Page 331 - Ship Construction.DJ Eyres 6Ed

Ch27-H8070.fm Page 320 Wednesday, October 18, 2006 7:01 AM
320 Ship Construction
that can impair its hydrodynamic performance and adversely affect the ser-
vice of the immersed fittings.
Fittings such as cooling water intake systems are often protected by
impressed current anti-fouling systems and immersed hulls today are
finished with very effective self polishing anti-fouling paints.

IMPRESSED CURRENT ANTI-FOULING SYSTEMS The functional
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principle of these systems is the establishment of an artificially triggered
voltage difference between copper anodes and the integrated steel plate
cathodes. This causes a minor electrical current to flow from the copper
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
anodes, so that they are dissolved to a certain degree. A control unit makes
sure that the anodes add the required minimum amount of copper particles
to the sea water, thus ensuring the formation of copper oxide that creates
ambient conditions precluding local fouling. A control unit can be con-
nected to the management system of the vessel. Using information from
the management system the impressed current anti-fouling system can
determine the amount of copper that needs to be dissolved to give optimum
performance with minimum wastage of the anodes.

ANTI-FOULING PAINTS Anti-fouling paints consist of a vehicle with
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pigments which give body and colour together with materials toxic to
marine vegetable and animal growth. Copper is the best known toxin used
in traditional anti-fouling paints.
To prolong the useful life of the paint the toxic compounds must dis-
solve slowly in sea water. Once the release rate falls below a level neces-
sary to prevent settlement of marine organisms the anti-fouling composition
is no longer effective. On merchant ships the effective period for tradi-
tional compositions was about 12 months. Demands in particular from
large tanker owners wishing to reduce very high docking costs led to spe-
cially developed anti-fouling compositions with an effective life up to
24 months in the early 1970s. Subsequent developments of constant emis-
sion 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 self-
polishing antifoulings with a lifetime that is proportional to applied thick-
ness 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.
The benefits of the first widely used SPC (self polishing copolymer)
anti-fouling paints could be traced to the properties of their prime ingre-
dients the tributylen compounds or TBT’s. TBT’s were extremely active