Question 64 GLI02 - Mate of Unlimited Tonnage

The Correct Answer is B **Explanation of why Option B is correct:** Tunnel bow thrusters operate by creating a lateral thrust (sideways force) across the vessel's hull. When a large vessel is moving astern (backward) at a slow speed, the main rudder system (which relies on water flowing past it, usually generated by the propeller driving forward) loses significant effectiveness. In this scenario, the bow thruster, which generates its own independent sideways force near the bow, becomes the primary, highly effective means of maneuvering the bow, thereby providing essential steering control and precise handling, particularly during docking or slow-speed maneuvers. **Explanation of why other options are incorrect:** * **A) They are effective on most vessels at speeds up to 10 knots.** Tunnel thrusters lose significant effectiveness rapidly as the vessel increases forward speed (typically becoming ineffective above 2 to 4 knots). The water flow past the hull disrupts the thruster's output, and the hydrodynamic forces generated by the vessel's movement quickly overwhelm the thruster's lateral force. * **C) They are fully effective at all drafts.** Tunnel thrusters require sufficient water depth above them to operate efficiently and prevent air ingestion (ventilation), which severely reduces or eliminates thrust. If the vessel is running light (at a shallow draft), the thrusters near the surface may draw in air, making them far less than fully effective. * **D) Because of their location, most modern installations have as much power as a tug.** While bow thrusters can be very powerful, they are designed to provide lateral movement for docking, not to generate the sustained, high-bollard-pull forces required of a powerful harbor tug. A tug's propulsion system and hull design are optimized for towing and pushing, resulting in significantly higher effective power output than a typical tunnel thruster installation.