Two nuclear submarines, one French and one British, collided somewhere in the Atlantic this month, according to the two countries' defense ministries. Both subs were damaged, but no one was harmed. In 2003, a British sub hit an Arctic iceberg during military exercises. In 2005, an American sailor was killed when a U.S. sub crashed into an "undersea mountain." Why are subs always running into things?
Because they're stealthy. So stealthy, in fact, that they don't use the equipment necessary to detect obstacles. Most subs have two types of sonar: active and passive. Active sonar sends out acoustic sounds, or "pings," which can reach thousands of yards. If the ping bounces back, that means it hit an object—like a whale, a ship, or another submarine. But stealth subs often avoid active sonar, since the ping could give away their location. Instead, they use passive sonar, which merely detects sounds. (Sophisticated passive sonar reaches dozens of miles and can even distinguish between different types of boat engines.) If two extremely quiet subs are using only passive sonar, there's a good chance they won't detect each other. That also explains why subs occasionally hit land masses and icebergs—those objects make no sound.
How do the subs get so stealthy? Ballistic-missile submarines are built to evade detection by making as little noise as possible. They move slowly—usually no more than 20 knots. They're coated in anechoic tile, a rubbery substance that absorbs sound and prevents sonar detection. And nearly every moving part is isolated so that it won't transmit sound. The deck where the engine runs, for example, is built on shock mounts, which absorb vibrations. Piping is suspended from rubber-lined isolation hangers, which keep the flow of water from making noise. When an engineer wants his sub to be really quiet, he can switch to heat convection instead of pumps to move water.
The biggest challenge for navy engineers is keeping the propeller quiet, since it can't be isolated. When the spinning blades reach a certain speed, they create bubbles, which make a lot of noise. One quieting technique is to use lots of blades—most sub props are seven- or eight-bladed. That way, each blade doesn't have to spin as fast to create the same propulsion. Engineers will also adjust the shape of the blades and the angle of the propeller to compensate for the flow of water around the hull. (Specifics about Navy propellers are supposed to be secret, but Microsoft's Virtual Earth caught a glimpse of one in 2007.)
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Explainer thanks Norman Polmar, Jon Rosamond of Jane's Navy International, and Capt. Barry Tibbitts.