Monday, December 27, 2010

Air Bubble Shipping

This was self evident to me four decades ago and the only question to me was why it was not been done.  The only reason that made much sense was that compressor technology itself stunk.  It still stinks, but a 500,000 ton container ship can afford to use jet engines to get the job done along with a simple conduit along the hull to allow escapement. And yes, parallel thin strips may help in slowing layer losses.

At worst a portion of the natural friction will be released.

Even better may be to simply deal with the bow itself to reduce its drag and allow normal momentum entrainment to keep the rest of the hull happy.  It is not as if these displacement hulls have any plans for increasing actual speed.

Actually placing a pair of bow props inside a driving channel could also be beneficial inasmuch as it would accelerate bow water under the hull and eliminate much of the bow wave drag.  On the other hand, I do not think it is significant enough, and air bubbles may be easier and cheaper.

At least they are paying attention today to the possibilities.

Airships of the sea

Naval architecture: Blasting a cushion of air bubbles under a moving vessel’s hull can reduce its fuel consumption

Dec 9th 2010 | from PRINT EDITION

IF YOU blow a lot of air bubbles under a ship, and keep them coming, “good things will happen”, says Steven Ceccio, an expert on bubbles at the University of Michigan’s mechanical-engineering department in Ann Arbor. When air is pumped rapidly out of small holes in a ship’s hull, the swarming bubbles will quickly join together and coat the hull with a layer of air a centimetre or two thick. This reduces drag, because air offers far less resistance than water.

As the ship moves forward, the layer of air slides back and out from under the hull. But blowing more bubbles to replenish it does not require much energy, so fuel savings of 5-10% are within reach, says Dr Ceccio. He studies air-lubrication systems, as the field is known, for the American navy, even though warships generally have V-shaped hulls, which facilitate fast travel but are unfriendly to bubbles. Almost all cargo vessels, by contrast, have flat bottoms, which allow a larger volume to be kept buoyant for a given amount of hull metal. Bubbles work well on these and, since the cost of fuel is often more than half of a cargo ship’s total operating expenses, the potential savings are huge.

Bubbles are wont to slip past the edges even of flat hulls, but efforts to hold them in place are paying off, says Uwe Hollenbach of the Hamburg Ship Model Basin, a facility that tests new naval technologies. One trick is to trap the blanket of air between two ridges that protrude a few centimetres downward from the port and starboard edges of the hull. Another is to shape the vessel’s stern in a way that stops air being sucked into the propeller, where it would reduce thrust by lessening the propeller’s grip on the water. It is also possible to design hulls that include air-trapping recesses a couple of metres deep.

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