# A Review of Advanced Air Lubrication Strategies for Resistance Reduction in the Naval Sector
## 2 Experimental Studies
### Tests Performed on ACDR
Four hull models (shown in Figure 12; the four models being A, B, C, and D) were meticulously designed and tested at various speeds and air flows, employing an evaluation method based on the Froude method. The tests examined the towing drag under conditions with and without air flow, thus enabling a comparison of the effectiveness of various air cavity configurations in the hull. Specifically, Figure 13 features two unpublished images from the experimental study conducted at the naval tank of the University of Naples Federico II by the authors [PERSON] et al. [41]. Figure 12: The 4 hull models under consideration (a), onboard elements of the model (b), and detail of the radial blower (c) (Ref. [41] reproduced under a CC BY license). From the analysis of the results, it becomes clear that air cavities in planing hulls lead to a significant reduction in drag when comparing tests conducted without air to those with air. It has been observed that energy savings become more pronounced at speeds exceeding 5.04 m/s, with Model B proving to be the most efficient under all conditions of speed and air flow. For Model C, to discern the effect of air injection, an air flow exceeding 7500 L/min is required at a speed of 5.04 m/s. The air entrainment coefficient (C\({}_{\text{Q}}\)) (3) used to represent drag data suggests that a value around 1 corresponds to a stationary air cavity. The article also highlights that a more complex hull geometry does not necessarily guarantee improvement, as planing hulls naturally develop low pressure at the bottom, aiding the maintenance of the air layer. Overall, the study provides a detailed analysis of the impact of different air cavity configurations on the drag and hydrodynamic behaviour of planing hulls. One of the most recent studies on ACS hulls was carried out by [PERSON] et al. [42] in a towing tank 135 m long, 7 m wide, and 3.6 m deep. The model (Figure 14a), made from wood-plastic panels, was carefully constructed and equipped with a transparent plexiglass window (Figure 14b). The tests included various combinations of speeds and initial stern inclination angles, utilizing an air cavity pressure of 50 kPa to examine its impact on the ship's drag. The model was subjected to drag tests with various air cavity pressures and initial stern inclination angles. Different speed and pressure condition combinations were explored. Drag was measured using a dynamometer, while pitch and inclination angles were detected with specific sensors. Data were collected in real-time via a data acquisition system. The conclusions of the high-speed Air-Cavity Craft study indicate that selecting an appropriate initial stern inclination angle is key for effectively reducing drag, and this  
Figure 14: The model of the air cavity craft (**a**) and the transparent plexiglass window (**b**) (Ref. [42] reproduced under a CC BY-NC-ND 4.0 DEED license). Figure 13: (**a**) The towing tank test with air injection. (**b**) The test without air injection. angle should vary with speed. Drag decreases as air pressure increases, stabilizing once optimal pressure is reached. At 50 kPa with a displacement of 125 tons, the minimum drag is observed at a 2.5\({}^{\circ}\) angle, showing an 18.3% reduction. Preliminary tests are crucial for validating feasibility and minimizing costs and time.