Does a helicopter hovering over an ocean displace its weight in water? Suppose an amphibious helicopter is floating on the water. Like any floating object, it displaces its weight in water. Then it starts engine, takes off, and begins hovering at a very low altitude above the water. At this point, does it still displace the same amount of water, indirectly by pushing down on the water surface with air currents? If not, how much water does it displace (in general terms; I'm not necessarily looking for an equation)? Does this change as it gains altitude?
 A: If the helicopter/air/ocean experiment were done in a cylindrical tube like this,

the water in the tube would definitely be displaced downward, because the pressure on the surface of the water would necessarily be equal to the weight of air + helicopter, divided by the surface area of the water.  And, the volume of displacement would be proportional to the weight of the helicopter if all other factors were unchanged. If the cylinder were made infinitely large, the same would be true. Of course, the larger they cylinder, the more spread out the displacement would be.  If a helicopter weighs, say, 2000 lb, then the average displacement over a radius of 100 feet would be only 0.12 inch.
Over the ocean, without an enclosing cylinder, it's a bit more complicated.  There are continents and islands; the air density varies from place to place, etc., but if there is more weight in the air, it is compensated by more pressure at the interface between water (+land) and air.
A: No, when it lifts off, it doesn't displace the same amount of water. The weight of the helicopter is lifted by the rotor blades corkscrewing their way into the sky, nothing to do with water displacement. There is, of course, an extremely shallow indentation and ripples caused by the down draft, but that will vary due to a number of factors like cargo and angle of ascent. Close to the water surface, the helicopter is aided by ground effect, just as a fixed wing aircraft would be.
A: The force that lifts the helicopter is not a buoyant force. Is is generated by the rotor hitting air molecules and sending this down at large speed. How their kinetic er nrgy us disdipated afterwards does not affect the lift, except in cases where the air pressure is altered. 
A: Let's try a similar situation as an example. You slap a person in the cheek. They will certainly feel force(and pain). Now if you stop your hand midway, does the person feel the same force?
Just before your muscles decelerate your hand, it is going at a velocity towards the cheek. This will produce an air current just as in the case of helicopter. But the air current is distributed on a much larger volume. So the energy will get spread accordingly. When altitude increases, this spreading also increases.
It is easier to push away air than water.
