Effect of cuprous oxide particles on the drag characteristics of marine coatings

Abstract

Cu2O is an active antifouling substance which is commercially produced with different particle sizes before being formulated into antifouling products, and this consequently leads to different surface finish roughness conditions. The further effects of particle size on the drag performance of antifouling coatings and hence on ship hull resistance, as well as biofilm attachment, has not been explored and studied systematically. Accordingly, about the question of how to make an optimised selection of Cu2O size remains for the ship owners or paint developers. In this research, a number of different sized Cu2O (2μm ≤D50≤250μm) were applied to Newcastle University’s (UNEW) flat test panels. The boundary layer characteristics of the coated surfaces and the uncoated reference surface were measured using two-dimensional Laser Doppler Velocimetry (LDV) in the Emerson Cavitation Tunnel. Pressure drop measurements were carried out using a turbulent flow channel under dynamic flow conditions. The effect of biofilm on the drag characteristics of the Cu2O surfaces under “in-service” conditions was investigated by mounting the test panels the research vessel, The Princess Royal, and examining them every six weeks during a six-month dynamic/static immersion period. The subsequent streamwise pressure drop measurements were conducted on all of the test panels along with uncoated reference panels. In addition, roughness characteristics were analysed by using an optical surface profilometer and the microstructure was studied using Scanning Electron Microscopy (SEM). Based on the roughness function results obtained from the tests, the additional resistance diagrams for full-scale ships were developed according to the Granville similarity scaling law by employing an in-house programme. The research study has systematically explored the effect of different sized Cu2O on the drag and roughness characteristics of marine coatings for the first time in the open literature and hence demonstrated the significant potential impact of this effect on ship performance in-service. It was found that the lowest roughness surface was not desmonstrated by the smallest sized particles because of aggregations which caused an unexpectedly rougher surface and higher fricitonal drag. Apart from the aggregated particles, the overall fricitonal drag was found to increase with particle size and this can be expressed by an empirical linear equation. Details of the time-dependent influence of biofilms on the Cu2O surfaces have been presented. Based on this, the frictional drag changes on different size full-scale ships under similar surface conditions may become more predictable. It was also observed that significant fluctuation occurs on the roughness and frictional drag values for surfaces coated with larger sizes of Cu2O particles, e.g. D50=60μm and therefore, similar behaviours are expected for ship surfaces which have similar or rougher characteristics than this particle size. It has to be noted that the test coatings were purely experimental, having exceptionally high Cu2O contents in the dry film that do not correspond to any commercial antifoulings.