Frictional resistance of antifouling coating systems

An experimental study has been made to compare the frictional resistance of several ship hall coatings in the unfouled, fouled, and cleaned conditions. Hydrodynamic tests were completed in a towing tank using a flat plate test fixture towed at a Reynolds number (Re-L) range of 2.8 x 10(6) -5.5 x 10(6) based on the plate length and towing velocity. The results indicate little difference in frictional resistance coefficient (C-F) among the coatings in the unfouled condition. Significant differences were observed after 287 days of marine exposure, with the silicone antifouling coatings showing the largest increases in CF. While several of the surfaces returned to near their unfouled resistance after cleaning, coating damage led to significant increases in CF for other coatings. The roughness function Delta U+ for the unfouled coatings showed reasonable collapse to a Colebrook-type roughness function when the centerline average height (k =0.17R(a)) was used as the roughness length scale. Excellent collapse of the roughness function for the barnacle,fouled surfaces was obtained using a new roughness length scale based on the barnacle height and percent coverage.