All Ships suffer from fouling: the build-up below the waterline of shellfish, seaweeds and other organisms.
This causes drag, which slows the affected craft and increases its fuel consumption.
Regular hull cleaning thus makes a considerable difference to the profitability of shipping.
It also results in a useful reduction in the amount of planet-warming carbon dioxide emitted by the world's merchant shipping--
an industry that many environmentalists think is notoriously dirty and which could therefore do with burnishing its green credentials.
Roar Adland, a shipping economist at the Norwegian School of Economics, in Bergen,
says that a midsized oil tanker's fuel consumption (and also, consequently, its emission of carbon dioxide)
drops by around 9% after its hull is cleaned at sea--something that happens, on average, once every six or seven months.
If the cleaning is done in a dry dock, which allows the process to be more thorough, that figure can be as much as17%.
At the moment, cleaning at sea is done by teams of divers.
In recent years, robots have sometimes been added to underwater cleaning crews, and have proved effective.
Jotun, a Norwegian coatings company, and Semcon, a Swedish engineering firm, propose, however, to go one step further.
They want to replace the divers completely with a machine. That machine, moreover, would not merely defoul a ship's hull, but stop it fouling up in the first place.
HullSkater, as the consortium dub their invention, is a 200kg hull-crawling robot.
It will reside permanently on-board ship, ready to be launched whenever the vessel is stationary--
for example, when it is waiting in the roads outside a port for a berth to unload and load. To deploy the robot, it is first lowered overboard by a crane.
Its four magnetic wheels, each of which is fitted with a motor, then clamp it to the hull and it can start trundling around.
After this, the ship's crew need do nothing. Wherever the vessel happens to be on the planet,
the robot is piloted remotely by an operator on land, who may be half a world away, via a 4G phone connection.
Inspecting and cleaning a hull takes HullSkater between two and eight hours, depending on the size and design of the ship.
The robot is fitted with special brushes that can probe the nooks and crannies of naval architecture--
the areas around propellers, for example--and its four cameras provide its operators with a detailed view of what is going on,
and also help to keep track of whereabouts on the hull the machine is.
The idea is to keep the hull permanently clean, by regularly removing from it the layers or slime-producing bacteria that are the first stage of the fouling process.