In-service measurements of the roughness induced drag-penalty of a tanker ship

A key element to remedying excess fuel consumption and emissions in the shipping industry lies in improving drag prediction of a roughened ship hull. This research aims to provide better predictions by improving: (i) empirical relationships for equivalent sand-grain roughness for realistic ship hull roughness, k_s and (ii) full-scale predictions based on lab-measurements. Laser Doppler Anemometry (LDA) is used to physically profile the turbulent boundary layer 69 m downstream from the bow on the flat bottom of an operational large chemical tanker. These measurements (Re_τ = O(1×10⁵)) demonstrate that the hull of this dry-dock cleaned and recoated vessel is transitionally rough with k_s⁺ ≈ 20 (k_s ≈ 0.1mm). Established roughness correlations, using surface topography obtained from imprints during dry-docking, provide a similar k_s to the full-scale experiment. The field measurements will be compared to laboratory experiments of a replicated surface to further validate k_s. The current LDA result indicates that the total integrated frictional drag of a freshly cleaned vessel is already more than 20% greater than a hydrodynamically smooth hull.