On the morning of 6 December 1917, the French cargo ship SS Mont-Blanc and the Norwegian vessel SS Imo collided in the harbour of Halifax, Nova Scotia, Canada. Laden with high explosives, Mont-Blanc caught fire and detonated, devastating the Richmond district of Halifax. At least 1,782 people, largely in Halifax and Dartmouth, were killed by the blast, debris, fires, or collapsed buildings, and an estimated 9,000 others were injured. The blast was the largest human-made explosion at the time. It released the equivalent energy of roughly 2.9 kilotons of TNT (12 TJ).
Mont-Blanc was under orders from the French government to carry her cargo from New York City via Halifax to Bordeaux, France. At roughly 8:45 am, she collided at low speed, about one knot (1.2 mph; 1.9 km/h), with the unladen Imo, chartered by the Commission for Relief in Belgium to pick up a cargo of relief supplies in New York. On Mont-Blanc, the impact damaged benzol barrels stored on deck, causing them to leak vapours, which were ignited by sparks from the collision, setting off a fire on board that quickly grew out of control. Around 20 minutes later at 9:04:35 am, Mont-Blanc exploded.
Nearly all structures within an 800-metre (2,600 ft) or half-mile radius, including the community of Richmond, were obliterated. A pressure wave snapped trees, bent iron rails, demolished buildings, grounded vessels (including Imo, which was washed ashore by the ensuing tsunami), and scattered fragments of Mont-Blanc for kilometres. Across the harbour, Dartmouth also sustained widespread damage. A tsunami created by the blast wiped out a community of Mi'kmaq, who had lived in the Tufts Cove area for generations.
Relief efforts began almost immediately, and hospitals quickly became full. Rescue trains began arriving the day of the explosion from across Nova Scotia and New Brunswick, while other trains from central Canada and the Northeastern United States were impeded by blizzards. Construction of temporary shelters to house the many people left homeless began soon after the disaster. The initial judicial inquiry found Mont-Blanc to have been responsible for the disaster, but a later appeal determined that both vessels were to blame. The North End of Halifax has several memorials to the victims of the explosion.
Background
Dartmouth lies on the east shore of Halifax Harbour, and Halifax is on the west shore. By 1917, "Halifax's inner harbour had become a principal assembly point for merchant convoys leaving for Britain and France." Halifax and Dartmouth had thrived during times of war; the harbour was one of the British Royal Navy's most important bases in North America, a centre for wartime trade, and a home to privateers who harried the British Empire's enemies during the American Revolution, the Napoleonic Wars, and the War of 1812.
The completion of the Intercolonial Railway and its Deep Water Terminal in 1880 allowed for increased steamship trade and led to accelerated development of the port area, The British garrison left the city in late 1905 and early 1906, and the Canadian government took over the Halifax Dockyard (now CFB Halifax). This dockyard later became the command centre of the Royal Canadian Navy (RCN) on its founding in 1910.
Just before the First World War, the Canadian government began a determined, costly effort to develop the harbour and waterfront facilities. The outbreak of the war brought Halifax back to prominence. As the RCN had virtually no seaworthy ships of its own, the Royal Navy assumed responsibility for maintaining Atlantic trade routes by readopting Halifax as its North American base of operations. In 1915, management of the harbour fell under the control of the RCN; by 1917, the naval fleet in Halifax was growing, including patrol ships, tugboats, and minesweepers.
The population of Halifax/Dartmouth had increased to between 60,000 and 65,000 people by 1917. Convoys carried men, animals, and supplies to the European theatre of war. The two main points of departure were in Nova Scotia at Sydney on Cape Breton Island and Halifax. Hospital ships brought the wounded to the city, so a new military hospital was constructed.
The success of German Untersee-boat attacks on ships crossing the Atlantic Ocean led the Allies to institute a convoy system to reduce losses while transporting goods and soldiers to Europe. Merchant ships gathered at Bedford Basin on the northwestern end of the harbour, which was protected by two sets of anti-submarine nets and guarded by patrol ships of the RCN.
The convoys departed under the protection of British cruisers and destroyers. A large army garrison protected the city with forts, gun batteries, and anti-submarine nets. These factors drove a major military, industrial, and residential expansion of the city, and the weight of goods passing through the harbour increased nearly ninefold. All neutral ships bound for ports in North America were required to report to Halifax for inspection.
Disaster
The Norwegian ship SS Imo had sailed from the Netherlands en route to New York City to take on relief supplies for Belgium, under the command of Haakon From. The ship arrived in Halifax on 3 December for neutral inspection and spent two days in Bedford Basin awaiting refuelling and supplies. Though she had been given clearance to leave the port on 5 December, Imo's departure was delayed because her coal load did not arrive until late that afternoon. The loading of fuel was not completed until after the anti-submarine nets had been raised for the night. Therefore, the vessel could not depart until the next morning.
The French cargo ship SS Mont-Blanc arrived from New York late on 5 December, under the command of Aimé Le Medec. The vessel was fully loaded with the explosives trinitrotoluene (TNT) and picric acid, the highly flammable fuel benzol, and guncotton. She intended to join a slow convoy gathering in Bedford Basin readying to depart for Europe but was too late to enter the harbour before the nets were raised. Ships carrying dangerous cargo were not allowed into the harbour before the war, but the risks posed by German submarines had resulted in a relaxation of regulations.
Navigating into or out of Bedford Basin required passage through a strait called the Narrows. Ships were expected to keep close to the side of the channel situated on their starboard ("right"), and pass oncoming vessels "port to port", that is to keep them on their "left" side. Ships were restricted to a speed of 5 knots (9.3 km/h; 5.8 mph) within the harbour.
Collision and fire
Imo was granted clearance to leave Bedford Basin by signals from the guard ship HMCS Acadia around 7:30 on the morning of 6 December, with Pilot William Hayes on board. The ship entered the Narrows well above the harbour's speed limit in an attempt to make up for the delay experienced in loading her coal. Imo met American tramp steamer SS Clara being piloted up the wrong (western) side of the harbour. The pilots agreed to pass starboard-to-starboard. Soon afterwards, Imo was forced to head even further towards the Dartmouth shore after passing the tugboat Stella Maris, which was travelling up the harbour to Bedford Basin near mid-channel. Horatio Brannen, the captain of Stella Maris, saw Imo approaching at excessive speed and ordered his ship closer to the western shore to avoid an accident.
Francis Mackey, an experienced harbour pilot, had boarded Mont-Blanc on the evening of 5 December 1917; he had asked about "special protections" such as a guard ship, given Mont-Blanc's cargo, but no protections were put in place. Mont-Blanc started moving at 7:30 am on 6 December and was the second ship to enter the harbour as the anti-submarine net between Georges Island and Pier 21 opened for the morning. Mont-Blanc headed towards Bedford Basin on the Dartmouth side of the harbour. Mackey kept his eye on the ferry traffic between Halifax and Dartmouth and other small boats in the area. He first spotted Imo when she was about 4,000 feet (1.2 km; 0.76 mi) away and became concerned, as her path appeared to be heading towards his ship's starboard side, as if to cut him off. Mackey gave a short blast of his ship's signal whistle to indicate that he had the right of way, but was met with two short blasts from Imo, indicating that the approaching vessel would not yield its position. The captain ordered Mont-Blanc to halt her engines and angle slightly to starboard, closer to the Dartmouth side of the Narrows. He let out another single blast of his whistle, hoping the other vessel would likewise move to starboard, but was again met with a double blast.
Sailors on nearby ships heard the series of signals and, realizing that a collision was imminent, gathered to watch as Imo bore down on Mont-Blanc. Both ships had cut their engines by this point, but their momentum carried them towards each other at slow speed. Unable to ground his ship for fear of a shock that would set off his explosive cargo, Mackey ordered Mont-Blanc to steer hard to port (starboard helm) and crossed the bow of Imo in a last-second bid to avoid a collision. The two ships were almost parallel to each other, when Imo suddenly sent out three signal blasts, indicating the ship was reversing its engines. The combination of the cargoless ship's height in the water and the transverse thrust of her right-hand propeller caused the ship's head to swing into Mont-Blanc. Imo's prow pushed into the No. 1 hold of Mont Blanc, on her starboard side.
The collision occurred at 8:45 am. The damage to Mont Blanc was not severe, but barrels of deck cargo toppled and broke open. This flooded the deck with benzol that quickly flowed into the hold. As Imo's engines kicked in, she disengaged, which created sparks inside Mont-Blanc's hull. These ignited the vapours from the benzol. A fire started at the water line and travelled quickly up the side of the ship. Surrounded by thick black smoke, and fearing she would explode almost immediately, the captain ordered the crew to abandon ship. A growing number of Halifax citizens gathered on the street or stood at the windows of their homes or businesses to watch the spectacular fire. The frantic crew of Mont-Blanc shouted from their two lifeboats to some of the other vessels that their ship was about to explode, but they could not be heard above the noise and confusion. As the lifeboats made their way across the harbour to the Dartmouth shore, the abandoned ship continued to drift and beached herself at Pier 6 near the foot of Richmond street.
Towing two scows at the time of the collision, Stella Maris responded immediately to the fire, anchoring the barges and steaming back towards Pier 6 to spray the burning ship with its fire hose. The tug's captain, Horatio H. Brannen, and his crew realized that the fire was too intense for their single hose and backed off from the burning Mont Blanc. They were approached by a whaler from HMS Highflyer and later a steam pinnace belonging to HMCS Niobe. Captain Brannen and Albert Mattison of Niobe agreed to secure a line to the French ship's stern so as to pull it away from the pier. The five-inch (125 mm) hawser initially produced was deemed too small and orders for a ten-inch (250 mm) hawser came down. At this point, the blast occurred.
Explosion
At 9:04:35 am, the out-of-control fire on board Mont-Blanc set off her cargo of high explosives. The ship was completely blown apart and a powerful blast wave radiated from the explosion, initially at more than 1,000 metres (3,300 ft) per second. Temperatures of 5,000 °C (9,000 °F) and pressures of thousands of atmospheres accompanied the moment of detonation at the centre of the explosion. White-hot shards of iron fell down upon Halifax and Dartmouth.
A cloud of white smoke rose to at least 3,600 metres (2,000 fathoms; 11,800 ft). The blast was felt as far away as Cape Breton (207 kilometres or 129 miles) and Prince Edward Island (180 kilometres or 110 miles). An area of over 400 acres (1.6 km2; 0.63 sq mi) was completely destroyed by the explosion, and the harbour floor was momentarily exposed by the volume of water that was displaced. A tsunami was formed by water surging in to fill the void; it rose as high as eighteen metres (59 ft) above the high-water mark on the Halifax side of the harbour. Imo was carried onto the shore at Dartmouth by the tsunami. The blast killed all but one on the whaler, everyone on the pinnace, and 21 of the 26 men on Stella Maris; she ended up on the Dartmouth shore, severely damaged. The captain's son, First Mate Walter Brannen, who had been thrown into the hold by the blast, survived, as did four others. All but one of Mont-Blanc's crew members survived.
The blast instantly killed more than 1,600 people and injured an additional 9,000, more than 300 of whom later died. It destroyed or badly damaged every building within a 2.6-kilometre (1.6 mi) radius, over 12,000 in total. Hundreds of people who had been watching the fire from their homes were blinded when the blast wave shattered the windows in front of them. Overturned stoves and lamps started fires throughout Halifax, particularly in the North End, where entire city blocks burned, trapping residents inside their houses. Firefighter Billy Wells, who was thrown away from the explosion and had his clothes torn from his body, described the devastation survivors faced: "The sight was awful, with people hanging out of windows dead. Some with their heads missing, and some thrown onto the overhead telegraph wires." He was the only member of the eight-man crew of the fire engine Patricia to survive.
Large brick and stone factories near Pier 6, such as the Acadia Sugar Refinery, disappeared into unrecognizable heaps of rubble, killing most of their workers. The Nova Scotia cotton mill located 1.5 km (0.93 mile) from the blast was destroyed by fire and the collapse of its concrete floors. The Royal Naval College of Canada building was badly damaged, and several cadets and instructors were maimed. The Richmond Railway Yards and station were destroyed, killing 55 railway workers and destroying and damaging over 500 railway cars. The North Street Station, one of the busiest in Canada, was badly damaged.