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ABSTRACT: There has been an increasing attention to the effect of hull roughness on ship resistance and powering. In conventional studies, the hull surfaces have been treated as uniform rough surfaces while the real ships’ hulls are exposed heterogeneous fouling accumulation. The work described here presents an experimental investigation into the effect of heterogeneous hull roughness on ship resistance. A series of towing tests were conducted using a ship model of the Wigley hull with various hull roughness conditions, including homogeneous conditions (i.e. smooth and full-rough conditions) and heterogeneous conditions (i.e. ¼-bow-rough, ¼-aft-rough, ½-bow-rough and ½-aft-rough conditions). The bow-rough conditions (e.g. ¼-bow-rough and ½-bow-rough) showed larger added resistance than aft-rough conditions (e.g. ¼-aft-rough and ½-aft-rough) with the same wetted surface area of the rough region. This finding suggests that the hull roughness of the forward part of the hull is more significant than the others in terms of the added resistance. Finally, a new method was proposed to predict the added resistance due to the heterogeneous hull roughness based on Granville’s similarity law scaling and the predictions were compared with the experimental result.