a trade secret. But the propulsion system, which he says generates 30 percent more thrust than any other propeller-based system, essentially “boils water underwater and generates steam vapor.” (I take this to mean the pressure directly behind the propeller blades is so low that the liquid water there “boils” off and becomes a gas—hence the bubbles.)
After doing some digging in the literature, I asked Sancoff whether what’s in the patent filing is really how it works—in terms of how the Ghost creates its mysterious supercavitation. His answer: “No.” (OK, so there’s more to the story here. But you know when you’re supercavitating, he says, because the engine efficiency actually improves as you go faster.)
In any case, the overall design makes the craft go fast, but Sancoff isn’t making any public claims yet about exactly how fast. “We don’t talk about speed, how many weapons [it can carry], or how far we can go,” he says. Yet its rumored speed is at least 80-100 knots—over 100 mph. That’s not going to challenge the top speedboat records—there have been hydroplane efforts (riding on the water surface) that have exceeded 200 mph (174 knots) and even 300 mph (261 knots), some with fatal results—but the Ghost is faster than any previous underwater vehicle, Sancoff says.
What’s more, he says, the Ghost provides a much smoother ride than what Navy SEALs are used to; many of them blow out their backs from the bumpiness of their boats, he says. “Our boat does not have impact from the waves. We cut through the wave,” Sancoff says. “That is critical science.”
Hydrodynamics experts I’ve talked to say the main challenges of such a craft are controlling it, stabilizing it, and making it quiet. Going superfast in a straight line might be doable, they say, but any sort of turning or maneuvering must be done very carefully, because if the bubble layer distorts or breaks down at high speeds, tremendous water forces will come to bear on the foils, which can be catastrophic.
To steer itself through the water and maintain stability, the Ghost uses four movable flaps on the front of each foil and four on the back of each foil, for a total of 16 flaps. (The flaps reach through the thin bubble layer into the surrounding water.) The struts are adjusted to keep the command module out of the water, and the foils stay submerged, so waves at the water surface should only hit the struts, which have a small cross-section.
“It’s computer controlled, like a modern F-18,” Sancoff says. “We’re boring what looks like two wormholes underwater, and we’re flying through foam.” Sancoff himself has been test-driving the ship over the past couple of years. “I have been learning an entirely new craft since then. It’s a totally new experience,” he says. “Just because you drive Grandpa’s boat, you’re not going to drive this one. It’s more like a helicopter.”
As for the craft’s audio profile, Sancoff is proud of its “silent propulsion” system that includes a sophisticated muffler system for the engines. You can’t hear it from 50 feet away, he says.
Coming Out of the Night
With any grand invention like this, some outside experts are going to be skeptical. “I wouldn’t say it’s not going to work. But I have concerns,” says Gary Balas, head of the department of aerospace engineering and mechanics at the University of Minnesota. Balas is an expert in flight and underwater control systems, but his main objection is that the propulsion system of the Ghost, with its forward propellers, is very unusual
