The curve assumed is the stability cross curve, specifically the KN curve. When KG is taken as zero , the righting lever is measured from the keel, so the plotted curve is KN , and the actual righting lever is then found by GZ = KN - KG \sin \theta.

In plain words

  • KG is the height of the ship’s center of gravity above the keel.
  • KN is the righting lever referenced from the keel.
  • If KG = 0 , the curve itself gives the lever directly from the keel, which is why it is called a KN curve.

Why this is done

Using KG = 0 avoids mixing in the ship’s actual center-of-gravity position while building the curve, so the same curve can be used later for different loading conditions by applying the correction KG sin θ.

If you want, I can also explain the difference between GZ , KN , and KM in one simple sketch-style explanation.