Over time a ship's hull in the water can become overgrown with barnacles, seaweed, and other marine organisms. This is called a fouled hull.
Fouled hulls are bad because they cause the once smooth surface of the ship's hull to be rough. This rough surface causes drag as the ship moves through the water. This drag casuses reduced top speed, and a need for more power to perform the same as a ship with a clean hull.
This fact that marine life see ship's hulls as a place to make their home has long been a problem to mariners. Antifouling goes back at least as far as the Pheonicians who attached copper bars to the hulls of their ships. Copper, as it breaks down in contact with water produces a chemical which is deadly to barnacles and similar organisms. Copper based powders were later added to marine paints to have a similar biocide effect. Of course, the 20th century saw advanced chemical biocides which were very effective at killing the unwanted organisms. Unfortunately, over time these deadly chemicals built up in coastal waters, and began to kill off oyster beds, and other desirablke life. This had the effect of having most of these "advanced" biocides banned by law, and put men back to using copper based powders in paint.
Scientists are scrambling for a new way to deal with this problem. Organic and earth friendly methods are being tried. The United States Navy is using powdered chemicals delivered by small tubes surrounding the ship's hull beneath the waterline through tiny holes. As water touches the chemical concoction natural biocides are produced that not only keep the hulls clean, but are also biodegradeable and earth friendly.
