or the clutch may be separate and the reverse gear may  be  combined. In  most  geared-drive,  multiple-propeller  ships, the  propulsion  units  are  independent  of  each other.  An  example  of  this  type  of  arrangement is  shown  in  figure  8-7. In some installations, the drive mechanism is arranged so that two or more engines drive a single propeller.  This  is  accomplished  by  having  the driving  gear,  which  is  on  or  connected  to  the crankshaft of each engine, transmit power to the driven  gear  on  the  propeller  shaft. Friction   clutches   are   commonly   used   with smaller,  high-speed  engines,  up  to  500  horsepower (hp).   Certain   friction   clutches,   however,   in combination  with  a  jaw-type  clutch,  are  used  with engines  up  to  1400  hp;  and  pneumatic  clutches with a cylindrical friction surface are used with engines  up  to  2000  hp. Friction  clutches  are  of  two  general  styles— disk and band. In addition, friction clutches can be  classified  as  dry  or  wet  types,  depending  on whether  the  friction  surfaces  operate  with  or without  a  lubricant.  The  designs  of  both  types  are similar, except that the wet clutches require a large friction area. The advantages of wet clutches are smoother operation and less wear of the friction surfaces. Wear results from slippage between the surfaces during engagement and disengagement and, to a certain extent, during the operation of the  mechanism.  Some  wet-type  clutches  are periodically filled with oil. In other clutches, the oil is a part of the engine-lubricating system and is  circulated  continuously. Twin-Disk Clutch and Gear Mechanism One  of  the  several  types  of  transmissions used by the Navy is the Gray Marine transmission mechanism.   Gray   Marine   high-speed   diesel engines are generally equipped with a combination clutch and a reverse and reduction gear unit, all contained in a single housing at the after end of the engine. The  clutch  assembly  of  the  Gray  Marine transmission mechanism is contained in the part of the housing nearest the engine. It is a dry-type, twin-disk  clutch  with  two  driving  disks.  Each  disk is  connected  through  shafting  to  a  separate reduction  gear  train  in  the  after  part  of  the housing.  One  disk  and  reduction  train  is  for reverse  rotation  of  the  shaft  and  propeller,  and the other disk and reduction train is for forward rotation. Figure 8-8.—Diagram of airflex clutch and reverse-reduction gear. Airflex Clutch and Gear Assembly On the larger diesel-propelled ships,  the clutch, reverse, and reduction gear unit has to transmit an enormous amount of power. To maintain the weight  and  size  of  the  mechanism  as  low  as possible, special clutches have been designed for large diesel installations. One of these is the airflex clutch  and  gear  assembly  used  with  engines  on LSTs. A typical airflex clutch and gear assembly for ahead and astern rotation is shown in figure 8-8. There are two clutches, one for forward rotation and one for reverse rotation. The clutches, bolted to  the  engine  flywheel,  rotate  at  all  times  with the  engine  at  engine  speed.  Each  clutch  has  a flexible tire (or gland) on the inner side of a steel shell.  Before  the  tires  are  inflated,  they  will  rotate out  of  contact  with  the  drums,  which  are  keyed to the forward and reverse drive shafts. When air under  pressure  (100  psi)  is  sent  into  one  of  the tires, the inside diameter of the clutch decreases. This causes the friction blocks on the inner tire surface to come in contact with the clutch drum, locking  the  drive  shaft  with  the  engine. Hydraulic Clutches or Couplings The fluid clutch (coupling) is widely used on Navy   ships.   The   use   of   a   hydraulic   coupling eliminates the need for a mechanical connection between  the  engine  and  the  reduction  gears. Couplings   of   this   type   operate   with   a   small amount  of  slippage. 8-7