The notion of flotation fillings persisted well into the Great War, when Q-ships (armed decoys masquerading as merchant vessels) had their holds packed with timber and empty wooden casks, in the hope that, if the ship’s girder structure held, they would stay afloat after a torpedo hit from the U-boat they were planning to lure to the surface. To give one example, USS Santee had been transferred from the Royal Navy in November 1917. On 27 December when on exercises for her decoy role, Santee was torpedoed off Ireland, probably by U 109. The torpedo made a hole 20 × 21ft (6.1 × 6.4m) in the number six hold, at the engine room bulkhead. Her cargo of timber kept her afloat, and in an awash condition Santee was towed to Queenstown and repaired. Her long and varied career ended in 1958, when she was scrapped in Hamburg.
An underwater contact explosion forms an expanding bubble of gas with external walls which move outwards from the point of ignition in all directions faster than the speed of sound in water. The wall of the bubble next to the ship’s hull will tear open the outer hull plating. When the force of the expanding gas has expended itself, the bubble collapses inwards towards the centre. As the outer expansion wall next to the ship has encountered the resistance of the ship’s hull plating it has expended much of its energy in tearing it open, and therefore does not collapse inwards to the centre of the point of explosion.
The hemispherical outer wall of water on the side away from the ship therefore meets no counter-force coming from the opposite direction. The water wall is directed in a high-speed stream directly into the hole caused by the blast, wrecking internal bulkheads and machinery. This effect is similar to the hollow charge effect of a hit on armour plate in an air environment, but the water is less focused and is moving more slowly than the stream of plasma in the hollow charge. The sheer mass of water, however, is extremely destructive. Venting to the top deck or to the bottom of the ship’s outer plating is of no use at all in resisting either the initial gas expansion or the stream of water, as they are moving too fast and cannot be diverted from their path.
No single individual metal bulkhead, whether armoured or not, even when backed by a liquid, will ever be capable on its own of resisting such forces. It will always give way. Accordingly, torpedo defence systems (TDS) aim to slow down the wave of expanding gas and the follow-up stream of water, absorbing energy in layer after layer in the hope of stopping the forces from reaching the inner vitals of the ship.
These are even more difficult to counter. Usually non-contact, and initiated by a magnetic influence or proximity fuse, such an underwater explosion acts in the same way as the contact explosion against the ship’s side plating. However, the under-keel explosion is aimed at producing a very different effect. The bottom hull plating may be torn open and the keel itself damaged by the initial gas explosion. In addition, the gas bubble acting directly beneath the ship will have the effect of causing the hull to lift in the water. The most serious effects are when this explosion takes place directly amidships.
Since the hull girder is designed to resist the downward weights imposed by the machinery and armament in a displacement condition, it is put under severe strain by the hogging effect. When the gas bubble collapses, the ship will tend to fall downwards into the void created, or suffer a sagging effect, only to be thrust upwards again under the shock of the wall of water beneath the centre of the explosion rushing upwards. Depending on the size of the torpedo warhead, the hull of a small to medium-sized ship, up to a frigate or a Second World War fleet destroyer, will be unable to resist the hogging, sagging and second hogging, and the ship’s girder will fail. Typically, the victim will be broken into two sections and will quickly founder.
There is no physical means of fully countering this effect in a smallish ship. It is unlikely to have the same effect on a 100,000-ton-plus nuclear aircraft carrier, although internal damage may be severe. Reinforcing the keel area at the expense of weight taken from elsewhere is a very limited solution. Adding additional double bottoms or even an armoured inner bottom may add resistance but, even so, the shock wave will be transmitted through the structure and may derange machinery and armament. The best remedy is to detect and avoid, or counter, the incoming torpedo.
In 1884 Sir Edward Reed, the previous chief constructor of the Navy, proposed to the Admiralty his patented design for anti-torpedo defence, comprising a double bottom. The inner bottom was to be composed of armour plate between 2.5 and 4in (63–105mm) thick, with an outer bottom 8ft (2.4m) from the armoured inner bottom, the space between to be closely subdivided. The proposals had the twin disadvantage of diverting weight from the main waterline armour belt, resulting in less protection against shellfire, and of raising the boilers and engines above the waterline where they were less well protected against shells. Although not adopted at the time, Reed’s plan was, however, the forerunner of internal vertical armoured bulkhead schemes conceived for dreadnoughts.
Early anti-torpedo bulkhead systems were limited in their effectiveness. Typically, the designers relied on the coal carried in bunkers on either side of the ship to minimise the effect of a torpedo hit. The inner bulkhead retaining the coal would then become the main holding bulkhead. This posed several problems. Although the coal could act as an energy absorber by being pulverised by the explosion, that held good only as long as the coal bunker was relatively full. When partially or mostly empty, the energy absorption was reduced to nil and, in addition, the presence of coal dust posed the risk of a secondary explosion, as was thought by Ballard to have been the cause of the Lusitania sinking so rapidly. Finally, the inner holding bulkhead was of necessity provided with scuttles to access the coal for feeding to the boilers, and even if these scuttles were closed when a torpedo struck it was likely they would be blown open, leading to flooding.
In fact, the realisation that designers’ faith in coal as protection was misplaced was as important a factor in deciding to switch from coal-fired to oil-fired boilers as was as the increased thermal efficiency of oil fuel. The fluid nature of the oil in the wing tanks could be used to absorb much of the energy of a torpedo explosion. At first, designers feared that the oil itself could be ignited by a torpedo explosion, but in fact this never actually occurred in action. As the fuel oil was used up, the bunker could be filled with water, thus preserving the energy-absorbing nature of the liquid-filled void.