Question 51 GLI03 - Master-LT 500-1600 GRT

The Correct Answer is B **Explanation for Option B (Wake fraction):** The term that defines the ratio of the wake speed (or more accurately, the speed of the water dragged by the hull, relative to the undisturbed water) to the ship's speed is the **wake fraction** (symbolized as $w_t$ or $w$). This dimensionless ratio quantifies the speed reduction of the water entering the propeller disk due to the boundary layer, wave-making, and eddy resistance of the hull. It is mathematically defined by the formula: $$w_t = \frac{V - V_a}{V}$$ where $V$ is the ship speed and $V_a$ is the speed of advance (speed of the water flowing into the propeller). In simple terms, it represents the degree to which the hull "wakes up" the water. **Explanation of Incorrect Options:** * **A) Speed of advance ($V_a$):** The speed of advance is the actual speed of the water flowing into the propeller. It is a velocity (speed), not a dimensionless ratio. It is related to the wake fraction by $V_a = V(1 - w_t)$. * **C) Wake distribution:** Wake distribution refers to the mapping or pattern of the water velocity field (wakes) at different points behind the hull, particularly across the propeller plane. It describes the spatial variation of the wake, not a single ratio relating wake speed to ship speed. * **D) Propeller velocity:** This term is ambiguous but usually refers to the rotational speed of the propeller (revolutions per minute, RPM) or the theoretical speed of the propeller relative to the ship, often used in propulsion calculations (e.g., $V_p$ or pitch speed). It is a speed or rotational frequency, not the specific dimensionless ratio described.