Analyzing powers and cross sections for the scattering of polarized protons from Si have been measured at eight energies between 17 and 29 MeV. The analysing powers show an "anomalous" behaviour and an optical model analysis yields a non-monotonic effect in the spin-orbit potential.Nuclear Reactions: fi-Si scattering 17-29MeV, measured a(0) and A(0); derived optical model parameters.