What correction must be applied if the deck-line is not in its correct position?

When determining the appropriate correction to apply if the deck line is not in its correct position, it is essential to focus on the vertical relationship between the actual and the intended positions of the deck line. The deck line serves as a reference point for measuring the vessel's draft and stability, and any deviation from its proper position can significantly affect the accuracy of stability calculations.

The vertical difference between the actual and correct deck line position directly influences the calculations for the vessel's stability, including the determination of the center of gravity, metacentric height, and the moments of inertia. By accurately identifying and applying this vertical correction, one ensures that the stability calculations reflect the vessel's true condition, allowing for safe navigation and operation.

The other options do not directly address the necessary correction related to the deck line's position. For instance, the horizontal distance from the keel to the deck line and the total height of the superstructure may be relevant in other contexts but do not specifically pertain to correcting the deck line's vertical discrepancy. Similarly, clearance under the deck, while important for various structural considerations, does not apply to the deck line's positional accuracy. Thus, the vertical difference is the key factor that must be corrected.