In this experimental and numerical study, the effect of the water flow in an open free-surface channel past different bluff bodies (prisms) is examined. Each prism was supported by a flexural plate and seven different shapes were experimentally tested: D-shape, inverse C-shape, circular, squared, triangular-120º, triangular-60º, and diamond-shape. All prisms had the same weight. The unsteady flow past the prism generated a prism span-wise motion whose amplitude and frequency was measured experimentally. The flow Reynolds numbers Re, based on the prism cross-wise width D, were varied between 400 and 1000. Hysteresis was also studied by decreasing the volumetric flow. Results show that the vibration amplitude strongly depends on the prism shape and the Re. In the range of this study, the D-shape, C-shape, and triangular-120º-shape were the most efficient in terms of the prism motion amplitude. For Re=800-1000 the minimum amplitude was obtained for the square section. Detailed analysis of the prisms wakes provided by the numerical simulations helped to understand the experimental results.