Histogram of the mean self-propelled speed (mean from nine trials for each test) for the silicone riblet material applied to a NACA 0012 foil surface. Histogram of the mean self-propelled speed (mean from nine trials for each test) for the silicone riblet material applied to a NACA 0012 foil surface. Error bars are ± 1 s.e.m. The motion program settings below each group of similarly colored bars show the programmed foil movement, defined by frequency (Hz), amplitude (cm) and pitch (deg). Silicone riblet material was tested with the biomimetic surface ridges parallel to the free-stream flow, perpendicular to free-stream flow and ‘inside’ or reverse orientation, with the riblet surface on the inside glued to the foil and the non-biomimetic (smooth) surface exposed to the water. Within each group of similarly colored bars, bars with # symbols are not significantly different from each other (P>0.05). All other comparisons have a level of significance between P<0.05 and P>0.000. On average, the foils with the ‘inside’ orientation propel at a 7.2 % slower self-propelled swimming (SPS) speed than the foils with parallel-oriented ridges. Johannes Oeffner, and George V. Lauder J Exp Biol 2012;215:785-795 © 2012.