Piezoelectric hydrophones
In 1914 the war at sea was still primarily a matter of bringing the enemy’s fleet to battle and destroying it. To this end, the main maritime weapon was the battleship, and typical of this was the Royal Navy’s Queen Elizabeth with her 15in (380mm) guns. Yet, while the British were successful in keeping the German Grand Fleet in harbour for much of the war, thanks to the Battle of Jutland in May 1916, the battle demonstrated that the Germans had superior sighting equipment and that the quality of the British armoured steel plate was inferior. There were, however, two other naval weapons which really came of age during 1914-18, the torpedo and the mine.
In naval parlance a mine is a weapon designed to explode against a ship, and in this sense there was and is no difference between the mine and torpedo. The first serious attempt to produce an effective mine was by an American, David Bushnell, during the American War of Independence (1775-83). He constructed a one-man submarine which carried a watertight keg containing some 150lb (68kg) of gunpowder. This could be released from inside the submarine and had a handscrew for attaching it to the target ship. Releasing the keg set a clock running in the submarine and this enabled the operator to get his vessel clear before the gunlock mechanism on the charge was actuated. Unfortunately, although his submarine was successful in getting close to British warships on a number of occasions, their bottoms proved too tough for the handscrew to penetrate. He also designed a floating mine, consisting of a gunpowder keg on a log float with an ingenious trigger-operated device, but this met with only very limited success. Robert Fulton, another American and contemporary of Bushnell, also used the submarine concept, in which he tried to interest both the British and French during the Napoleonic Wars. The inventor of the Colt revolver also invented an ingenious system for electrically detonating moored mines and in a trial sank a ship at 6.4km (4 miles) range. This idea was taken up by the Prussians during their war with Denmark 1849-50 and the Russians during the Crimean War (1854-6). At the same time a detonation system for the moored contact mine was invented by the father of Alfred Nobel which would remain in use for the next 100 years. This was made up of a hollow lead horn on the outside of the mine which enclosed a mixture of sugar and potassium chlorate, together with a glass phial filled with sulphuric acid. If a ship struck the mine and bent the horn the phial would break and the resultant chemical reaction produced a flame sufficient to detonate the gunpowder charge.
The American Civil War saw many types of mine used, including Nobel-type chemical fuses and electrically detonated mines. Bushnell’s submarine concept was also used in the form of the `David’, a small hand operated semisubmersible craft carrying an explosive charge, later types being steam driven. Two types of torpedo were also used. The first was the `spar’, which was an explosive charge on a pole suspended over the bows of a small vessel, which, on approaching the target was lowered on a wire and detonated through contact with the hull. It was successfully used against the Confederate ironclad Albemarle in 1864. The second type was the `towing’ torpedo, which was towed by a boat but at a 45° angle rather than directly astern, for obvious reasons. Just after the American Civil War the first self-propelled torpedo was invented by Robert Whitehead. It was driven by a compressed air engine and had two contra-rotating screws to keep it on course, as well as a hydrostatic pressure system to keep it at the correct depth. It did have a number of teething problems, but its first successful use in war came in January 1878 when the Russians, using two Whitehead torpedoes, sunk a Turkish ship in Batum harbour. The new explosives developed by Nobel helped to perfect the torpedo because of their greater power than gunpowder, and very soon torpedoes were being used in two different ways, fired either from a submarine or by a new type of surface warship, the torpedo boat destroyer.
During 1914-18 the sea mine underwent several further refinements. They could either be anchored to the seabed by a weight, or be free floating. In the latter case, various types were used. There were drifting mines suspended from a float, creeping mines on the end of a chain that dragged along the sea bottom, and the oscillating mine, which used a hydrostatic valve to vary its depth below the water surface. Later in the war an ingenious anti-submarine mine, the antenna mine, was developed by the British. This was moored at a depth of some 15m (soft), with a 12m (40ft) copper antenna suspended on top of it by means of a float. If this contacted the steel hull of a submarine or surface vessel it created a sea cell through a copper element within the mine and this fired the electrical detonator.
The war quickly showed that it was the submarine rather than the surface launched torpedo which was the greater threat, and this was dramatically brought home by the sinking of three elderly British cruisers within 75 minutes by the German U-9 on 22 September 1914. The torpedo, too, had become more efficient, especially as a result of the adoption of the gyroscope for directional control by the Austrian Ludwig Obry in 1895. Greater range was also achieved by the use of alcohol or paraffin as a fuel. The threat became especially severe to the British in 1917 when the Germans embarked on unrestricted submarine warfare against merchant shipping and ways had to be found to defeat the menace. The adoption of the convoy system was one, and, apart from the antenna mine, two other methods of destroying the submarine under the surface were the paravane charge, which was towed behind a surface vessel in the hope that a submarine might hit it, and the more effective depth charge which made its first appearance in 1914, although it did not achieve any results until two years later. It was, in essence, a mine launched by a thrower and set, through a hydrostatic fuse, to detonate at a certain depth, usually 25m (80ft). The submerged submarine still had to be detected, however, and a solution to this was not found until 1918 with the invention of ASDIC, named after the Allied Submarine Detection Investigation Committee and later called sonar (Sound Navigation and Ranging) by the Americans. This consisted of a radio transmitter/receiver, which transmitted sound impulses. If these hit a solid object they would be reflected in the form of an echo or `ping’. From the transmission-reception time interval the range to the submarine could be worked out and depth charges set accordingly. Antisubmarine warfare had arrived.
