An Air Cavity Hull (ACH) uses broad, shallow recesses built into the underside of the ship's hull in order to reduce water drag and improve fuel efficiency. The innovation is being developed primarily by the DK Group, a marine engineering firm located in Rotterdam in the Netherlands.
ACHs use the same basic principle as supercavitating torpedoes, in that an object or hull encased in a bubble of air can slip through water much more easily than a hull slicing the water directly. Most drag in water is caused by friction created by the hull and water. Air has less than 1% the viscosity of water, so it basically "lubricates" the ship as it moves through the thicker medium, allowing much easier passage.
But where supercavitating torpedoes completely encase the torpedo, an ACH uses a broad cavity filled with compressed air to allow a large percentage of a ship’s subsurface hull to glide through the water with less resistance. This produces less drag and increases fuel efficiency.
A compressor near the bow would draw in air from the deck and pump it into the subsurface cavity. Air would build up in the cavity under pressure, giving the vessel a large swath of its ‘hull’ that produces far less drag than the rest. Air would also slowly seep away along the sides and the stern of the cavity, but under most circumstances can be readily replaced by the compressor to maintain optimal pressure. Tests have shown that the compressor would use about one percent of the ship’s available power, which is more than made up for in the increased engine efficiency the air cavity hull provides.
The DK Group’s test vessel, which used its patented Air Cavity System, showed a 7% increase in fuel efficiency. Larger vessels, which would be more stable in the water and would lose less air from less pitching and rolling at the surface, would see an even larger increase. Other research programs studying in both the Netherlands and Russia indicate that a 20% increase in efficiency is theoretically possible, but the best tests have ever been able to achieve is about 10%.
These figures are dependent on the ship moving in more or less calm waters. In choppy or stormy waters, much more of the air in the subsurface cavity will escape, making it more difficult to maintain optimum pressure. But then, ships in rough waters rarely operate at full efficiency anyway, no matter what systems they use.
Originally designed to be used in newly designed ships, DK Group has recently developed a retrofit process that allows them to modify existing vessels with ACHs in about 14 days. The average ship can expect to recoup the cost of the retrofit with fuel savings after about 2 years at current fuel prices.
http://dkgmt.com/http://www.scientificamerican.com/article.cfm?id=air-cavity-system http://bentley-marine.com/Advanced_Hulls.htm http://www.marinetalk.com/articles-marine-companies/art/Air-Cavity-System-Might-Save-Fuel-DKG00271646TU.html http://www.marinelink.com/News/Article/Air-Cavity-System-for-Existing-Vessels/333581.aspx
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