Question 65 TV02 - Towing Vessels - Great Lakes and Inland

The Correct Answer is B **Why option B ("The vessel's displacement") is correct:** When a vessel reduces engine speed (or stops the engines) without engaging reverse (backing), the rate at which her speed through the water decreases is determined primarily by the forces resisting her forward motion. The most significant of these resisting forces is hull drag, which is directly related to the vessel's size and mass. **Displacement** is the measure of the mass (weight) of water the vessel pushes aside, and thus, it is the primary indicator of the vessel's inertia (resistance to changes in motion) and her inherent hull drag characteristics. * A larger, heavier vessel (high displacement) has high inertia, meaning it takes longer for the resisting forces to overcome the forward momentum, resulting in a slower rate of speed decrease and a longer stopping distance (high *coasting* ability). * A smaller, lighter vessel (low displacement) has low inertia and typically a lower momentum, meaning drag forces slow it down much faster. Therefore, displacement is the fundamental factor determining how quickly a vessel "dead in the water" loses speed. **Why the other options are incorrect:** * **A) The sea state:** While the sea state (waves, wind, current) influences the *actual* speed over the ground and affects drag, it is an external, variable factor. Displacement is the inherent, primary factor dictating the vessel's characteristics (inertia and hull resistance) that determine the *rate of decrease* in speed through the water under normal assumptions. * **C) The number of propellers:** The number of propellers relates to propulsion and maneuverability. Once the engines are reducing speed or stopped, the propellers are no longer actively contributing propulsion (or backing force). They become part of the drag profile, but the effect of the overall hull inertia (displacement) far outweighs the localized drag of the screw(s). * **D) The vessel's horsepower:** Horsepower dictates how quickly the vessel can *increase* speed and achieve a maximum speed. When the engines are reducing power or stopped, horsepower is irrelevant. The issue becomes one of momentum and drag, both of which are primarily dictated by displacement.