The HMS Sidon was launched in September 1944, one of the third group of wartime diesel electric S-class submarines built by Cammell Laird & Co Limited, Birkenhead. It is now one of Portland in Dorset’s classic submarine dives. At a maximum depth of 34m, this is a great dive for suitably experienced sports divers.

The Sidon was sunk as an ASDIC (Anti-Submarine Detection Investigation Committee) target on 14 June 1957 almost two years after a tragic accident involving experimental high-test peroxide (HTP) torpedoes, which resulted in the boat sinking alongside in Portland Harbour. The 50th anniversary of this tragedy was commemorated in 2005 and a memorial close to the Portland Heights Hotel was dedicated to those who died in the accident.

At 66m long, the wreck is smaller than the nearby M2, which makes it easier to explore on a single dive. However, this is a dive that will keep you coming back for more.

With slack conditions and in good visibility we descended through the plankton, as shoals of bib parted to reveal the clean lines of an upright and intact submarine, with a pronounced list to starboard. Touching down aft of the conning tower on the sloping deck at a depth of 30m, the shot disappeared, draped over the casing. We followed the shot over the side to check it was free for recovery, and reached our maximum depth of 34m on the sea bed. From there we swam forward past the conning tower at deck level, on our way to the sleek and surprisingly sharp bow. We paused to inspect the torpedo tube hatches, three on either side, hunting out conger eels with our torches before swimming back along the casing, towards the stern, in search of the twin screw shafts. With dive time ticking by, we retraced our fin strokes back along the higher port side, ascending to view the bridge area where we found a suitable point from which to deploy our delayed SMB.
HMS Sidon’s HTP Accident.

On the morning of 16th June 1955, British submarine HMS Sidon was moored alongside the depot ship HMS Maidstone in Portland Harbour and was loading a new design of torpedo, code-named “Fancy,” Two experimental torpedoes with an HTP supported motor, had been loaded aboard for testing. 56 officers and crewmen were aboard.

Quite unexpectedly at 8:25 am, an explosion in one of the Fancy torpedoes (but not the warhead) burst the number-three torpedo tube it was loaded into and ruptured the forward-most two watertight bulkheads killing 12 submariners – 7 others were seriously injured. As always in explosions aboard submarines, fire, toxic gases, and smoke accompanied the blast.

After the accident was investigated, it was found that direct cause of the accident was determined to have been malfunctioning of the “Fancy” torpedo, and that torpedo program was terminated

An engineer was readying a torpedo for the morning test firing and had begun a “hot-run”. Its engine had started while it was still inside the submarine and was over-speeding, over-pressurising the torpedo’s internal pipework and its fuel system. The “Fancy” torpedo used “High Test Peroxide” (HTP) as an oxidizer. When an oxidizer line burst, HTP sprayed onto the copper fittings inside the torpedo, decomposing into oxygen and steam. The torpedo’s warhead did not detonate, but with the massive increase in volume in the sealed torpedo casing, an explosion was the consequence – not of the warhead, but a catastrophic rupturing of the metal casing under pressure, sending large pieces of the torpedo through the submarine with enormous force and causing the flooding that destroyed the boat.

The force of the blast inside the tube blew open the bow cap and internal door allowing water to get in. As the boat was on the surface water entered only relatively slowly and she did not sink immediately. Attempts to stop her from sinking were unsuccessful.

The torpedo was an experimental one using High Test Peroxide, the same type of fuel used in the torpedo which is believed to have caused the more recent tragedy aboard the Russian Submarine Kursk on 12 August 2000

The submarine started to settle by the bows with a list to starboard, and her commanding officer, Lieutenant Commander Verry, ordered the ship evacuated from the engine room and after escape hatches. Within a few moments HMS Sidon sank at the quayside, also claiming the life of the Medical Officer of the depot ship alongside which she was moored, who had rushed onto the stricken submarine to help in the rescue of the on-board crew.

Thanks to a rescue party from Maidstone, everyone not immediately killed escaped except the medical officer who had gone aboard with the rescue party, assisted several survivors, and collapsed, unnoticed, in the control room after everyone else had left. At about 8:50 Sidon sank to the bottom of the harbour.

One week later the wreck was raised and towed into a causeway on Chesil Beach. The bodies of the 13 casualties were removed and buried with full honours in the Portland Naval Cemetery on top of the cliffs overlooking the harbour. A Court of Inquiry cleared anyone aboard Sidon for the loss of the boat.