Influence of roughness on vessel efficiency

Fouling attached to the hull

Fouling attached to the hull

To propel a vessel at a given speed, you need to overcome a series of resistances, including those due to wave-making, frictional, form, appendage and air (there are others but these are the main ones). The key influence on frictional resistance is roughness, in which coatings have a significant role to play.

Depending upon ship type and speed, frictional resistance can account for between 50 and 70% of the total resistance. For a container ship traveling at 20 knots, the propulsion system is roughly 15% efficient (converting 15% of the energy in the fuel into usable thrust, the rest going as waste). Of that 15%, 70% can be needed to overcome frictional resistance; so 1, 2 or 3% reductions in frictional resistance can have significant impacts on performance.

The classic view of roughness is that it includes fouling (slime, weed and animal) and surface effects (detachment of coating, corrosion and cracking, repairs, and cold flow of coating). The correlation between hull roughness and ship efficiency can be made with a variety of equations derived from empirical studies, e.g. the equations for increase in the frictional resistance coefficient due to hull roughness proposed by Marintek or Bowden and Davidson, or the increase in power due to hull roughness proposed by Townsin et al. Hull roughness is typically measured using a hull roughness gauge, giving (Rt50) which is the average peak-to-trough height over a distance of 50 mm.

Rt(50) parameter

Rt(50) parameter.

More about Rt(50)…

Typical coating surface roughness after three years in service has increased with 300+ microns for rosin-based biocidal anti-fouling, 200­ – 300 microns for SPC biocidal anti-fouling, and 60­ – 100 microns for foul-release coating.

Real-life anti-fouling performance can be merged together with the Townsin roughness equation to show the potential impact of different coating technologies on vessel performance. Here Jon showed a slide illustrating the fact that, after 60 months, there will be a fuel penalty of about 8% if using a  self-polishing co-polymer, while the penalty will be more like 12% if using a controlled-depletion polymer.

Foul-release technologies add a different perspective to vessel efficiency. They are self leveling and so automatically create smoother surfaces. Commercial systems have a typical roughness of 60­ – 100 μm and, according to Townsin, give a fuel benefit of around 1­3% compared to biocidal anti-foulings. However, surface roughness is not everything with FR coatings. Candries et al. showed that silicone’s had lower drag coefficients compared to freshly-applied SPC coatings. Their study also showed that rougher FR surfaces (created through surface defects) still maintained the drag reductions. There is strong evidence that roughness of FR coatings is not the controlling factor. An AMBIO EU project showed that fluoropolymers had lower skin friction than silicone’s.

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