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After reviewing the methods by which frictional resistance of ships is calculated, this report explores the sources of hull roughness and how roughness affects the speed of ships over a time scale. The report discusses the role of coatings and cathodic protection in controlling roughness and establishes a relationship between the size and shape of roughness and the percentage of increase in hull resistance. Model resistance testing, using fiberglass castings from corroded ship plates is described, with some resultant data given, and related papers on roughness measurement are discussed.