Question 57 ONC04 - Mate of LT 500-1600 GRT

The Correct Answer is B. ### Explanation of Why Option B is Correct Option B, "Upward and inward flow," correctly describes the primary components of the flow generated by a ship's wake (specifically, the flow influenced by the propeller and the boundary layer effects). 1. **Inward Flow (Transverse Component):** The propeller of a ship (in forward motion) acts to push the water *aft* (backward). However, the water immediately surrounding the propeller hub is drawn from the surrounding area. Furthermore, the low-pressure area immediately behind the ship's stern (the region where the two sides of the flow meet, known as the viscous wake) causes the water that has been pushed aside by the hull to flow back toward the centerline of the ship. This motion is called the **inward flow** or **inflow** component. 2. **Upward Flow (Vertical Component):** As the water converges inward toward the stern and the propeller draws water into its disc, there is also a significant component of flow that moves **upward**. This is due to several factors: * **Propeller Suction and Thrust:** The propeller creates a jet (slipstream) that is directed mostly astern. However, the water drawn into the propeller disc (the suction side) often comes from below and around the hull, resulting in an upward motion component. * **Hull Boundary Layer:** The water that has been slowed down and dragged along by the hull (the boundary layer) has momentum and must flow up and away from the hull as it moves past the stern, contributing to the upwelling of the water surface. This is often associated with the surface turbulence observed in the immediate wake. Therefore, the combination of the water converging back toward the ship's centerline and simultaneously moving toward the surface results in an **Upward and inward flow**. *** ### Explanation of Why Other Options Are Incorrect * **A) Upward and outward flow:** This describes the flow pattern near the **bow** (the wave diverging away from the ship), not the flow pattern directly behind the stern/propeller (the wake). The water behind the stern converges (inward), it does not diverge (outward). * **C) Downward and inward flow:** While the inward component is correct, the downward component is incorrect for the primary wake flow at the surface. The flow generated by the hull's boundary layer separation and the propeller generally results in an overall upward velocity component as the water seeks to fill the void behind the moving hull. * **D) Downward and outward flow:** Both components are generally incorrect for the main wake. The flow is upward (seeking the surface) and inward (converging toward the centerline). Downward flow might occur slightly ahead of the propeller (or in the far field of specific ship types), and outward flow describes the bow wave, not the stern wake.