And informed reader is ahead of me if they thought of supercavitation.
From Hakai Magazine, November 12:
A technology called air lubrication offers a way to make large ships more efficient.
Water is not as yielding as you think, says Noah Silberschmidt, founder and CEO of UK-based Silverstream Technologies. For more than a century, gigantic steel vessels have been ploughing the oceans, generating seemingly unavoidable—and surprisingly costly—friction between ship and sea. But this friction can be reduced in an innovative way, says Silberschmidt, with the help of millions of tiny bubbles, each just a millimeter across.
With the push to make shipping more efficient, ship owners are looking for new ways to reduce fuel consumption and emissions. One contender is Silverstream Technology’s eponymous Silverstream System, a device installed in a ship’s hull near the bow that generates a carpet of air bubbles flowing all the way to the ship’s stern.
The concept underpinning the device—air lubrication—is not new, but advancing technology is allowing the company and its competitors to retrofit existing ships with air lubrication systems, or include them in new vessel constructions.
Air is less dense than water, which means that the bubbles reduce the resistance between the ship and the sea around its hull. It’s a bit like gliding your hand through a gently bubbling hot tub versus a still bathtub.
Silberschmidt says that, over time, air lubrication can reduce fuel consumption by five to 10 percent. Fuel savings of a few percent might not sound like much, but Silberschmidt says shipping firms can spend between US $5- and $10-million on fuel for a single average-sized vessel every year....
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And from Popular Mechanics, June 2016:
Bubble-Enclosed Submarines Could Go Really, Really FastSupercavitation promises airplane speeds--under the sea.
Researchers at Penn State are working to enclose the submarines of the future in a bubble of gas, allowing them to achieve top speeds faster than what is possible while moving through regular water. If successful, it could mean submarines capable of speeds of up to hundreds of miles per hour.
As submarines and torpedoes travel through seawater they are naturally at the mercy of physics, and objects traveling through sea travel encounter much more drag than objects traveling through air. The end result is that the practical speed limit of submarines is somewhere around 30 to 40 knots. Even with nuclear propulsion, that's about the best many military submarines can do.
But what if it were possible to enclose a submarine in a gas as it travels through liquid? That's the principle behind supercavitation.
There is a precedent: the Russian VA-111 Shkval torpedo. Developed in the 1970s, Shkval is equipped with a bubble generator in the nose that envelops the torpedo in a gas membrane while a solid rocket fuel engine provides thrust. The Shkval is capable of speeds in excess of 200 knots—up to five times faster than conventional torpedoes....
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