def prop_power(self, propulsion_eff=<NUM_LIT>, sea_margin=<NUM_LIT>):
PP = (<NUM_LIT:1> + sea_margin) * self.resistance() * self.speed/propulsion_eff<EOL>return PP<EOL>
Total propulsion power of the ship.

:param propulsion_eff: Shaft efficiency of the ship
:param sea_margin: Sea margin take account of interaction between ship and the sea, e.g. wave
:return: Watts shaft propulsion power of the ship