Torpedo protection

[paul.mercer]

Gentlemen,
Most of the battleships sunk in WW2 appear to have been sunk by torpedo, Bismarck, Yamato, Musashi, Barham, Scharnhorst, PoW, Repulse etc. I realise that some like Bismarck and Scharhorst had taken a fearful battering from heavy shells before they were torpedoed but it does make me wonder why ships were designed with sufficient armour to take punishment from battleship size shells and still float, but the armour around where a torpedo would hit was often penetrated. Again, I realise that the force of an underwater explosion is directed against the hull (as was the bouncing bomb when it went off against the dam) but surely Navy designers must have recognised this threat, so why did they not beef up the underside armour?

[tommy303]

It was not realistic to armour the underwater portion of a ship in the same way you did against shells, as the dynamics of a shell hit are quite different from those of a contact or non contact underwater explosion. Not only would the armour itself have been prohibitively heavy, but so too would the backing structure to carry it (if too weak, it would allow the plates to be shifted inwards causing flooding anyways). Since armouring was impractical, designers sought to limit the amount of flooding to outer areas of the hull and provided an inner armour material bulkhead or bulkheads to contain the flooding and prevent it from penetrating to the vitals of the ship. Outboard of this bulkhead was a large void area in the most basic design; in more elaborate designs this outer area was divided into alternate layers of void and liquid filled compartments. The void was intended to allow the explosion bubble to expand in a controlled manner, losing some of its force before impacting either a liquid loaded layer or the main torpedo bulkhead; the liquid loaded layer was expected to absorb more of the energy and to spread the stress forces over the torpedo bulkhead which would deform elastically without rupturing. Generally the hull plating was kept as thin as structurally possible to avoid or minimize a torpedo detonation's generating secondary splinters which might pierce the torpedo bulkhead.

Some designs worked well, some less so, and some were never put to the test of combat. The main problem was that the systems even when performing well amidships, might perform less well fore and aft where the width of the system narrows to follow the hull contours until it was no longer possible. Since it was recognized that a torpedo hit would cause off center flooding, quick counterflooding and the ability to pump large amounts of water or fuel to the unaffected side would help minimize loss of stability. However, here again, some ships such as the Iowas had excellent liquid transfer capacity, while others, such as Yamato, were less satisfactory. No matter how good a system was, it was recognized that enough hits in rapid succession along one side of a ship would probably lead to loss of stability and capsizing well before loss of buoyancy. It was also recognized that some areas were impossible to adequately protect, such as the bow, and most particularly the stern where propeller shafting of necessity penetrates the side defence system and causes an inherent achilles heel.

[Mostlyharmless]

I seem to remember reading that counter flooding in Yamato was controlled from a station outside the armoured citadel. Naturally Murphy's Law caused that station to be hit by a bomb from a dive bomber.

Although the USN and the RN must have made extensive tests of various possible Torpedo Defence Systems or Side Protection Systems, details of the Japanese tests are perhaps easiest to find online at http://www.fischer-tropsch.org/primary_ ... S-01-9.pdf.

[tommy303]

True. Since Murphy's Law has yet to be repealed, one can never discount it in factoring anything.

[RF]

Is the fact that a torpedo - or at least the ship or submarine launched torpedo - has a heavier warhead than a shell not relevant here? A 16 inch shell weighing around a ton, whereas the 21 inch torpedo was about three tons?

[alecsandros]

I don't think so.
Not the total weight - but the weight of the explosive charge. The torpedoes carried warheads of 200-300 or even more kg of TNT equivalent, whereas common AP shells had bursting charges of ~ 20kg of TNT.

[Keith Enge]

The vulnerability to torpedoes is explained by the old adage: bomb and shell hits let in air; torpedo hits let in water. Tommy303 explained torpedo defense systems well. You don't want rigid armor to counter torpedoes. This just transmits the shock, displaces stuff, and causes other internal damage. Instead, you want something to cushion the blow. Voids allow expansion of the explosion while liquid-filled compartments slow splinters so they can't penetrate the innermost torpedo bulkhead. That torpedo bulkhead must be able to flex elastically so has a minimum of supports (the supports are rigid and so the bulkhead tends to tear rather than flex near supports).

In their design of the South Dakota class and then the Iowa class, the US made the mistake of trying to incorporate armor into their torpedo defense system. They continued their armor belt downward, thinning as it dropped deeper. They found that this actually weakened their TDS, not strengthened it so they had to make some last minute changes to the design to compensate.