A vessel's propeller works by thrusting a column of water away from the main body of the ship, thereby producing a reactive force that moves the boat forward. Most naval vessels use a screw-type propeller that transmits thrust through the main shaft to the thrust bearing. This reactive force reacts to the backward force of the water column to push the boat forward.
Propeller blades rotate downward while water rushes in to fill the space left behind to create a pressure differential. Water accelerates from the front of the propeller towards the back, creating a column of water slightly bigger than the propeller itself. Higher velocity creates a water jet out of the rear of the propeller, which adds momentum and thrust to the water. This thrust moves the boat forward. Propeller blades work in water much like fan blades work in air.
The main thrust bearing is located at the forward end of the main shaft within the gear casing. A reduction gear connects to the main shaft which, in turn, connects to the propeller. Reduction gears reduce high rotational speeds of the engine so the propeller functions at lower rotational velocities. Gears allow the engine and propeller to work efficiently.