code: def prop_power(self, propulsion_eff=0.7, sea_margin=0.2):
          
          PP = (1 + sea_margin) * self.resistance() * self.speed/propulsion_eff
          return PP
docs: 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