Answer
These are naval architecture curves used to determine a vessel's survivability.
KN Curve (Cross Curves of Stability): 'K' is the Keel, 'N' is the horizontal line of action of the buoyancy force. The KN curve plots the righting arm against the angle of heel for an assumed Center of Gravity (KG = 0). It depends only on the ship's underwater hull form (displacement and draft), not on how the cargo is loaded.
GZ Curve (Curve of Static Stability): This is the actual Righting Lever curve for a specific voyage condition. It is calculated using the formula: GZ=KN−KGsin(θ), where KG is the actual height of the center of gravity for the loaded ship.
Damage Stability: If the hull is breached, buoyancy is lost, causing sinkage, trim, and heel. The original GZ curve drastically shrinks. Damage Stability Criteria mandate that the residual area under the damaged GZ curve must still meet statutory minimums (under SOLAS/MARPOL) to prevent capsizing and allow safe evacuation.
The diagram is structured into two main columns for a direct comparison:
Intact Stability (Left): This establishes the baseline. It illustrates the standard definitions, shows the ship's internal points (K, G, B, M) and the GZ righting lever on an intact cross-section. Crucially, it displays the illustrative graph of KN Cross Curves (plotting KN against Displacement for fixed angles) and the resulting, healthy Intact GZ Curve (plotting GZ against the Angle of Heel). It includes the mathematical formula GZ=KN−KG×sin(θ) to show the relationship.
Damage Stability (Right): This illustrates the consequences of flooding. A ship cross-section with a 'FLOODED COMPARTMENT' shows how the center of buoyancy B shifts to B', resulting in a reduced G'Z' lever and the development of an angle of equilibrium (θe). The corresponding graphs highlight the critical shifts:
Damaged KN Cross Curves show illustrative 'RESIDUAL KN VALUES' that are reduced or shifted compared to the intact state, reflecting the altered underwater form.
Damaged GZ Curve (Solid Red/Orange): This is the key visualization, overlaid on the dashed blue Intact GZ Curve. It clearly labels the devastating effects of damage: Reduced Maximum GZ, Decreased Range, and Decreased Area Under Curve (lost stability energy). It explicitly points to the 'Angle of Equilibrium (θe)' which may be non-zero (indicating a stable, non-upright position). The diagram reinforces that the final curve must meet RESIDUAL STABILITY CRITERIA from SOLAS/MARPOL.
A central explanation box reiterates the GZ formula, emphasizing that while G might change slightly, the fundamental change is the loss of buoyancy and its distribution which is reflected in the modified KN curves and a severely degraded GZ curve.