Take a look at these three ships sunk as targets for torpedoes.
None of the ships was actually struck by a torpedo. Rather, the torpedo passes beneath the target, exploding under the keel of the ship. The first blast wave does significant damage. But an effect known as the bubble jet is the true shipkiller. The explosion causes a bubble in the water. As the bubble collapses, it pushes water upward, in effect shooting a massive water hammer into the hull of the ship. This effect has long been known, since the early days of naval mines.
You’re probably familiar with the troubles US submariners (and to a lesser extent, destroyermen and torpedo bomber pilots) had with torpedoes in the first year and a half of World War II in the Pacific. One of the major problems was that the US torpedoes were designed to operate just like those in the video, that is, explode under the target’s keel. First, the magnetic exploder was tested in unrealistic conditions before being fielded, and turned out to be wholly unreliable. Further, the depth control mechanism of US torpedoes also consistently sent torpedoes about 10 feet deeper than set, further exacerbating the unreliability of the magnetic exploder.
The remedies for those problems were relatively simple; the depth would be set with the inaccuracy in mind, and the magnetic exploder was removed, relying instead on a contact exploder to detonate the warhead along the side of the ship. This method would not produce the same terminal effects, but an explosion alongside was better than no explosion at all.
That, however, turned out not to be the answer after all. It turned out, the contact exploder was itself flawed. Sub skippers would patiently, and with great skill, stalk their targets to achieve the perfect shot into the enemy flank, only to hear a thud, or worse, a series of thuds, as torpedoes slammed into the side of Japanese ships… and failed to explode. The faulty contact exploder relied on a firing striker that was too thin, too flimsy to withstand a solid hit. A glancing blow would usually yield an explosion. It would be late in 1943 before an improved contact exploder would be fielded.