However, when World War I broke out, the Royal Navy requisitioned most warships under construction in Britain for foreign powers and refunded the two-thirds of Bjørgvin's £370,000 purchase price already paid by the Norwegians.

Addition of the bulges cost 2 knots (3.7 km/h; 2.3 mph) in speed, but prevented the extra weight resulting from all of these changes from deepening her draft.

She was powered by two vertical triple expansion steam engines, which developed a total of 4,000 indicated horsepower (3,000 kW) from four Yarrow watertube boilers and gave a maximum speed of 12 knots (22 km/h; 14 mph).

The ship's captain, Commander N. W. Diggle had been walking along the cliffs with Vice-Admiral Keyes when they heard the explosion and both men quickly returned to the harbour.

Cossack's first 18-inch (460 mm) torpedo struck the anti-torpedo bulge amidships, but failed to explode because it had been fired too close to Glatton.

[6] The Antarctic explorer Surgeon Lieutenant-Commander Edward L. Atkinson, although rendered unconscious by the first explosion and burned and blinded, was able to rescue several men before escaping, and was awarded the Albert Medal.

[7] A Court of Enquiry held immediately afterwards found that the explosion had occurred in the midships 6-inch magazine situated between the boiler and engine rooms.

The red lead paint on the bulkhead was blistered beneath the lagging and tests at the National Physical Laboratory demonstrated that it had been subject to temperatures of at least 400 °F (204 °C).

Part of the cork was missing and folded newspapers were found in the empty space which were left there by the dockyard workers during construction.

Furthermore, a number of rivets were entirely missing which meant that 0.5 inches (12.7 mm) holes were present, which could have allowed the hot ashes to ignite the newspapers.

The forced-draught pressure in the boiler room would have supplied air through the rivet holes, causing the cork to give off flammable gases, and eventually ignite the cordite charges.

It was necessary to seal all of the holes on her topside and pump air into each compartment at a rate of 70,000 cubic feet (2,000 m3) per minute to restore her buoyancy.

The first attempt to lift her began on 2 December 1925 and was successful in breaking the suction holding her to the bottom in combination with the rising tide.