Mairne gearbox is transfer output from marine main engine to propeller, and reduce the speed.  So the gearbox selection is to learn the marine engine and propeller information.  

1.  Marine engine power and speed,   it will be decided on large or small.  the engine power is large, the gearbox will be large.  Gearbox must be working on rated speed range.  

2.  Transfer capacity.   Power/Speed,   the unit is KW/RPM or HP/RPM,  the gearbox must meet the transfer capacity, other wise, it is not enough. 

3.   Ratio:  gearbox reduction ratio is input speed/ output speed. that means engine speed and propeller speed. each gearbox must be confirmed ratio.
 The ratio have nominal ratio and real ratio,  different factory use different.

4.   Flywheel and housing,  gearbox flywheel must be in the consistent with engine, otherwise, can not install.  if the engine cancel housing, the gearbox also cancel housing.  

5.  Flexiable coupling,  large gearbox should be install on flexiable coupling or tooth block connection,  small gearbox without coupling.  install the coupling can increase transfer capacity about 8%. 

6.  Propeller push thrust:  Propeller push is larger, the gearbox should be large