Part 3 of the series “The Science of Sailing” is dedicated to the topic of viscous drag. Viscous drag is the prime component of hydrodynamic drag for most sailing craft (and most other boats and ships). Its assessment is still surrounded by notions and assumptions that are at best only partially correct – notwithstanding the fact that the study of this subject has been ongoing since William Froude presented his paper entitled: “The Fundamental Principles which Govern the Behaviour of Fluids, with Special Reference to the Resistance of Ships”, in 1875.