How can anything travel at a speed of over 200 mph underwater? Read on!

By Aussiegirl

Recently there was news of a torpedo that Iran has tested that could travel at the alarming speed of over 200 mph -- under water! You might find this hard to believe, since as everybody knows, you can't run through water at nearly the speed that you can through air -- so how could a torpedo travel so fast? Well, this article from The Nav Log (subtitle: News and Commentary on Military Technology, National Defense, and Other Matters) explains it as being based on the physics of what is called supercavitation (here's a link to the Wikipedia entry on supercavitation).

This Nav Log article explains it thus: Basically an underwater missile, the solid-rocket propelled torpedo achieves its speed by producing an envelope of supercavitating bubbles from its nose and skin, which coat the entire weapon surface in a thin layer of gas. This drastically reduces what would be metal-to-water friction.

Untitled

In the last week of March, 2006, Iran announced the successful test of a new, high-speed torpedo code-named “Hoot” by NATO. Iran accompanied the news with the boast that at a speed of 210 knots, the new torpedo is four times as fast as such US stalwarts as the Mk 46 and Mk 48 torpedoes and is thus “unavoidable” by its intended target.

The new Iranian weapon is apparently based upon Russia’s VA-111 Shkval (Squall) torpedo. The Shkval is a high-speed supercavitating rocket-propelled torpedo originally designed to be a rapid-reaction defense against US submarines. Basically an underwater missile, the solid-rocket propelled torpedo achieves its speed by producing an envelope of supercavitating bubbles from its nose and skin, which coat the entire weapon surface in a thin layer of gas. This drastically reduces what would be metal-to-water friction. The torpedo leaves the torpedo tube at 50 knots, then lights its rocket motor. In tests in the 1990s the Shkval reportedly had an 80 percent kill probability at a range almost four nautical miles, although steerability was reportedly limited.

The reliability of such rocket-propelled torpedoes remains uncertain. The much publicized loss of the Russian submarine “Kursk” was, according to some sources, likely due to an accidental rocket motor start of such a torpedo while still aboard the boat.