A marine vessel hull comprising an air induction system for introducing air into a cavity under the hull, the hull having an underside and first and second sidewalls and fore planing surface located toward a bow of the hull and an aft planing surface located toward a stern of the hull, the cavity being a volume bounded by the underside and the first and second sidewalls and the fore and aft planing surfaces, the system comprising: an air inlet arranged to receive air at ambient air pressure; a plenum arranged to receive air from the air inlet; a fore planing surface having a trailing edge, the fore planing surface being locatable toward a bow of the hull, the trailing edge and the plenum defining a step therebetween having a finite height and extending spanwise substantially a width of the trailing edge, the step being arranged to communicate pneumatically and hydraulically with the plenum, and a first mixing chamber arranged to communicate pneumatically and hydraulically with the step, the first mixing chamber having a predetermined length; a fore end and an aft end;, characterised in that: the first mixing chamber is located underneath the plenum; and the step is located at the fore end of the mixing chamber such that when the marine vessel moves forwardly through water: a first pressure is generated at the step that is less than ambient pressure; a stream of free water moves past the step; and air communicated from the plenum is entrainable in the stream of free water; and wherein the aft end of the first mixing chamber is arranged to communicate pneumatically and hydraulically with the cavity such that when the marine vessel moves forward, and entrained air moves along the length of the mixing chamber, the kinetic energy of the free stream of water increases pressure of the entrained air so that at the aft end of the first mixing chamber, the entrained air exerts a second pressure that is greater than ambient pressure and air is introduced to the cavity. an air inlet arranged to receive air at ambient air pressure; a plenum arranged to receive air from the air inlet; a fore planing surface having a trailing edge, the fore planing surface being locatable toward a bow of the hull, the trailing edge and the plenum defining a step therebetween having a finite height and extending spanwise substantially a width of the trailing edge, the step being arranged to communicate pneumatically and hydraulically with the plenum, and a first mixing chamber arranged to communicate pneumatically and hydraulically with the step, the first mixing chamber having a predetermined length; a fore end and an aft end;, characterised in that: the first mixing chamber is located underneath the plenum; and the step is located at the fore end of the mixing chamber such that when the marine vessel moves forwardly through water: a first pressure is generated at the step that is less than ambient pressure; a stream of free water moves past the step; and air communicated from the plenum is entrainable in the stream of free water; and wherein the aft end of the first mixing chamber is arranged to communicate pneumatically and hydraulically with the cavity such that when the marine vessel moves forward, and entrained air moves along the length of the mixing chamber, the kinetic energy of the free stream of water increases pressure of the entrained air so that at the aft end of the first mixing chamber, the entrained air exerts a second pressure that is greater than ambient pressure and air is introduced to the cavity. the first mixing chamber is located underneath the plenum; and the step is located at the fore end of the mixing chamber such that when the marine vessel moves forwardly through water: a first pressure is generated at the step that is less than ambient pressure; a stream of free water moves past the step; and air communicated from the plenum is entrainable in the stream of free water; and wherein the aft end of the first mixing chamber is arranged to communicate pneumatically and hydraulically with the cavity such that when the marine vessel moves forward, and entrained air moves along the length of the mixing chamber, the kinetic energy of the free stream of water increases pressure of the entrained air so that at the aft end of the first mixing chamber, the entrained air exerts a second pressure that is greater than ambient pressure and air is introduced to the cavity.