A Need for Underwater Speed (Supercavitation)

When I was a young lad, I had the privilege of being a mate on a 17^th century replica sailing ship. I'll keep the name of the vessel withheld, but in the course of an autumn I enjoyed New York Harbor and the Jersey coast underway by power of sail and wind. It was one of the few incredible academic experiences offered by my inner city school.

However, as a preteen and a student, my duties were undemanding. In shifts: keep watch for floating debris; steer the helm; night watch; climb the masts; check ropes for faults; galley duty; cleaning; and finally the segue for which I'm searching - take measurements. Here is where I learned the meanings behind fathoms and knots, two nautically-specific units of measure.

The U.S. remains one of the few nations to measure water depth in fathoms. Originally, one fathom was the length of rope between a sailor's outstretched arms where he would tie a knot. The sailor would then tie off a lead and cast the line overboard to provide an estimate of water depth. Today, the fathom has been standardized as six feet.

The knot, as you might have guessed, also pertains to a count of binds in a rope. A line was tied to a triangular piece of wood and thrown behind the ship; the wood would remain stationary while the vessel sailed onward. Over the course of 28 seconds the line extended from a reel, and these lines had knots placed every 47 feet 3 inches. The sailor would record the number of knots for a numerical representation of vessel speed. Why 47 feet and 3 inches in 28 seconds? It's a ratio of traveling 6,080 feet (nautical mile) in 3,600 seconds (an hour); or one nautical mile per hour.

Of course, these measurements are useless without something to measure, and very boring without something fast to measure. In 1978, Australian Ken Warby piloted the Spirit of Australia to 275.98 knots (317 mph), a record that stands today. This record has lasted 35 years in large part due to the 85% fatality rate of attempts to set the watercraft speed record. It's a dangerous game of physics, engineering, and courage.

What could be more extreme? Consider the speed record for submarines.

A submarine is never going to be able to attain the 276 knots achieved by Ken Warby; fluid mechanics just doesn't work that way. Yet there is clearly value in having submarines that can attack navy fleets and abscond from the scene. The Soviet K-222 Papa-class submarine had a maximum speed of 44 knots, which is blistering fast for underwater boats. This submarine also required a titanium hull to account for the damage caused to the hull by its speed. The K-222 was dismantled in 2010 after over 20 years in reserve service.

But a new generation of engineers and military experts are dreaming up innovative, high-speed vehicles capable of more than doubling the pace of the K-222, and it relies on the property of supercavitation to achieve this goal.

Around the same time as the K-222's production, Soviet engineers began production of the Shkval torpedo. This torpedo and its variants travel at 200 knots -underwater - by the use of supercavitation. Supercavitation is the process where the torpedo envelopes itself in a bubble of gas and water vapor to significantly reduce drag. In the Shkval toperdo's instances, the kerosene and high test peroxide rocket engine provide exhaust gases and vapor which expand around the torpedo. The only part of the weapon that actually contacts water is the nose cone which is especially designed to deflect water at wider angles than a typical torpedo. The result is a torpedo which is actually used to combat other torpedoes, as well as more conventional targets. Russia remains the only nation to have an arsenal of supercavitating torpedoes, though Iran has been developing one since at least 2006. There are some limitations to these torpedoes however, as they can only be shot in a straight line.

Now, American marine engineers are constructing two crafts that utilize supercavitation to enhance the Navy's mobility.

In 2011, the U.S. Navy released photos of its GHOST prototype, a vehicle claimed to be subject to 900 times less drag than a typical vessel. This craft functions similarly to a catamaran, as two hulls suspend the structure of the boat above the water. In this instance, the GHOST has gas turbines that envelop the hulls in exhaust gas. GHOST is designed to provide an extremely-mobile deterrence in littoral combat zones and around the perimeters of battle fleets, though there isn't any word on its production or induction to service.

Also in development in recent years is the Underwater Express, a ship commissioned by DARPA. This program calls for a supercavitating submarine that is controllable at speeds of 100 knots. Travelling underwater at a high speed has significant tactical and logistical advantages, but General Dynamics' Electric Boat is still working on the project. After a few years of updates regarding the Underwater Express, the media seemed to altogether forget about the multimillion dollar research, even though initial tests were planned for 2010/2011. Media requests to Underwater Express's program director have not been returned, but if/when they are, I will update this entry appropriately.

Regardless of the current feasibility of supercavitating ships and subs, it's clear that militaries worldwide are interested in having the fastest, most capable strike capabilities - and unlike the surface speed record, it's unlikely to be 35 or more years until superspeed submarines are a reality.

Resources

(Image credits: Ship Wiki; Roots Web/Ancestry; Pakistani Defense Forum; Business Insider; Aviation Week)

Aviation Week - Run Deep, Run Very Damn Fast

Wikipedia - VA-111 Shkval; Supercavitation

Defense Industry Daily - All Aboard the Underwater Express!

IIAI - Underwater Express...

GizMag - Prototype GHOST military watercraft claims a world's first