Maritime Emission Manager Course

Causes of Surface Roughness and Hull Roughness Reduction

The main factors that influence hull fouling rates are:

• Initial roughness of the hull
• Quality of hull coating
• Robustness of the coating with respect to mechanical damage
• The areas of the hull where there is sunlight, along the sides of the hull and near the waterline.
• Temperature of water (colder water generally means less fouling)
• The salinity of the water (performance coating will be a function of salinity of water)
• Number of algae in the water
• Ship speed and its operation profile (hull moving, speed, at berth, at anchor, layby, etc. or static)
• Hull maintenance

Taking care of the underwater portions of the hull is all about minimizing roughness. This can be achieved via smooth surface finish, more appropriate paint, more appropriate operation and more appropriate hull and propeller maintenance. In order to minimize a ship’s frictional resistance, the owner must address both physical and biological roughness (MariEMS 2017).

1.16.2 Hull Coatings

Controlled Depletion Polymer (CDP): Typical life before recoating is 3 years, but because of the build-up of this leached layer and reduced biocide release micro fouling (green slime or weeds) can become a problem in less than two years. The average hull roughness increase is estimated at about 40μm per year in surface profile, but this can vary greatly.

Self-Polishing Copolymer (SPC): An insoluble metallic or organic synthetic polymer. The chemical reaction provides good control of the rate of dissolution and results in a much thinner leached layer and smoother surface profile than possible with CDPs. No ship movement is required as there is no residual ‘skeleton’ and the surface is actually self-smoothing. Five years of service for high quality systems can be achieved. Average hull roughness increase is estimated at about 20μm per year.

Foul-release Coating: A biocide-free coating that uses non-stick properties to control fouling. It is usually silicone or fluorosilicone based and designed to shed any micro or macro growth when the vessel is underway. Coating gains some of its effectiveness from its extremely smooth surface and this must be maintained for best performance.

In general, application of a good high-quality coating can yield an average reduction of up to 4% in propulsion fuel consumption. Reducing an already rough hull to smoother one and applying advanced coating even can provide 10-12% decrease in fuel cost and this also decreases emissions significantly (MariEMS 2017).

Propeller Roughness and Energy Efficiency

Similar to the hull surface, propellers suffer degradation in performance due to surface roughness. The absolute magnitude of the reduction in ship efficiency due to propeller roughness is less than those experienced with a rough hull surface, but it still has been estimated that it could cause an increase of as much as 6% of total fuel consumption. The effect will be greatest for propellers with large area aspect ratios and for propellers running at high rates of rotation (MariEMS 2017).

There have been important advances in foul-release coatings for propeller blades made in the last 15 years. These new coatings can have better surface properties than the polished propeller surface. Even though they have evolved with very good adhesion properties, the coatings are subject to damage by cavitation erosion and leading edge impingement (MariEMS 2017).