Guided waves are commonly used to control large components such as plates and tubes since they have the advantage to spread over long distances with little loss of energy. Because of the dispersive and multimodal behaviour of guided waves, simulation becomes a very helpful tool for a proper analysis of these tests. The purpose of this paper is to numerically investigate the effect of axial preload on the propagation of elastic waves in helical waveguides. Typical applications are springs and multi-wire strands. The study of wave propagation in prestressed helical structures requires the development of the equation of dynamics in a helical coordinate system. The three dimensional equilibrium equations of prestressed dynamics are rewritten in the helical covariant and contravariant bases. The problem then becomes translationnally invariant along the waveguide axis and can be solved with a semi-analytical finite element (SAFE) technique. The problem is hereby reduced to the two-dimensions of the cross-section. The SAFE eigenproblem can be solved by setting the wavenumber and finding the angular frequency or inversely. This numerical model is validated by an analytical model of wave propagation in a helical Timoshenko beam under axial preload. Dispersion curves for single straight and helical wires are computed to quantify the effect of prestress. For small lay angles, the dispersion curves for wavenumbers as well as energy velocities present small differences with the infinite straight cylinder. The effect of prestress is significant for lower frequencies. Some results are also obtained for helix with larger lay angles.