Question 57 GLI02 - Mate of Unlimited Tonnage

The Correct Answer is C ### 1. Explanation for why Option C is Correct: When a vessel, especially one making headway, enters shallow water (a phenomenon often called the "shallow water effect" or "squat"), the hydrodynamics around the hull change significantly. The most relevant effects concerning maneuverability are: * **Increased Wake and Boundary Layer:** The restricted flow underneath the hull forces the water to move faster and create higher pressure differentials. This altered flow pattern changes the wake entering the propeller and, crucially, the flow of water around the rudder. * **Reduced Propeller and Rudder Efficiency:** The proximity of the seabed restricts the flow of water that the propeller can draw upon and expel, reducing its thrust efficiency. More importantly for rudder response, the velocity of the water flowing past the rudder (which is essential for creating lift and turning force) is often reduced or disrupted by the powerful suction effect (squat) and the altered wake, particularly at lower speeds relative to the seabed. * **Increased Resistance:** Squat increases the effective draft, and the constricted flow dramatically increases wave-making and friction resistance. To maintain the same headway, the propeller must work harder, or the vessel's speed will drop. Since rudder effectiveness is directly proportional to the square of the water speed flowing past it, any reduction in speed or disruption of the flow causes the rudder to generate less lift for a given angle. * **Conclusion:** The net effect of these factors is that the vessel becomes less responsive to helm orders, making the rudder response feel **sluggish**. ### 2. Explanation for why Other Options are Incorrect: * **A) Your vessel will tend to ride higher:** This is the opposite of what occurs. The dramatic increase in flow velocity between the hull bottom and the seabed causes a significant drop in pressure (Bernoulli's principle). This reduced pressure results in the vessel sinking lower in the water, a phenomenon known as **squat** (trimming by the bow, stern, or overall reduction in under-keel clearance). * **B) Your speed will increase without a change in your throttle:** This is incorrect. While the water velocity *relative to the vessel's center* may increase due to the restricted flow, the overall resistance (drag) on the hull increases dramatically in shallow water. This requires significantly more power to maintain the same speed. If the throttle setting is unchanged, the vessel's speed (speed over ground) will **decrease** due to the massive increase in resistance. * **D) Your vessel will have better rudder response:** This is incorrect and contradicts C. Due to the increased resistance, decreased efficiency, and disruption of flow past the rudder (especially due to squat), the vessel's maneuverability is generally impaired, leading to a sluggish and poor rudder response.