Experimental Investigation on the Effects of Air-Lubrication in a Stepped Planing Hull

In the constant search for new methods to reduce the resistance of ships and, consequently, overall emissions, active boundary layer ventilation through air injection is currently one of the most effective techniques. While its application to displacement units has reached industrial maturity, its effectiveness in planing units has yet to be confirmed. To this end, in the context of the NAUSICA project funded by the MISE [Ministry of Economic Development], a test program was conducted to assess the effectiveness of the proposed technique on a model of high-performance, double-stepped, 16.5 m long planing hull by measuring the total resistance and its potential reduction with forced air flow injected under the hull, at the steps. In addition to the measurement of resistance and fore and aft immersions, i.e. trim and sinkage, embedded into the resistance test according to ITTC standards, visualizations of the hull bottom were carried out to correlate the measurements of resistance to the shape and geometry of the air layer that develops under the hull. To this purpose, a fixed underwater video-acquisition system, composed of a high-speed camera and a group of LED lighting strips, was placed approximately in the middle of the basin for visualizing the hull bottom. In the first part of the study, reported in this work, we focused on reducing the ship's resistance in the boat speed range of 10-35 knots (FN∇ from 1 to 3.7), which includes the pre-planing speed range. A reduction of up to 20% of the base value was appreciated for the maximum air flow rate injected at both steps in the speed range (FN∇ from 1.4 to 2.5), which tends to decrease as the injection decreases. At 30% of the maximum air flow rate, the reduction is no longer experimentally appreciable. At speeds below the identified range, this reduction is extremely limited (<5%) while at higher values the reduction appears to be negligible, probably because the hull is already significantly naturally ventilated and/or the active ventilation is insufficient.