Question 59 1AE01 - First Assistant Engineer

The Correct Answer is C. ### Why Option C is Correct A "tractor" type azimuthing thruster is defined by the positioning of its propeller relative to the supporting structure (the gear unit, or skeg/foot) in the direction of intended thrust. 1. **Tractor Configuration:** In a tractor configuration, the propeller is positioned forward of the gear unit/skeg. It "pulls" the unit through the water. 2. **Azimuthing Angle of $0^\circ$:** This angle generally refers to the forward direction of the vessel (ahead). The propeller is rotating to push water aft, generating forward thrust. 3. **Relative Position:** When the propeller is pulling the vessel forward, the water flows over the propeller first. Consequently, the gear unit (which houses the vertical shaft and steering mechanisms) is positioned **abaft** (behind) the propeller. 4. **Wake:** Since the gear unit is located downstream of the propeller in the direction of water flow, it operates entirely within the high-velocity, turbulent water generated by the propeller. Therefore, the gear unit is **in the wake** of the azimuthing thruster's propeller. Thus, statement C ("With an azimuthing angle of $0^\circ$, the gear unit is directly abaft of and in the wake of the azimuthing thruster's propeller.") accurately describes the defining characteristics of a tractor thruster operating in the ahead direction. --- ### Why Other Options Are Incorrect **A) With an azimuthing angle of $0^\circ$, the azimuthing thruster's propeller is directly abaft of and in the wake of the gear unit.** * **Incorrect:** This describes a "pusher" configuration. In a pusher thruster, the gear unit is forward (in front) and the propeller is abaft (behind) and in the wake of the gear unit. This contradicts the definition of a tractor thruster. **B) With an azimuthing angle of $0^\circ$, the gear unit is directly abaft of the azimuthing thruster's propeller, but the azimuthing thruster's propeller is in the wake of the gear unit.** * **Incorrect:** The first half ("the gear unit is directly abaft of the azimuthing thruster's propeller") is correct for a tractor configuration. However, the second half introduces a physical impossibility and contradicts the tractor design: the propeller cannot be in the wake of the gear unit if the gear unit is abaft of the propeller. The water flows over the propeller first. **D) With an azimuthing angle of $0^\circ$, the azimuthing thruster's propeller is directly abaft of the gear unit, but the gear unit is in the wake of the azimuthing thruster's propeller.** * **Incorrect:** The first half ("the azimuthing thruster's propeller is directly abaft of the gear unit") describes a pusher configuration (A). Although the second half ("the gear unit is in the wake of the azimuthing thruster's propeller") is correct for a pusher if the thruster were operating astern, or generally incorrect for the configuration described in the first half when moving ahead, the core error lies in misidentifying the relative positions of the propeller and gear unit, confusing a tractor with a pusher.