An anonymous reader writes: For decades, researchers have been trying to build boats, submarines, and torpedoes that make use of supercavitation---a bubble layer around the hull that drastically reduces friction and enables super-fast travel. Now a company in New Hampshire called Juliet Marine Systems has built and tested such a craft, and says it is the world’s fastest underwater vehicle. The ship, called the “Ghost,” looks like two supercavitating torpedoes with a command module on top, and can carry 18 people plus weapons and supplies. The company is in talks with the U.S. Navy to build a version of the ship that can guard the fleet against swarm attacks by small boats. The question is how well it really works, and whether it can be used reliably and effectively on the high seas.