Sagging
Sagging is a common hull deformation that shows up when a vessel carries most of its weight amidships. The hull bends downward in the middle, forming a convex shape. Think of a beam resting on supports at both ends with a heavy load placed at the center. Ships behave in a similar way when their central holds carry more weight than the bow or stern.
In a draft survey, sagging becomes clear when the midships draft is greater than the mean of the forward and aft drafts. This difference indicates that the hull has flexed downward. Because sagging increases the underwater volume, it also increases the vessel’s displacement. If left uncorrected, this leads to an understated cargo weight.
To correct for sagging, surveyors use a representative mean draft. For most conventional ships, the three-quarter mean draft is applied because it gives more weight to the midships reading. Large box-shaped vessels, such as Panamax and Cape Size carriers, may have dedicated deflection correction tables that provide the correction directly in tonnes. These tools are especially important since heavy central loading can sometimes create a double sag, a condition that the usual three-point draft method may not capture accurately.
Detecting sagging early and applying the right correction is essential for reliable displacement calculations. It ensures the survey reflects the vessel’s true underwater volume, especially when the loading pattern is uneven.
