Flotation

Flotation  : In marine technology, flotation can be divided into materials intended for use on the surface and those for use underwater. For surface flotation, buoys are fabricated out of steel or other metals, molded plastic or plastic foams. Ionomer foam is a popular material because of its reduced weight and high durability.
Underwater, subsea flotation is often needed to achieve neutral buoyancy for subsea vehicles and platforms or to provide a positively buoyant upward force. Because of the compressive force of water pressure, subsea flotation is a more challenging problem then it is on the surface.
For shallower depths, foamed materials may be used. Foams compress as they descend into the depths and thus the amount of buoyancy they provide is not constant. Ultimately at some depth they will become so compressed that they will lose all of their buoyancy. For deeper depths, non-compressible materials that are less dense then water should be used.
The bathyscaphe Trieste solved this problem by using a large volume of aircraft fuel, which is less then water and compresses very little. In 1963, under the supervision of Dr. Andreas Rechnizter, this submersible carried Don Walsh and Jacque Piccard to the deepest point in the world—The Challenger Deep.
In more recent years, non-compressible foams made from a matrix of small glass spheres embedded in epoxy have been used. Known as syntactic foam, this material can be cast into shapes and sanded or machined. Syntactic is available in various densities with different depth ratings. Hollow ceramic spheres are also available for deepsea flotation.
Subsea buoys are made from a variety of materials, ranging from PVC plastic pipes with end caps to titanium spheres. Glass spheres are popular for deep depths, because they are relatively inexpensive. Plastic “hard hats” are often used to protect the glass.