The motion of a vessel impacts its stability. Which of these motions shown in the illustration affects the governing "motion ahead and astern" stability? See illustration SF-0048.
• Six degrees of freedom of ship motion shown: surge, sway, heave (linear) and roll, pitch, yaw (rotational) • The phrase "motion ahead and astern" refers to movement along the ship’s lengthwise (fore-and-aft) direction • Difference between linear (straight-line) motions and rotational motions around an axis
• Which arrow in the illustration points generally from bow toward stern and back again, along the longitudinal axis of the vessel? • Of the motions listed, which one is a straight-line translation rather than a rotation or vertical movement? • When a vessel speeds up or slows down moving ahead or astern, which specific type of motion is changing?
• Match the term in the choices to the correct arrow and label on illustration SF-0048 • Confirm that your chosen motion is fore-and-aft, not vertical or sideways and not a rotation • Verify that the motion you pick would directly affect the vessel’s ability to move ahead and astern in a seaway
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