According to application and ustage,  there are four types of marine generator sets.  

1.  Marine main generator,
     To supply power in the whole vessel.  Main generator is laid in engine room.  This generator is large power and size, and often use sea water exchanger cooling system,  open type. 
     

2.  Emergency generator 
     Generally speaking,  emergency generator use radiator cooling, and closed type. emergency generator with double starter,  electric starter and air starter or electric starter or hand starter.  when the main generator don't work or emergency matter, can open emergency generator.  emergency power is smaller than main generator. 

3.  Habour generator or habor generator 
   Habour generator works when the vessel will arrive at habour or closed to shore,  Habour generator can install on deck or engine room.  
  If it is installed on deck, can use radiator cooling system,  but without double starters,  if in the engine room, can use double cycle water cooling.
  Not at all of vessel install on habour generator,  some times emergency generator can be used habour generator.  large ocean vessel have independent   habour generator 

4.  Shaft generator  (PTO /PTI operation)
    The shaft generator is applied in marine diesel engine output to generating.  to save diesel fuel oil, reduce lubricating oil consumption, etc.  Deep sea vessel or cargo can be used Shaft generator system.