This paper considers the relationship between the resistances of small models of surface vessels as obtained in a towing tank, and the resistances of the corresponding full-size hulls. Tests of the bare model hulls are compared with tests of the same models roughened at the bow by applying narrow strips of comparatively coarse sand. The effects of the artificial stimulation of turbulence caused by this roughening, both in hastening the transition from laminar to turbulent friction and in altering the point of separation of the boundary layer, are discussed. Resistance curves for full-size hulls, as developed from tank tests of their models made with induced turbulence, are compared with direct measurements of full-size resistances, as obtained in open-water tests of the full-size hulls. The test results are presented graphically, and the work of other investigators in the field is included.