Question 51 GLI04 - Mate of LT 500-1600 GRT

The Correct Answer is D. **Why Option D ("The effect of the propeller flow on the rudders") is correct:** Flanking rudders (also known as leading rudders) are positioned forward of the propeller, typically used on specialized vessels like towboats or integrated tugs, especially those operating on inland waterways. When the vessel is moving astern (reversing), the propeller is churning water forward, creating a powerful wash or "propeller race" flowing directly over these forward-mounted rudders. This substantial, high-velocity flow over the flanking rudders makes them highly effective for steering, allowing the operator to direct the thrust and control the vessel's heading even while reversing. **Why the other options are incorrect:** * **A) The water flow due to the vessel's movement through the water:** While the vessel's movement does create water flow, when going astern, the primary, high-velocity flow over the flanking rudders is dominated by the propeller wash (the thrust being directed forward). The relatively slower ambient flow created by the hull moving through the water is secondary to the powerful propeller effect. * **B) The discharge current being channeled to impinge on the vessel's deadwood:** Deadwood refers to the structural filler material between the sternpost and the keel. Rudder effect is about directing the flow onto the rudder blades to create lift (steering force), not impinging the current onto the deadwood. Furthermore, the "discharge current" (propeller wash) is used to activate the rudders, not simply directed at the hull structure. * **C) The tunnel effect of the water flow past opposing rudders:** The "tunnel effect" is not a standard hydrodynamic principle governing flanking rudder operation. While rudders work in pairs, their effectiveness astern is fundamentally determined by the high-speed flow supplied by the propeller, not a specialized "tunneling" action between the opposing blades.