Question 57 RVR03 - Mate of LT 500-1600 GRT

The Correct Answer is B **Explanation for Option B (Correct Answer):** Option B is correct because of the hydrodynamic principle known as **Squat**. Squat is the combined effect of the vessel's stern sinking (trimming by the stern) and the overall mean draft increasing (mean bodily sinkage) when moving through the water, especially in shallow or restricted waters (like a narrow channel). As the vessel moves through the water, it pushes water aside (a 'return flow' current is generated around the hull). In narrow channels, this return flow must accelerate significantly beneath the hull and between the hull and the channel boundaries. According to Bernoulli's principle, this increased velocity results in a decrease in water pressure around the hull. This reduced pressure is less effective in supporting the vessel, causing the vessel to sink deeper into the water. Therefore, the **mean draft will increase** in comparison to the static sailing drafts recorded before getting underway. **Explanation of Incorrect Options:** * **A) It will decrease in comparison to the sailing drafts:** This is incorrect. The hydrodynamic effect of moving through the water (Squat) always causes a vessel to sink deeper (increase draft), not rise higher (decrease draft). A decrease in draft is known as "planning" and only occurs at extremely high speeds, typically beyond the capability of conventional merchant or naval vessels operating in a channel. * **C) It will increase if you decrease speed:** This is incorrect. Squat is directly proportional to the square of the vessel's speed ($V^2$). If you decrease speed, the squat effect will diminish, and the draft will decrease (the vessel will rise) towards its static sailing draft. * **D) It will remain the same as the sailing drafts:** This is incorrect. A vessel moving through a channel will never maintain its static sailing draft due to the pressure differential and accelerated water flow caused by its motion (Squat). The draft only remains the same when the vessel is stopped (static).