The draft (or draught) of a ship's hull is the vertical distance between the waterline and the bottom of the hull (keel), with the thickness of the hull included; in the case of not being included the draft outline would be obtained. Draft determines the minimum depth of water a ship or boat can safely navigate. The draft can also be used to determine the weight of the cargo on board by calculating the total displacement of water and then using Archimedes' principle. A table made by the shipyard shows the water displacement for each draft. The density of the water (salt or fresh) and the content of the ship's bunkers has to be taken into account. The closely related term "trim" is defined as the difference between the forward and aft drafts.
The draft of a ship can be affected by multiple factors, not considering the rise and fall of the ship by displacement:
The drafts are measured with a "banded" scale, from bow and to stern, and for some ships, the average perpendicular measurement is also used. The scale may use traditional English units or metric units. If the English system is used, the bottom of each marking is the draft in feet and markings are 6 inches high. In metric marking schemes, the bottom of each draft mark is the draft in decimeters and each mark is one decimeter high.
Larger ships try to maintain an average water draft when they are light (without cargo), in order to make a better sea crossing and reduce the effects of the wind (high centre of velic force). In order to achieve this they use sailing ballasts to stabilize the ship, following the unloading of cargo.
The Plimsoll Line is the line where the hull of a ship meets the surface of the water, in concept or reality. Specifically, it is also the name of a special marking, also known as the International Load Line or water line (positioned amidships), that indicates the draft of the ship and the legal limit to which a ship may be loaded for specific water types and temperatures in order to safely maintain buoyancy, particularly with regard to the hazard of waves that may arise. Temperature affects the level because warm water provides less buoyancy, being less dense than cold water, as does salinity because fresh water is less dense than salty seawater. For vessels with displacement hulls, the hull speed is determined by, amongst other things, the waterline length. In a sailing boat, the waterline length can change significantly as the boat heels, and can dynamically affect the speed of the boat.
The purpose of a load line is to ensure that a ship has sufficient freeboard (the height from the water line to the main deck) and thus sufficient reserve buoyancy, indisputable seen from the outside.
In aircraft design, the term "waterline" refers to the vertical location of items on the aircraft. This is the (normally) "Z" axis of an XYZ coordinate system, the other two axes being the Fuselage Station (X) and Buttock Line (Y).