Structure Preventing Submarine Collapse
In relation to the large amount of pressure on each
side of a submarine when submerged, the closer to the ocean floor a
submarine goes, the stronger the pressure. What is allowing the
submarine itself not to become crushed or concave?
Your understanding of water pressure is correct. If a submarine dives too
deep it will be crushed.
Roughly speaking, a submarine is rated for three types of diving depths.
There is the normal operating depth, a safe excursion depth, and then
Naval attack submarines must be built with great structural strength to be
able to withstand the enormous pressures at the depths they use for tactical
purposes. This is one reason for the cylindrical shape of submarines as
cylindrical shapes are best for withstanding high pressures. The reason for
that is the external pressure puts the shell of the submarine under straight
compression, which the steel is best able to withstand. If the sub had
large flat areas, the pressure could bend the metal relatively easily.
Nonetheless all submarines have a maximum depth; if they venture below that
depth, the hull is in great danger of being crushed. The maximum depth for
modern naval submarines is 800 to 900 feet; I do not believe any naval attack
submarine can go below a depth of 1,000 feet with safety.
For every 10 meters (32.8 feet) increased depth in water the pressure
increases by one atmosphere (about 15 lb/foot^2). That is about 2160 pounds
per square foot (2160 lb/ft^2) or about 1 ton per square foot. At a depth
of 1,000 feet, the pressure increases to 450 lb/inch^2 or 30 tons/foot^2.
Research submarines can go much deeper. The record I have found is "Alvin"
which is owned and operated by the Woods Hole Oceanographic Institution.
This has a maximum operating depth of 4500 meters (14,764
feet)! The pressure
there is around 450 atmospheres (6750 lb/inches^2 or 450 tons/foot^2).
The short answer to your question is that submarines are engineered and
constructed to withstand the pressure up to their maximum operating depth.
Best, Dick Plano,
Professor of Physics emeritus, Rutgers University
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Update: June 2012