Unpredictable fuel prices and environmental requirements in recent years
have forced the maritime community to reduce fuel consumptions of ships by
improving their propulsive efficiency. In order to do this for ships, it is
important to not only keep the hull free from biofouling, but also its propeller.
Since ancient times, marine fouling has
been mainly combated by chemical methods in combination with various toxic
compounds such as; Arsenic, Zinc, Tin and Copper. The most effective chemical
method to date has been Self Polishing Co-polymer (SPC) technology containing
Tri Butyl Tin (TBT-SPC). In the 1990’s however, it was demonstrated that TBT
caused harmful side effects on non-fouling marine life. Therefore, “International
Convention on the Control of Harmful Anti Fouling Systems on Ships” was
declared by International Maritime Organizations, banning the application of
TBT paints on newly built ships from 2003 and the use of TBT paints on existing
ships from 2008. This has led to a new range of more environmentally friendly
antifouling systems being introduced to the market.
In the early 1990’s, a new and economically friendly combating system
was introduced, making use of silicon based, low surface energy coating
materials and is known as “Foul Release” technology. Matsushita and Ogawa
(1993) showed that a foul release system can protect propellers from fouling
for at least one year, while Atlar et al. (2002) and (2003) showed that these
coatings have the equivalent drag of a new or well polished propeller and
quantified that the coated propeller can display up to 6% gain in the
efficiency against the same propeller without a coating.
Cavitation inception itself is a complex phenomenon which is far from
being completely understood at present. The mechanisms underlying this
phenomenon are thought to be threefold: water quality (mainly nuclei content
and its statistics); the growth of the boundary layer over the blade sections;
and the type of cavitation to be developed. Amongst them, it is most likely that
the growth of the boundary layer will be most affected by the presence of the
coating while the type of cavitation may also be affected. The foul release
coatings are expected to delay transition from the laminar to the turbulent
flow and hence the associated delay in cavitation inception will also be
expected. This study found that the foul release coating slightly reduced the
cavitation development on the blades.
As far as propeller noise is concerned, the role of the turbulence in
the boundary layer is a crucial parameter, which will be affected by the
presence of the coating; this noise component would suffer from the effects of
possible fouling with an uncoated propeller. Coating of the propeller blades in
this experiment reduced the noise levels.
Overall it seems that foul release coatings are effective in reducing
fouling on propeller blades, while increasing performance, although experiments
are still being conducted at the University of Newcastle, using different
blades of different sizes, differing amounts of coating and application
technique. Keeping propellers free from biofouling will increase the ships
propulsive efficiency and decrease the amount of fuel consumption, which has
obvious environmental impacts.
1 comment:
Hey Nikie,
Interesting paper, I can still hardly believe the costs that biofouling has on the shipping industry, I read somewhere it can create up to 40% drag on the hulls of ships costing thousands per year, the main problem with anti fouling technologies (chemicals)is the damage they can do to marine life (as you said) and requires a lot of investment to produce, although a worthy investment if production can be cost effective.
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