FREEBOARD literally means the amount of the vertical side of a vessel which is above water—(free meaning clear, and board being the side of the vessel, e.g., starboard or steering side).
The freeboard is measured vertically at the middle of the length, generally from the line of the uppermost continuous deck to the centre of a circular disk, I 2in. in diameter, which is marked on the side of the ship, and which is called the freeboard mark, or sometimes, the Plimsoll mark, as defined in the Merchant Shipping act, inspired by Samuel Plimsoll.
The centre of the disk gives the limit of loading for the summer season. Certain further marks, away from the disk, give the ap propriate limits for other seasons, such as winter, winter in the North Atlantic, and the Indian summer. Nearly all nations now require sea-going vessels to have a freeboard mark, which is deter mined in accordance with the conditions described in the article entitled SHIPPING: Registration, Classification and State Regula tion.