Meaning of margin line

mar'gin line"

Pronunciation: [key]
  1. an imaginary line used in making calculations regarding the flooding of hulls, running fore-and-aft 3 in. (8 cm) below the upper surface of the bulkhead deck at the side.
  2. the line along which the planking on a transom-sterned vessel terminates aft.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.