To prolong the useful life of the paint the toxic compounds must dissolve slowly in sea water. Once the release rate falls below a level necessary to prevent settlement of marine organisms the anti-fouling composition is no longer effective. On merchant ships the effective period for traditional compositions was about 12 months. Demands in particular from
large tanker owners wishing to reduce very high docking costs led to specially developed anti-fouling compositions with an effective life up to 24 months in the early 1970s. Subsequent developments of constant emission
organic toxin antifoulings having a leaching rate independent of exposure time saw the paint technologists by chance discover coatings which also tended to become smoother in service. These so called selfpolishing antifoulings with
a lifetime that is proportional to applied thickness and therefore theoretically unlimited, smooth rather than roughen
with time and result in reduced friction drag. Though more expensive than their traditional counterparts, given the claim that each 10 micron (10−3 mm) increase in hull roughness can result in a 1 per cent increase in fuel consumption their self polishing characteristic as well as their longer effective life, up to 5 years protection between drydockings, made them
attractive to the shipowner.