Buoyancy and Altitude
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.
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.
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.
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.
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Update: June 2012