The most disastrous accidental boiler explosion in the annals of the Royal Navy occurred in the battleship Thunderer on 14 July 1876, and it led to the death of more than 40 persons. The Thunderer, the sister of the turret ship Devastation, had been supplied with eight rectangular box boilers, arranged in two stokeholds, constructed by Humphrys, Tennant and Company for a working pressure of 30 lb. per square inch. On the day of the accident, the ship proceeded out of Portsmouth Harbour to Stokes Bay to carry out a full-power trial. Many officials were on board to watch the trials, and the ship was just being put on her course, when without any warning, the upper part of the front of the starboard forward boiler was blown out and the stokeholds and engine rooms were immediately filled with the release of steam. The explosion killed fifteen men instantly, including her commanding officer, who was in the engine room at the time, and seriously wounded seventy others, thirty of whom subsequently died of their injuries. Owing to the extraordinary nature of the explosion, the inquiry that followed was very thorough, and many eminent engineers gave evidence. The reasons for the explosion, however, were not far to seek, and they had nothing to do with construction or condition of the boilers. It was show conclusively that no one had opened the stop valves, and that the pressure gauge, having been found out of order, had been shut off, while examination and experiments with the safety valves in the shop left no doubt that they had become inoperative having seized in their seats through corrosion. The safety valves were of the dead-weight type, and while the valves and seatings were of brass, the safety-valve box was a large iron casting. Had the stop valves been open, or the safety valves been in working order, the accident could not have occurred, while if the pressure gauge had been in use, attention doubtless would have been called to the undue rise in pressure. As it was, no one could say at what pressure the boiler gave way. For the inquiry, a model of the boiler as it appeared after the accident was made in Portsmouth Dockyard, and this is now preserved in the Science Museum, London.
The Thunderer, which was launched in 1872 and completed in 1876, was the last capital ship in the Navy to have rectangular boilers. Whilst such boilers had proved satisfactory for steam pressures up to 30 lb. per square inch cylindrical boilers were required for higher pressures required for higher powers. There had been some reluctance to adopt higher steam pressures for fears of the consequences of an accident but, since this explosion showed that even low pressure steam could be lethal, it seems to have been argued that high pressure steam would be no worse. The accident also brought about the introduction of the spring-loaded safety valve, with an alarm whistle fitted which blew when the pressure reached a few lb. per square inch above the set pressure of the main valve; it was also arranged that safety valve easing gear could be worked from the deck above the boilers.
In 1879 the Admiralty issued the Steam Manual for Her Majesty's Fleet; containing Regulations and Instructions relating to the Machinery of Her Majesty's Ships.
The following was how the inquiry was reported in the local press, September 2nd, 1876:
The inquest held by the local Coroner, at Haslar hospital upon the death of the forty-five men killed by the explosion of a steam-boiler on board HMS. 'Thunderer', at Portsmouth, on July 14th, 1876, has been proceeding many days. One of the most valuable witnesses was Mr. F. J. Bramwell, the engineer appointed by the Lords of the Admiralty to examine the engines and boilers of the ship. His examination occupied three entire days of last week He ascribed the explosion simply to an excessive pressure of steam caused by the accidental sticking-fast of a safety valve, and he felt convinced that the valve had not been tampered with. The starboard forward boiler had exploded, blowing away nearly the whole of the top front plate. This plate was about 15ft. long and 4ft. 3 in. deep. It was broken into two pieces. The top plate of the boiler was bent upwards in front. There had been three wrought-iron columns, or stanchions, each eight inches in diameter, supporting beams over the stoke-hole and in the deck above. The after stanchion was broken through at the level of the stoke-hole plates, and again ten or eleven feet above them. The second was much bent, and the third was considerably indented. The construction of the boilers was described as follows: They were nearly rectangular boxes, about 15 ft. 3 in. long-ways by 13 ft. high, and 10 ft. 6 in. wide at the level of the furnaces. Each boiler contained four furnaces. The furnace was a box about 4 ft. high, 3 ft. 2 in. wide, and 7 ft. long. At its hinder end was placed the combustion-chamber, or flame-box. Two furnaces united in one combustion-chamber, and the other two united in another combustion-chamber, the fire being made upon the bars, at a level about half way up the fire-box. The flames and gases went into the combustion-chamber, and returned to the front of the boiler through the tubes extending from the combustion-chamber to the smoke-box. In this boiler there were 306 tubes when the boiler was in its complete condition. The front of the smoke-box was furnished with five doors, which lay at an angle. The roof of the smoke-box rose gradually from the forward end of the boiler towards the after end: it terminated in a vertical opening called the up-take. From the four uptakes, suitable conduits rose to the base of the chimney, where the super-heater was, and thus conveyed from the four boilers in one stoke-hole the production of combustion into the common chimney. The furnaces, combustion-chamber, and tubes were entirely surrounded by water. To enable the rivets to bear the required pressure, they were strengthened by a cap. Fore and aft in the boiler were three rows of stays, containing five stays in a row: they were made of double-eyes which laid hold of the web of the vertical T-bars. The tables of these T-bars were riveted to the side-plates, and in that way the side plates were held together. To hold in the side plates, and the flat sides of the fire-boxes, there were four rows of horizontal stays, eight stays in a row, at each side of the fire boxes, and also to the plates. There were similar stays to stay the top and bottom of the boiler together and to stay the back and front. The front plate of the up-take was stayed to the front plate of the boiler by screwed stays, the stays being 1 3/8 in. diameter over the thread. There is no kind of boiler, except a spherical or a cylindrical boiler exposed to internal pressure only, which could dispense with stays, for even a cylindrical figure subject to external pressure, such as the cylindrical flues of boilers, must be stayed unless they are very short. The flat ends of cylindrical boilers, and the fire-boxes of locomotives, all require to be stayed. Such as we have described is the ordinary type of marine boiler employed for low and medium pressures where compound engines are not used. Each of the four boilers, like the exploded one, had a pair of safety valves contained in one box. The valves were 5 7/8 in. diameter of bore. It is considered that the real cause of this great disaster was the failure of one of these valves, which might be due to corrosion. A separate inquiry has been made by a scientific commission, on behalf of the Board of Admiralty.
Thunderer was and unlucky ship. In January 1879 one of her 12in, 38 ton muzzle-loading guns in the fore turret exploded whilst conducting firing practice in the Sea of Marmora, killing 11 men and injuring thirty-five others. The inquiry initially raised doubts about the current form of gun barrel construction but concluded that the barrel had been double loaded following a misfire. The lack of recoil had not been noticed because the guns were run in hydraulically immediately after firing (she was the first ship so fitted). This particular accident could not have happened with a breach-loader and served to sound the death knell for the muzzle-loader.
