Her short career came to an end when she broke her back and sank near South Dowsing Shoal, off Ingoldmells, on 18 September 1901.

Cobra was constructed by Armstrong Whitworth and company as a private venture and was one of two which it offered for sale to the British Admiralty on 12 December 1899.

The Director of Naval Construction, Sir William Henry White inspected the vessel and although considering it to be less strong than would have been the case had it been specified by the navy, could find no particular objections to its hull design.

Lieutenant Bosworth Smith — with a crew of two officers and 48 men — was appointed to collect the ship from Newcastle and take her to Portsmouth where she would be fitted with guns and ammunition.

Wind from the NNW and a rough sea caused the ship to roll heavily, so that there was difficulty feeding coal to the boilers and two had to be extinguished, reducing speed to 5 kn (5.8 mph; 9.3 km/h) by around 22:00.

It was reported that another sailor, reaching the boat after an exhausting swim, saw that "if he added his weight all would be lost, so he said "It's one for many, good-bye all," and he loosed his grip, sinking to rise no more".

[6][7] John Smith, master of the steam fishing lighter '15' left Great Yarmouth harbour at 08:00 and heading towards the South Dowsing light.

He was asked to take news ashore about what had happened, but first returned to Cobra's whaler, which was floating upside down and recovered more bodies from the sea.

On 20 September, Commander Storey of the naval vessel Hearty attempted to tow the wreck — which was still visible — into shallower water.

A Swedish diver — Mr. Frank Carlson — made several descents to the wreck, which was lying bottom up with the stem raised off the sea bed.

He testified that the ship was slightly longer than Swordfish, thus weaker, but to compensate for this had been increased in depth making it 17% stronger.

Carpenter's Mate E. Privett testified that he had felt two slight shocks while standing on the bridge as though the ship had grounded, and that shortly afterwards the stern had risen 20-30°.

Captain Shilling from the steamer Oakwell reported hitting floating wood in the nearby area.

[9] An enquiry was set up in November 1901 under Vice-Admiral Rawson because of ongoing concerns that if Cobra had been dangerously weak, other vessels might also be.

[10] The court martial and enquiry reports were classified as secret for 50 years, preventing outside experts examining the detailed evidence.