompanied by a Billingsgate fish market boat that had also been requisitioned. Passing the Royal Albert Docks, the crew observed a lock full of ships’ lifeboats, an early indication that a major operation was underway. Upon arrival at Westminster Pier, crowded with officers and naval ratings, the crew was sent to local ship chandlers and shops to exchange chits for provisions sufficient for four days. They purchased tea, bread, eggs, bacon, sugar, meat, candles, and cigarettes, which were then loaded onto the vessel.

With provisions aboard, the vessel proceeded first to Southend Pier, where no free berths were available, resulting in Tigris I being moored alongside another boat. Initially believing they could return home by nightfall, the crew prepared to leave and take the train back. However, orders to proceed to Sheerness altered these plans. The vessel lay outside Sheerness harbour until nightfall. Once inside, naval ratings with fixed bayonets ordered the crew ashore and marched them into an office where they were asked to volunteer for service at Dunkirk. After telephone calls to the Admiralty and to alert relatives, the crew signed on with the Royal Navy at approximately £20 pay for one month. Harry Hastings later remarked, "We joined up for a month, but one bloody day was enough!"

Hastings also recounted the story of a man who refused to sign up. After returning to the Barley Arms at Twickenham and explaining the mission to a woman who overheard him, the police were alerted. The man was arrested and, in a closed court due to the secrecy of the operation, sentenced to three months in prison.

At Sheerness, naval personnel were installed both in command and as additional crew. Further provisions and two ladders intended for use across the Channel were loaded. The convoy then proceeded to Ramsgate, where it moored overnight before sailing to Dover the following morning. The convoy was led by a fish-cutter armed with two machine guns and displaying a triangle of three red lights on its mast. At Dover, a voice over the Tannoy announced: "Does anyone want to turn back? Those who want to go on, follow the light on the top of my mast." Forty-five boats set off in convoy, aware that losing the guiding light could leave them isolated in a minefield.

Partway across the Channel, the same voice declared: "I'm turning back now. Where you see the smoke coming up, that's Dunkirk. Make for it." Tigris I followed behind Princess Freda, another peacetime passenger boat, and Mears’ Margherita. The latter did not complete the journey; a destroyer’s wake caused by a large wave capsized Margherita, which sank. Two ratings aboard survived by surfacing through a hole in the aft section and were rescued.

Bill Clark described his first impression of Dunkirk: "Destroyers were coming towards us loaded with troops, some fully dressed, some half dressed. On the beach to starboard, there were thousands of men, some lined up in companies, columns, and groups down to the water’s edge. Others lay on the sand or sheltered in the dunes. Ahead of us, there was much noise as the Germans dropped bombs from the air." When bombs were not falling, the sea was calm with no wind and a blue sky.

Approximately one and a half miles offshore, large destroyers awaited, unable to approach closer to land. About 250 yards out, Tigris I touched the sand and lowered her ladders. Being wooden, the ladders floated on the water, complicating troop embarkation. The first man, leading a column, boarded after 72 hours stranded on the sands. Others climbed using rubbing strakes on the hull, spaced two feet apart and barely wide enough for a foothold, to hoist themselves aboard with packs and rifles. The first run rescued over 400 men; in total, Tigris I is credited with saving approximately 900 lives from the beaches of Dunkirk.

Realizing they could not approach the beaches closely enough to assist further, the vessel entered Dunkirk harbour under hazardous conditions to ferry troops to larger ships offshore. Bill Clark recalled, "On the way out to our destroyer, we had to dodge many small boats, either loaded with troops or empty and heading back to the beaches."

During the fourth run, a bomb exploded near Tigris I, blowing Harry Hastings out of the wheelhouse while machine guns strafed the boat. Rescued from the oily, foul-smelling sea amid floating corpses, Hastings survived despite bullet holes in the deck and funnel and shrapnel damage to the hull. Though badly shaken, the crew returned to Dunkirk. Hastings described the troops as being in a terrible state, having been on the beaches for days without washing or shaving.

Tigris I sustained further damage later that day in a collision with a Thames barge. The breakwater used for embarkation had been directly hit, and lorries had been driven into the water to form a makeshift jetty. The vessel used this improvised structure to embark French troops, with the final load transferred to one of Nolan’s Tilbury mud-hoppers.

With Tigris I severely damaged, it was deemed unable to return home. Naval command ordered the engine to be dismantled and the ship to be beached and abandoned. Holed by enemy action, opened at the seams by collision, and with pumps unable to manage flooding, the vessel was left amid the debris and carnage of Dunkirk. Harry Hastings and his crew were transported back to Ramsgate, where they rested during the journey. Bill Clark described their condition: "We felt like death. I don’t know when we last had a wash. No shave since the Saturday. My face was beginning to itch; my feet were swollen and my body sore all over. We were all the same."

At Ramsgate, the quay was lined with stalls offering tea, coffee, sandwiches, chocolate, and tobacco, guarded by naval ratings. Hastings recalled being escorted to the Merry England for a wash and refreshment. The crew feared being sent back to Sheerness or arrested for desertion if they left for home. Nevertheless, they proceeded to Ramsgate station with only six shillings between them. They traveled to Rochester, where they borrowed thirty shillings from Warren Hastings’ mother-in-law, which enabled their return to Kingston. There, they were celebrated as heroes in their local pub.

Tigris I was not lost. A group of French sailors found and refloated the vessel, plugging her holes. She was towed across the Channel to Ramsgate harbour by a naval tug after being discovered crippled off the Goodwin Sands. Subsequently taken to Sheerness and then to Teddington, Tigris I was repaired and returned to civilian use after the war as a houseboat. The vessel was finally broken up in 1985.

• Subject of the 2017 book The Tigris of Dunkirk (ISBN 9781973928249)
• Featured in the 2015 Palau postage stamp set The Little Ships of Dunkirk