A novel indicator for ship hull and propeller performance: Examples from two shipping segments

Ship hulls and propellers suffer from both mechanical defects and marine growth, which increase surface roughness and thus lead to significant power/speed penalties. A recent standard, ISO 19030:2016, aimed at establishing a transparent and accurate method to compare a vessel's performance to itself over time. However, the standard performance value, percentage speed difference V-diff, is both vessel-specific and sensitive to operation point, most importantly vessel speed. The current paper thus proposes a new performance indicator, based on the concept of equivalent sand-grain roughness height k(s), which would enable comparisons between vessels, increased accuracy for comparison of a vessel to itself over time, and calculation of penalties under operating conditions differing from past data (e.g. slow steaming). Sand-grain height k(s) is determined iteratively, through modelling roughness penalties on propulsive power (Granville's similarity law scaling), comparing modelled and measured penalties, and stopping the iterative process at a given tolerance on power penalty. This new performance indicator is applied to onboard-collected data from a Panamax tanker and two Roll-on/Roll-off sister vessels, yielding qualitative agreement with hull condition reported from diving inspections. Comparative advantages and limitations of ks are discussed in relation to the ISO 19030 performance value.