airforce airscrew cutout engine fan fly isolated propeller propellor rotation rotational screw symbol thrust boat Burrard canada churn coastline direction foam force inlet island move ocean propel travel wake water wave ACV aerodynamic aerodynamics air air-cushion air-cushion-vehicle air-pressure amphibious craft cushion drive engines fans flexible flexible-skirt flight friction high-air-pressure hovercraft hull hybrid-vessel impeller inflated inflated-skirt low-frictional-force MTV multi-terrain-vehicle multiple-terrain on-board pilot propellers rise sightseeing skirt skirted specialised-transport speed stationary surface surfaces tourism tourists transport transporting travelling vehicle