Question 63 ONC03 - Master LT 500-1600 GRT

The Correct Answer is C ### Explanation of Correct Option (C) **Why Option C ("The hull has a greater pressure on the land side of the bow") is correct:** When a vessel moves through shallow water close to a bank (a fixed vertical or sloped obstruction), the water flow between the hull and the bank is restricted. This restriction causes the water velocity in that gap to increase significantly, especially near the narrowest parts of the vessel (usually amidships and toward the bow). Simultaneously, the movement of the vessel displaces water, creating a phenomenon known as the **bank cushion** (or bow cushion) near the fore part of the vessel. The bow pushes water ahead of it, and because the water cannot easily escape between the bow and the bank, a mound of high-pressure water builds up in this area. According to Bernoulli's principle, increased velocity corresponds to decreased pressure, and decreased velocity (or stagnation/piling up of water) corresponds to increased pressure. The piling up of water on the land side of the bow means the water pressure exerted on the hull in this specific area is **greater** than the pressure on the offshore side. This pressure differential is what pushes the bow away from the bank, contributing to the overall hydrodynamic interaction known as bank effect. ### Explanation of Incorrect Options **A) The hull has a greater pressure on the land side of the stern:** This is incorrect. While the stern also experiences significant pressure effects, the primary high-pressure zone (the bank cushion) is at the bow. At the stern, the vessel's movement often creates a secondary, low-pressure zone (suction or bank suction) between the stern and the bank due to the rapid efflux of water being drawn into the area vacated by the hull, exacerbated by propeller action. This low pressure actually pulls the stern *toward* the bank. **B) The hull has a lesser pressure on the land side of the bow:** This is incorrect. A lesser pressure would imply increased velocity (Bernoulli effect), but the physical action of the bow moving into a restricted channel causes the water to pile up, creating a high-pressure cushion, not a low-pressure area. **D) The hull has a lesser pressure on the land side of the stern:** This option describes the phenomenon known as **bank suction** or the low-pressure zone often found near the stern (as mentioned in the explanation for A). While this phenomenon *does* occur and is a major component of the bank effect, the question asks about the effect of the **bank cushion**. The bank cushion is specifically the high-pressure area at the bow. Therefore, while D describes an associated effect, it does not describe the effect of the *cushion*, which is defined by the high pressure at the bow.