The World's First Supercavitating Boat?

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."

Re:So fast it outran the Link !

[cachimaster]

Great, another public relations company "news". No reporter was involved on this, 100% paid advertisment.
This is journalism today. You want to be on the news, just pay for it. Even slashdot is part of the system now.
BTW this work for universities too, that's why MIT makes the new every time they wire a microcontroller to a dishwasher.

Re:So fast it outran the Link !

[gstoddart]

Great, another public relations company "news". No reporter was involved on this, 100% paid advertisment.

Did you actually RTFA? They're citing people who are casting doubts on the claims, they're talking about people who refused to comment.

So, I'd be more inclined to believe you read the first paragraph and have decided it's a press release.

The presence of things like "I am dubious about the application of supercavitating propellers" tells me this wasn't simply word-smithed to provide only glowing praise.

Re:So fast it outran the Link !

[Solandri]

So a couple years ago I was recollecting to a friend who is in the U.S. Coast Guard about a science program I had seen on TV about a new boat the CG was experimenting with which used hydrofoils to lift the main hull clear of the water when the boat was at speed. I asked him whatever happened to that program as it looked super interesting and promising for high speed water craft. He said they were abandoned because they would routinely be cruising along and strike a submerged log floating in the water which would rip one or more of the hydro foil skis off, and that would be the end of that boat. It happened *all* the time.

The Navy had a bigger hydrofoil project - the Pegasus class [wikipedia.org], built by Boeing. Hitting a submerged log is a euphemism. The story I heard (from some of the guys who helped design and test it for the Navy) was that they were averaging one whale strike a year per ship.

Boeing even took that into consideration with its design. The foils need to rotate up anyway for slow-speed operation in shallow harbors. So on the front foil, they added what they called a structural fuse. Like an electrical fuse is designed to burn out before the wiring does, they added a big metal bar to the linkage holding the foils in place. The bar was designed to break before the foil or its mounting points on the hull if the foil struck anything (a whale or a log). Once the bar broke, the foil would be free to collapse upward. The ship wouldn't be very operational afterwards and would suffer minor damage, but at least it wouldn't sink and an expensive foil wouldn't be ripped off. From what I was told, it worked pretty well. But the frequency of whale strikes*, and the downtime associated with recovering and repairing the ship after one, was just too much and they canceled the program.

Boeing adapted most of the technology into a passenger hydrofoil [wikipedia.org] which I believe is still in service in a few areas around the world. They eventually sold the design and rights to Kawasaki Heavy Industries. I got to ride one when I visited Japan, and it feels more like flying in a plane that it does riding a ship. There's a slight rocking motion, but it's very muted compared to a regular monohull or catamaran. The hull is above the waves and swells, so the ship is mostly unaffected by them.

* The foils are basically wings "flying" underwater and are bound by much of the same physics as aircraft wings. If you go too slowly, they stall and the ship sinks bank into the water. A twist though is that you can get cavitation along the top of the foil. In the air, a wing creates a low pressure zone on top, causing the air underneath to lift the plane. In the water, this low pressure zone can drop so much in pressure that the water boils into vapor and cavitates. Once it cavitates, the water flow is disrupted and the foil loses its lift. (Same problem that propellers suffer, unless they're designed to supercavitate - generate thrust despite cavitating.)

However, since water increases in pressure rapidly with depth, this can be solved by simply running the foils at a deeper depth. Beyond a certain depth, the ambient water pressure is enough to prevent cavitation. This does mean though that you cannot simply "fly" the ship along the top of the water thus minimizing the danger from whale strikes. The minimum depth of the foils will be determined by their geometry and the weight of the ship. So the foils are usually running several meters underwater, making a whale strike a catastrophic event.