Hydrodynamic slamming is one of the utmost problems in Ocean Engineering. Wave breaking, in the open ocean as well as near the shore, is always accompanied by large energy dissipation. The high pressures exerted on marine structures along with the small time duration make the scrutiny in this research field absolutely important. Hydrodynamic impact problems can by studied analytically, numerically and experimentally as well. However, analytical solutions are of particular interest due to the mathematical challenges involved. A major difficulty that arises from this kind of problems is due to the fact that mixed boundary conditions (Neumann and Dirichlet) hold in the two different portions of the wave front. In this aspect, some rational simplifications have to be made in the conceptualization of the physical problems. In this presentation, the steep wave impact on a vertical cylinder is examined, within the original Wagner theory. A steep wave, is defined as a rectangular mass of water with vertical wave front and it is a formulation exploited to simulate a breaking wave. The problem is solved in 3D. In order to calculate the slamming force on the cylinder, we need firstly to determine the contact line between the solid and the fluid at any time instant.