This work addresses the experimental and numerical study of a stepped planing hull and the related ﬂuid dynamics phenomena typically occurring in the stepped hull in the unwetted aft body area behind the step. In the last few years, the interest in high-speed planing crafts, with low weight-to-power ratios, has been increasing signiﬁcantly, and, in such context, naval architects have been orienting toward the stepped hull solution. Stepped planing hulls ensure good dynamic stability and seakeeping qualities at high speeds. This is mainly due to the reduction of the wetted area, which is caused by the ﬂow separation occurring at the step. This paper presents the experimental results of towing tank tests in calm water on a single-step hull model, which is the ﬁrst model of a new systematic series. The same ﬂow conditions are analyzed via Reynolds Averaged Navier-Stokes (RANS) and Large Eddy Simulations (LES), with different moving mesh techniques (overset/chimera and morphing grid), performed at different model speeds. The numerical results are in accordance with experimental data, and overset/chimera grid is found to be the best approach between the analyzed ones. The ﬂow patterns obtained numerically through LES on a reﬁned grid appear similar to the ones observed in towing tank investigations through photographic acquisitions. These ﬂow patterns are dominated by a rather complex 3D arrangement of vortices originating from air spillage at both sides of the step. The understanding of these phenomena is important for the effectiveness of stepped hull designs.