Passive air vent system for a marine propulsion engine

1. Field of the Invention A marine propulsion system is provided with a hydrophobic membrane disposed across a port of its cooling system which allows air to pass through the membrane, but inhibits water from passing through it. During draining procedures, air can freely pass into the cooling system through the membrane to replace evacuated water and avoid the formation of low pressure regions within the cooling system. During initiation of the marine propulsion engine, following a period when what has been drained from the cooling system, air can freely pass through the membrane to be expelled from the cooling system as it is replaced by water pumped in the cooling system by a sea pump.