Fouling-release of barnacles and algae from a ship hull

Paper in proceeding, 2012

Fouling-release of various life stages of the barnacle Balanus improvisus and the green alga Ulva sp. was measured from different surfaces on a boat hull. Barnacles settle as 1 mm cypris larvae and green algae as 10 µm spores that grow into adults of cm-size. The hydrodynamic forces acting on the different life stages of fouling organisms vary as the organisms change size and shape during development. It is of interest to find the flow speeds required for fouling-release of the different life stages. In measurements of fouling-release under defined hydrodynamic conditions and flow speeds of 20 knots significantly more newly settled cypris larvae were removed than metamorphosed barnacles. The detachment failure mode is believed to be different between the two barnacle stages. The material tested were two silicone based products and a reference Plexiglas (PMMA). For the alga Ulva sp. newly settled (12 h) spores were difficult to remove in 20 knots while a large proportion of the 3 week old sporelings were removed in 10 knots. The density of barnacles was not found to influence detachment, whereas for algae detachment increased with higher density. The importance of bacterial film composition for the early growth in Ulva sp. is also described. Based on field trials, laboratory experiments and calculations, the flow speeds critical for fouling-release of various life stages of Balanus improvisus and Ulva sp. is presented and discussed. The results are important from coating development and user perspective. Ships spending only short periods in port may be self-cleaned of barnacle-larvae while for metamorphosed barnacles a considerably higher speed is needed for detachment.