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Hempasil put to the test


Significant fuel savings are claimed for Hempel’s new low resistance anti- fouling coating.


A


recent study from FORCE Technology suggests that Hempel A/S’s silicone-based fouling release


coating Hempasil can result in a fuel savings of 10.6% for a large container vessel. Hempel says that one of its main targets


is to develop products with less impact on the environment. Hempasil, the company says, is such a product. Te silicone-based fouling release system features a ‘non-stick’ smooth and flexible surface with very low surface tension. Fouling organisms do not easily stick to the surface, while its smoothness reduces drag, thus lowering fuel consumption and costs. Hempasil is biocide free. In the FORCE Technology tests, the


silicone-based antifouling coating was compared with a conventional tin-free self-polishing surface coating (SPC). A dedicated test rig was engineered with 2.5m long test plates attached to the towing carriage.


Initial tests in a 220m long towing tank


were done using uncoated hydraulically- smooth plates, which demonstrated that the rig could achieve measurement errors as small as 0.3% at 15knots speed. The hydraulically-smooth plates also compared favourably to the classical theory propounded in textbooks. Coating surfaces for three different


conditions were compared: a surface of a newbuild vessel, and two ship hulls with longer service time manifesting different levels of corrosion. Te ‘medium roughness’ condition corresponded to a ship with several years service, and the ‘high roughness’ for an even older vessel. Each of these surfaces was coated with either Hempasil or with a conventional SPC coating. The test results consistently showed


that the skin friction for Hempasil at three roughness levels was significantly lower than the corresponding skin friction values for the SPC antifouling, according


to the manufacturer. Te re-calculation to full-scale was done using hydrodynamic and turbulence theories and computer calculations of the resistance of real hull shapes of various ship types, including the skin friction component originating from the difference in surfaces texture and coating, the wave resistance component, as well as wake and air resistance components. Te results were translated into power,


fuel, and environmental savings to show the full effect on fuel consumption, and CO2


and NOx emissions. Te results were


also calculated for four different vessel types, in addition to the three different hull roughness categories. The improvement in skin friction


was up to 5% for a Reynolds number corresponding to that of a large container vessel. Fuel savings of between 2% and 11% were achieved for the various types and sizes of vessels.NA


FORCE Technology developed a dedicated test rig with specially- designed 2.5m long test plates which were towed while attached to the towing carriage.


The Naval Architect April 2008


27


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