On the morning of Thursday, 16th June 1955, HMS Sidon, carrying a crew of 56, was preparing to sail from alongside the depot ship, HMS Maidstone, moored in Portland harbour. Earlier that morning she had embarked two experimental practice torpedoes, fuelled by High Test Peroxide, which were to be fired on the torpedo range that day.
At 0825, as the crew was closing up at Harbour Stations, the highly volatile fuel in one of the torpedoes exploded. The blast wrecked the torpedo tube, damaged two watertight bulkheads, devastated the forward compartments and jammed debris against the forward control room bulkhead. Twelve men in the forward compartments died in the explosion and elsewhere in the boat, seven were injured, others suffered from concussion and all were affected by smoke, gasses and shock.
On the bridge, shortly after the explosion, the First Lieutenant gave Surgeon Lt. Rhodes (pictured opposite) a fast lesson in the use of a DSEA set. The submarine began to settle by the bows, with a list to starboard. The CO ordered the engine room and after escape hatches to be opened and all hands evacuated. With only emergency lighting to guide them, the able bodied assisted the injured out of the boat through the conning tower and after hatches.
Despite the heroic efforts of the crew and a rescue party from the depot ship, it proved impossible to reach the victims in the forward compartments or to control the flooding. Two would-be rescuers actually made it through the debris but were forced back by the toxic fumes. As the water reached the hatches, the rescue party was evacuated. At approximately 0850 Sidon sank to the bottom of the harbour.
When Sidon sank, she took a thirteenth victim with her. The medical officer from the depot ship had gone aboard with the rescue party. He assisted several survivors out of the submarine and, unnoticed, collapsed in the control room and died of asphyxiation, after everyone else had evacuated. Seven days later, the wreck was raised and beached in Portland harbour, allowing a party of volunteers to remove the bodies of their shipmates. A few days later they were buried with full military honours in the Portland Naval Cemetery, on the cliffs overlooking the harbour
A Court of Enquiry absolved anyone aboard Sidon of blame for the loss of the boat. The direct cause of the accident was determined to have been the malfunctioning of the safety features of the torpedo. This particular experimental torpedo programme was terminated shortly afterwards. Two years later Sidon was scuttled in Lyme Bay as a sonar bottom target