This paper presents a numerical analysis on the induced relation between hull surface roughness and ship performance and discuss how to maintain hull-surface with cost and environmental impact in mind. The analysis is based on CFD simulation of the ship performance due to change of hull surface roughness condition before/after dry-docking and in-water hull cleaning. A typical tanker ship, KVLCC2 is investigated for 14 different partial cleaning cases. The attainable reduction of propulsive power by hull surface treatment is estimated as an index, Cleaning Efficiency Index (CEI). A clear understanding is obtained how hull geometry has profound implication for the effect of roughness on the change of power. Partial hull cleaning of fore-end and stern-aft part of the hull was found to give higher relative CEI than entire hull cleaning. The present study provides guidelines which part of the hull to treat during dry-docking and hull cleaning process with better quality or higher priority if necessary