Page 543 - Practical Design Ships and Floating Structures

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which prevents fouling of the hull by a well-controlled release of a copper-based toxin, and for a Foul
Release system. The Foul Release system is a non-toxic silicone elastomer onto which fouling
organisms have great difficulty attaching. If vessels are stationary for extended periods, settlement can
occur, but there is only weak bonding between the fouling and the Foul Release surface. Consequently,
the organisms can be removed by the hydrodynamic forces against the hull when the vessel is
travelling fast enough. The speed at which most fouling organisms can release has been measured at
the Florida Institute of Technology by towing experiments (Kovach and Swain, 1998). These trials
have shown that, with the current Foul Release technology, speeds in excess of 15 knots are rcquired to
prevent most fouling types settling.

The following sections describe two sets of towing tank experiments, which have been carried out with
flat plates to study the resistance increase caused by the application of the different paint systems. A
detailed roughness analysis was carried out with a stylus instrument and an optical measurement
system. Both findings have been correlated.

2 DRAG EXPERIMENTS

The first set of experiments involved a 2.55m long plate that was towed in the 40m long, 3.75m wide
and 1.2m deep tank of the University of Newcastle-upon-Tyne. The aluminium plate was towed over a
speed range up to 2ds. The measurements were taken with the three different surfaces, which were the
aluminium reference surface, the 3-coat SPC antifouling scheme and the 3-coat Foul Release system.
The measurements showed that the drag increase for the Foul Release was significantly lower than for
the SPC surface (Candries et al., 1998). Because of the limited speed range and run-length, it was
decided to repeat the experiments with a large plate over a much larger speed range.
The second set of experiments was carried out over a speed range up to 8ds, with a 6.3m long plate,
in the 320m long CEHIPAR Calm Water Tank. The design of the aluminium plate was based on the
NSRDC friction plane model 4125, which has been used for similar experiments at the David Taylor
Model Basin (West, 1973). The total resistance of the plane was measured with the dedicated
dynamometer of the carriage for the same three different surfaces: the aluminium reference surface, the
surface coated with a 3-coat SPC antifouling scheme and the surface coated with a 3-coat Foul Release
system. Figure 1 shows the total resistance coefficients for the three surfaces plotted against the
Reynolds number. Above a Reynolds number Re = 2-10’, the Foul Release surface exhibits a drag
which is on average 1.56% higher than the aluminium surface, and the SPC surface exhibits a drag
which is on average 2.91% higher than the aluminium reference. In other words, the total drag
coefficient of the Foul Release surface was on average 1.41% lower than the SPC surface (Candries
and Atlar, 2000).

2 ROUGHNESS MEASUREMENT
The roughness measurements were initially carried out with the BMT Hull Roughness Analyser, which
is the standard instrument for use on ship hulls. The stylus instrument measures Rt50, which is the
highest peak to lowest valley perpendicular to the mean line over a length of 5Omm. When the stylus
has traversed the evaluation length, fifteen readings of Rt50 and an average, the Mean Hull Roughness
(MHR) are printed out. The instrument was used throughout both sets of the towing tank experiments.
In general, 10 and 20 MHR values for the small and large plate respectively were averaged to obtain
the overall Average Hull Roughness (AHR). It was observed from the beginning that the measurement
of the Foul Release surface required a special treatment in that the coated surface had to be wetted
slightly in order to get meaningful readings. If the surface was dry, the stylus hopped over the rubber-