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I believe it is the line above which the sea should not rise. It's marked out the outside of the craft and if the line is below water the craft is too overloaded. I could be wrong, my sea-faring...
The ship you are looking for might be the Mary Rose, of the English Navy in the 16th Century. The Mary Rose may not have been called a ship-of-the-line, but for all intent and purpose that is what...
To tell how high or low the ship is resting in the water.
Also by examining the plimsoll line you can see how heavy a load is that the ship is carrying and is sometimes used as a warning against...