Effect of moderate fouling on ship drag penalty

Biofouled ship hull surfaces have been subject of focus for decades as they give rise to higher frictional drag, which results in less efficiency, more fuel consumption thus more emissions. These surfaces are home to various marine species such as slimes, calcareous tubeworms and barnacles, which for hydrodynamicists represent rough walls with a range of scales that can interact with the growing turbulent boundary layer developing over the hull. In order to assess the drag penalty due to the moderate biofouling, a test coupon coated with calcareous tubeworms and barnacles was scanned, scaled and replicated for wind-tunnel testing to determine the equivalent sandgrain roughness ks. Direct drag measurements were performed using a floating element balance, while flow field measurements were obtained by PIV. Fully rough conditions were met in the drag measurements and this allowed the estimation of ks. The wake parameter of this rough wall was found to be different from that of classical smooth walls. This information was used in conjunction with the equivalent sandgrain roughness to predict the drag for a full scale cruising ship.