A bow thruster is a device used on large ships and yacht-sized boats to aid docking and exiting a dock without the assistance of a tug boat. The most common style of bow thruster is a propeller system that is mounted in the lower area of the ship's bow. Directed from port to starboard, or left to right, the bow thruster is used to push the bow area of the ship away from the dock when exiting port and towards the dock when tying up the ship to the dock. When the ship is sitting still, the captain is able to direct the ship's movement by using the bow thruster, which is able to turn the ship without the aid of forward motion.
When considering the addition of a bow thruster to a yacht or a boat typically too large to be trailered, the first consideration is the method of powering the unit. Whether it be a forward-mounted unit or a stern-mounted unit, electric or hydraulic power are the most common. Electric power is the preferred method of powering a thruster, or side steer unit, as it is commonly called. Many companies manufacture both through-the-hull versions of the bow thruster as well as externally-mounted units, which clear the water once the vessel is up on plane.
One disadvantage of the bow thruster is the slight reduction in forward motion it creates, which causes a vessel to use more fuel to travel the same distance as a non-thruster-equipped ship. The cost is often offset by the thruster's ability to save on tug boat fees in order to dock at some harbors. Another precaution for ships equipped with a thruster involves maintaining clean and clear thrusters. Contact with a muddy bottom in shallow ports can leave the thruster inoperable or damaged. Frequent cleaning and inspection of the thruster and its housing is required while the ship is empty.
An empty ship often has its thrusters visible or nearly visible due to the reduced depth in which the ship sits in the water when unloaded. The thruster on most ships is marked with a white cross inside of a red circle. This warns smaller craft to avoid this area of a larger ship's hull in order to prevent damage from turbulent water. While most bow thruster designs involve the use of a propeller or impeller, some advances in jet propulsion thrusters have been made and are beginning to be used worldwide.