CHAPTER 17
Passive Defence
The simplest form of passive defence was to prevent the torpedo craft from coming in range of your ship, and then to prevent its torpedo from making contact. This gave rise to various static installations, which varied in their effectiveness.
HARBOUR DEFENCES
Chains stretched across rivers to deter unwanted entry have been known since classical times. In fact the popularity of the traditional London waterfront pub name of ‘Cat and Wheel’ — often sited one facing the other across the eastern stretches of the Thames in London — has been traced back to the old Roman and medieval ‘catanea wheel’, the capstan which stretched the chain taut each night: a romantic but vaguely plausible suggestion.
Wider estuaries to be protected in times of war, and especially harbour entrances, were protected by booms, constructed from old ships’ spars, chained together. It was long thought that the only way to break through was to send boat parties to hack away at the fastenings or break the boom with gunpowder charges.
This idea was literally dashed to pieces when, during the naval manoeuvres of 1885, the Royal Navy’s experimental torpedo ram HMS Polyphemus smashed her way through the boom defending the entrance to Berehaven, the Royal Navy’s anchorage in southern Ireland. Starting out to sea and building up to her maximum speed of 18 knots, the ram had evaded several torpedo boats sent to intercept her — they can be seen to the right of the small insert view in the engraving — and easily broke through the boom.
Booms were still in vogue up until the Second World War to close the entrance to a harbour, but they were now used to support anti-submarine nets. A far more effective deterrent to surface torpedo-boat attack was the laying of electrically controlled observation minefields, the siting of QF guns, and the provision of defensive torpedo installations such as the Brennan.
These were heavy nets supported by buoys, and were quite effective for protecting individual warships, such as the Tirpitz, from torpedo attack, whether from submarines or aircraft. They would not, however, be able to offer protection from attack by underwater stealth craft such as Chariots and midget submarines, whose crews would simply dive beneath the nets, or, if they extended to the seabed, would cut through them.
These were originally intended for use not only at anchor, but also when at sea. The early RN ‘Bullivant’ types consisted of 6.5in (165mm) diameter steel wire rings joined by small steel rings, and weighed only one pound per square foot. They hung down level with the hull bottom, and proved able to stop the original, slow-speed 14in torpedoes.
HMS Mars was the first ship to experiment with net booms lowered almost to sea level. She also carried the heavier Gromet net, which weighed 5lbs per square foot. The Bullivant nets introduced in 1906, had much smaller rings only 2.5in (63.5mm) in diameter and interlinked without separate smaller rings. It hung down some 25ft (7.6m). In 1906, the Channel fleet steamed at 6 knots with their nets down.
With the advent of larger, faster and more powerful torpedoes, often with net cutters fastened to their heads, however, the nets became seen in most navies as a mere encumbrance, threatening to entangle propellers if damaged by gunfire. The Royal Navy held onto them for a long time, and the High Seas Fleet for even longer. Significantly, the US Navy never used them.
TORPEDO PROTECTION IN HULL DESIGN
As soon as the effectiveness of the first torpedoes was conclusively demonstrated, ship designers began to attempt to counter the effects of underwater explosions. Surface-running torpedoes, either by design or by faulty depth mechanisms might, with luck, strike not a sensitive part of the hull such as the unprotected bow or stern areas but directly on the massive belt armour. Given the thickness of the early belts, and their backing by flexible wood layers, the chance of the explosive force penetrating the belt and venting inside the ship was limited. However, as Whitehead had correctly surmised, a ship’s most vulnerable part was the underwater portion of her hull. Protecting against explosions underwater was a much more difficult problem, never fully solved.
The Italian battleships Italia and Lepanto launched in 1880 had foregone extensive armour protection to concentrate on massive main guns and high speed — probably the most extreme examples of what would later become known as the ‘Fisher doctrine’ which he applied to RN battlecruisers. To save these Italian battleships from sinking under the effects of concentrated shellfire, the ends of their hulls were closely subdivided in the hope that they would continue to float even when perforated like colanders. Some Victorian ironclads even had whole areas of the hull packed with cork.
Royal Navy designers toyed with the idea of fitting cellulose inside the hulls of battleships, as suggested by the French, in the hope of minimising the effect of a torpedo hit, given that watertight bulkheads often held. The cellulose was supposed to swell up and help seal the hole. Unfortunately, when this ‘protective’ system was tested on the old ironclad HMS Belleisle on 3 September 1903, the 18in torpedo warhead blew the cellulose filling to bits, and scattered lumps of coal from the bunkers all over the ship. Needless to say, she sank. The results of this trial made designers install armoured screens covering the magazines in the new HMS Dreadnought.