Part 4 of the "Science of Sailing" addresses the phenomena and drag arising from the air-water interface experienced by sailing craft. The topics that are dealt with are wave causation and its consequences on drag, the properties of regular waves in deep and shallow water, the wave pattern generated by the hull of sailing craft, wave drag, interference of wave systems, running trim and sinkage/rise, breaking waves, spray, wind-generated waves and spectra, motions displayed by sailing yachts, wave-added drag, surfing behaviour, cavitation, and ventilation.