2 Historical types of ships
3 Measuring ships
One can measure ships in terms of overall length, length along the waterline, beam (breadth) and tonnage.
A number of different tonnage definitions exist, the majority of which measures volume rather than displacement. Displacement most frequently applies to naval vessels and equals the actual weight of a ship under specific conditions. "Light ship" tonnage measures the actual weight of the ship with no fuel, no persons, no cargo, no water on board, just as it first entered the water. The term "displacement" occurs due to the basic physical law, discovered by Archimedes, that the weight of a floating object equates exactly to that of the weight of the water that would otherwise occupy the "hole in the water" displaced by the ship.
In Britain, up until the end of the 19th century, shipowners could load their vessels until their decks were almost awash, resulting in a dangerously unstable condition. Additionally, anyone who signed onto such a ship for a voyage and, upon realizing the danger, chose to leave the ship, could end up in gaol.
Samuel Plimsoll, a member of Parliament, realized the problem and engaged some engineers to derive a fairly simple formula to determine the position of a line on the side of any specific ship's hull which, when it reached the surface of the water during loading of cargo, meant the ship had reached its maximum safe loading level. To this day, that mark, called the " Plimsoll Mark", exists on ships' sides, and consists of a circle with a horizontal line through the center. Because different types of water, (summer, fresh, tropical fresh, winter north Atlantic) have different densities, subsequent regulations required painting a group of lines forward of the Plimsoll Mark to indicate the safe depth (or freeboard above the surface) to which a specific ship could load in water of various densities. Hence the "ladder" of lines seen forward of the Plimsoll Mark to this day.
