Resistance Reduction by Air Cavities on a High Speed Craft

Air lubrication is an effective method for reducing the frictional resistance encountered by marine vessels. The Damen Air Cavity System (DACS) achieves this by decreasing the wetted surface area of the hull, thereby lowering hydrodynamic resistance. This study investigates the implementation of air cavity system on a high-speed catamaran, which features a non-flat bottom geometry. The findings indicate that the application of air cavities can reduce the vessel's resistance not only in calm water conditions but also in the presence of waves. Furthermore, the study outlines the limitations associated with the applicability of the system. INTRODUCTION Reducing the energy required to propel a ship can be achieved by minimizing hydrodynamic resistance, which consequently lowers both fuel consumption and greenhouse gas emissions. Traditional hull form optimization primarily targets the reduction of residual resistance. However, frictional resistance—largely dependent on the wetted surface area—remains a significant and often unavoidable component. The contribution of frictional resistance to total resistance varies with the Froude number and vessel type. At low Froude numbers, frictional resistance is the dominant component. In the intermediate range, wave-making resistance becomes more significant, peaking at the so-called resistance hump. At higher Froude numbers, frictional resistance once again becomes the prevailing factor. Air lubrication is a well-established technique for reducing frictional resistance, thereby enhancing calm water performance (Pelzer & Mäkiharju, 2022; Zverkhovskyi & de Jong, 2020). Two principal methods of air lubrication exist. The first involves the injection of dispersed air bubbles near the hull, which reduces the local density of the boundary layer. The second method entails the formation of stable air layers that physically separate the hull from the surrounding water, effectively decreasing the wetted surface area. The Damen Air Cavity System (DACS) is a patented air lubrication technology designed to reduce hull resistance by generating stable air cavities along the vessel's bottom surface. These cavities are formed by injecting air beneath the hull, downstream of a cavitator—a small vertical plate that separates the flow and facilitates cavity formation (Figure 1; Zverkhovskyi (2014)).