Air Lubrication

Three numbers describe the same technology. A flat plate in the United States Navy's Large Cavitation Channel, with air fed underneath it, sheds more than 80 per cent of its skin friction along its whole length. A vendor brochure offers up to 10 per cent fuel. The first full-scale trial to measure a net saving, on a 116 metre Japanese training ship in 2001, measured the local friction falling where the bubbles hugged the hull and still lost speed on most of its runs, because the bubbles also added resistance and found the propeller. All three are honest. They are measurements at different points along a chain that runs from wall shear stress to fuel in a tank, and almost every argument about air lubrication is really an argument about which point a number came from.

This course builds that chain and then makes you carry a number along it. The free survey course Energy-Saving Technologies introduces the family in outline; this one engineers it, on one ship, with the arithmetic shown. What is inside: decomposing calm-water resistance to size the fraction an air layer can physically reach, and why hull form decides that before anything else does; the three flow regimes the laboratory work found, and the threshold that separates a film which persists downstream from a curtain of bubbles that fades within metres; what sets the air flux a ship needs, and how roughness, fouling and the shape of the bottom move it; the three architectures actually sold, and which hull can take which; the delivery pressure the draught imposes, the compression power that follows, and how a resistance saving must be converted before it can be set against a compressor load; where the air goes once it leaves the covered plating, and what it does to the propeller, the speed log, the coating and the radiated noise; and the yard, class and regulatory scope of a retrofit, including the several different percentages the same system earns depending on whose ledger it lands on.

The opening lesson, on how much of a ship's resistance is friction and how little of it an air layer can reach, is free to read as a sample. The rest of the course follows one 208,000 dwt Newcastlemax bulk carrier from her resistance prediction to her payback, so that each lesson tightens the same number rather than starting a new one.

The house voice here is a sceptical one, and deliberately so. The full-scale record is broader than the sceptics allow and far more thinly instrumented than the vendors allow, the honest range for a net saving is wide, and this course would rather hand you that range with its reasons than a single confident figure. By the end you should be able to work out for yourself whether a given hull is a candidate at all, size the air supply and the machine that has to deliver it, subtract the parasitic load from the right quantity, and read a supplier's proposal closely enough to name the assumption that is carrying it.