*Article* **The Effect of Longitudinal Rails on an Air Cavity Stepped Planing Hull**

**Filippo Cucinotta 1,\*, Dario Mancini 2, Felice Sfravara <sup>1</sup> and Francesco Tamburrino <sup>3</sup>**


**Abstract:** The use of ventilated hulls is rapidly expanding. However, experimental and numerical analyses are still very limited, particularly for high-speed vessels and for stepped planing hulls. In this work, the authors present a comparison between towing tank tests and CFD analyses carried out on a single-stepped planing hull provided with forced ventilation on the bottom. The boat has identical geometries to those presented by the authors in other works, but with the addition of longitudinal rails. In particular, the study addresses the effect of the rails on the bottom of the hull, in terms of drag, and the wetted surface assessment. The computational methodology is based on URANS equation with multiphase models for high-resolution interface capture between air and water. The tests have been performed varying seven velocities and six airflow rates and the no-air injection condition. Compared to flat-bottomed hulls, a higher incidence of numerical ventilation and air–water mixing effects was observed. At the same time, no major differences were noted in terms of the ability to drag the flow aft at low speeds. Results in terms of drag reduction, wetted surface, and its shape are discussed.

**Keywords:** computational fluid dynamics; hull design; air cavity ships; hull ventilation; stepped planing hull
