Friction Increase due to Roughness of Ship Hull Paint (Part 2) - Frictional Drag Reduction by Hull Coatings

Abstract

The IMO adopted a revision of MARPOL Annex VI to apply the Energy Efficiency Design Index (EEDI) to newly built ships in July 2011. The target EEDI will be tightened incrementally until 2025. The Energy Efficiency Existing Ship Index (EEXI) was also introduced to existing vessels in June 2021. Ship hull roughness is one of the most important factors that affect ship resistance and efficiency. In this study, portable 3D hull roughness measurement equipment was developed. A method to estimate friction resistance added by hull roughness was devised by using a cylinder test, which can reproduce the speed of a ship in service. The projected area of the “cones” above the non-effective thickness δs to stream direction per unit area which is called the cone projected area (CPA), was calculated using the roughness parameter Rc (i.e.,the average height of roughness components) and the wavelength parameter RSm (i.e., the average length of roughness components). A strong positive correlation was confirmed between the friction increase rate FIR (%) and CPA. Using this relationship, a friction resistance increase caused by “wavy roughness” or roughness of a paint film on a hull in service can be easily estimated from Rc, RSm and δ_s.