There are many robotic professionals and researchers who are working with Underwater Unmanned Vehicles or UUVs. Many of these folks wish to use such robotic underwater units for ship hull cleaning. These units hug the hull while cleaning and scrub the bottom.

Most of this has more military applications for surveillance of harbor and ship assets, but such a robotic system can also serve as a cleaning device, inspection device. These are good for let's say the US Coast Guard to inspect incoming ships from foreign lands or US Naval Ship Yards to inspect its own assets. They are mounted on Robotic UUVs.

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It is moved by the brushes as they spin along the surface of the hull, thus propelling itself as it works. Similar systems exist in the truck and trailer, bus washing industry, although hardly as good as the Interclean or Ryko systems currently leading their transportation sector. Here is the Portable truck washing System, which is somewhat similar although above ground:

http://www.bitimec.com/

Here is the information on the Interclean type systems:

http://www.truckwashguy.com/08072003_1.shtml

Right Now the divers who do this usually with snorkels on hoses and attached to the side of the boat, do it in private marinas when no one is watching. The best way to keep the boat surface of the hull correct is to coat it with;

http://www.Adsil.com

or a ceramic type coating or use a layered surface like a shark so the boundary layer continually moves or use a sound wave on the surface to vibrate to slightly. Like they do to get ice of windshields of cars, Company out of Detroit is doing this now? You see the service is desired so there will be people doing it, even if it becomes illegal due to issues with environmental pollution. A small tethered system like the Space Shuttle inspection system for the tiles underneath could work. Drop the thing in the water and a little video cam shows you, your cleaning process. Folks interested in boat or ship hull cleaning should not necessarily be scared away only due to the environmental aspects and legal issues, as there are people willing to pay lots of money to add the 3-5 Kts to their vessel's speed and protect the Gel Coats. An equipment engineer or entrepreneur should probably think it thru and come up with the BMP and device to do it. Market it and sell it as it solves a perceived real problem? There will be many buyers for this I am certain. Think on all the applications and potential with underwater robotics.