Air lubrication for ships has been around for a long time (decades now?). It's a great idea. This study gives a single point design as an example, stating that

Air lubrication for ships has been around for a long time (decades now?). It's a great idea. This study gives a single point design as an example, stating that in some configurations you save energy and in others you don't. The challenge is that to realize benefits in practice, the air bubbles must maximally coat and remain near the skin of the hull for as long as possible. The main issues I've always seen is that there are a number of factors that work against this objective. Namely: a.) Ships can have very different hull shapes and while flat bottoms can allow an air bubble curtain traversing fore to aft along the vessel bottom, it leaves the sides 'uncoated', and V-hulls tend to allow bubbles to escape upwards leaving the aft of the hull uncoated, b.) ships tend to operate over a range of speeds through water, so a configuration at speed X may not be ideal for a speed Y, c.) there and ocean currents are incident at any angle and speed (relative to the hull heading), and there are additional wave action and ship motion (pitching, rolling, etc.) forces at play that can all disrupt bubble adhesion The cumulative reduction in hull resistance needs to be enough to counter the cost of compressing air (which is somewhat costly), not to mention the acquisition & maintenance cost of the air lubrication system machinery itsel (compressors, valves, piping, etc) - and the bit of real estate/internal volume that the system consumes. Don't get me wrong, I love the idea of air lubrication systems, but I believe the uncertainties of being able to keep and air curtain entrained along the hull such that a net-reduction energy consumption is realized is probably why this isn't more prevalent. Who wants to be the Navarch/marine engineer that talked their customer into buying a system that promissed savings, but ended up costing instead?