use  compressed  air  for  the  operation  of  the instruments.   Compressed   air   is   also   used   in diving  bells  and  diving  suits.  Our  following discussion on the use of compressed air as an aid in the control of submarines will help you under- stand  the  theory  of  pneumatics. Submarines  are  designed  with  a  number  of tanks that may be used for the control of the ship. These tanks are flooded with water to submerge, or they are filled with compressed air to surface. The compressed air for the pneumatic system is maintained in storage tanks (called banks) at a  pressure  of  4500  psi.  During  surfacing,  the pneumatic system delivers compressed air to the desired control tanks (the tanks filled with water). Since the pressure of the air is greater than the pressure of the water, the water is forced out of the  tank.  As  a  result,  the  weight  of  the  ship decreases. It becomes more buoyant and rises to the surface. METALS As  you  look  around,  you  see  not  only  that your  ship  is  constructed  of  metal,  but  also  that the boilers, piping system, machinery, and even your  bunk  and  locker  are  constructed  of  some type of metal. No one type of metal can serve all the needs aboard ship. Many types of metals or metal alloys must be used. A strong metal must be   used   for   some   parts   of   a   ship,   while   a lightweight  metal  is  needed  for  other  parts.  Some areas require special metal that can be shaped or worked  very  easily. The  physical  properties  of  some  metals  or metal  alloys  make  them  more  suitable  for  one  use than  for  another.  Various  terms  are  used  in describing the physical properties of metals. By studying   the   following   explanations   of   these terms, you should have a better understanding of why  certain  metals  are  used  on  one  part  of  the ship’s  structure  and  not  on  another  part. BRITTLENESS is a property of a metal that will allow it to shatter easily. Metals, such as cast iron or cast aluminum and some very hard steels, are brittle. DUCTILITY  refers  to  the  ability  of  a  metal to  stretch  or  bend  without  breaking.  Soft  iron, soft  steel,  and  copper  are  ductile  metals. HARDNESS  refers  to  the  ability  of  a  metal to  resist  penetration,  wear,  or  cutting  action. MALLEABILITY  is  a  property  of  a  metal that allows it to be rolled, forged, hammered, or shaped  without  cracking  or  breaking.  Copper  is a very malleable metal. STRENGTH  refers  to  the  ability  of  a  metal to   maintain   heavy   loads   (or   force)   without breaking. Steel, for example, is strong, but lead is weak. TOUGHNESS is the property of a metal that will not permit it to tear or shear (cut) easily and will  allow  it  to  stretch  without  breaking. Metal preservation aboard ship is a continuous operation,  since  the  metals  are  constantly  exposed to fumes, water, acids, and moist salt air. All of these elements will eventually cause corrosion. The corrosion of iron and steel is called rusting. This results  in  the  formation  of  iron  oxide  (iron  and oxygen)  on  the  surface  of  the  metal.  Iron  oxide (or  rust)  can  be  identified  easily  by  its  reddish color. (A blackish hue occurs in the first stage of rusting  but  is  seldom  thought  of  as  rust.) Corrosion can be reduced or prevented by use of better  grades  of  alloyed  metals.  Chromium  and nickel  are  commonly  used.  Coating  the  surface with  paint  or  other  metal  preservatives  also  helps prevent rust. Metals   and   alloys   are   divided   into   two general classes: ferrous and nonferrous. Ferrous metals  are  those  composed  primarily  of  iron. Nonferrous metals are those composed primarily of  some  element  or  elements  other  than  iron. One way to tell a common ferrous metal from a nonferrous  metal  is  by  using  a  magnet.  Most ferrous metal is magnetic, and nonferrous metal is  nonmagnetic. Elements must be alloyed (or mixed) together to  obtain  the  desired  physical  properties  of  a metal.  For  example,  alloying  (or  mixing) chromium and nickel with iron produces a metal known as special treatment steel (STS). An STS has great resistance to penetrating and shearing forces.  A  nonferrous  alloy  that  has  many  uses aboard  ship  is  copper-nickel.  It  is  used  extensively in  saltwater  piping  systems.  Copper-nickel  is  a mixture  of  copper  and  nickel.  Many  other different metals and alloys are used aboard ship that will not be discussed here. 2-19