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Name: Alisdair
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Does air pressure affect how high a ship will float above the surface of the water? For example, If I had a ship and sailed it on distilled water in a perfect environment in the Himalayas would it float lower over the water than if I did the experiment at sea level?

Air pressure will have very little effect. The bouyant force that supports the ship is determined by the weight of the water that is displaced. Changing the density of the water by going from sea water to distilled water, as in your example, would make some change, however. Technically, there is a small bouyant force contributed by the air, and that would change with air pressure, but it is so small compared to the contribution from the water that even removing it altogether would probably not make a noticeable difference.

David Brandt

For practical purposes, no. The height at which a boat floats depends only on the density of the submerged volume of the boat and the density of water. Water is very nearly incompressible, so its density will increase only very slightly as air pressure increases. The air in the submerged volume of the boat will increase in density as air pressure increases, but the effect of this change will also be very slight, because air makes only a small contribution the boat's weight. So both density changes would be very small, and their effects would oppose each other, making the combined effect negligible compared to unrelated effects, such as the change in water's density with temperature.

Tim Mooney
Beamline Controls & Data Acquisition Group
Advanced Photon Source, Argonne National Lab.

A ship floats when its weight is equal to the volume of water displaced by the ship. This depends upon the ship's mass and the shape of the ship. Since water is essentially incompressible, the barometric pressure has little effect on the volume of water displaced and the air pressure is the same on both the water and the ship as well.

Vince Calder

Yes it would, but by a very small amount. The higher the air pressure, the more dense it is. The more dense the air is, the more it weighs in a given volume. Since ships have lots of air in them, the ship itself would weigh just a little bit more, and would ride lower in the water. (If you could increase the air pressure enough to make the air as dense as water, the ship would sink!) However, the weight or mass difference of pressurised air is relatively tiny when compared to the weight or mass of the materials used to build a ship, as these are usually metals such as aluminum or steel.

Ryan Belscamper

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