A floating object will sink if overloaded. The more mass on board a ship, the lower it rests in the water. Generally, the lower a ship sits the water, the less able it is to navigate and survive strong weather. Today, the need to balance the load on board with the actual carrying capacity of the vessel is reflected in the Plimsoll line. Established in 1875, this line is seen on the sides of cargo ships and has a series of hatches, showing the maximum depth to which that ship may safely be loaded.
The weight of a ship is often described in terms of displacement, the amount of water that must be displaced to keep the ship afloat. A ship's displacement light is the weight of the ship essentially empty. The ship's displacement loaded is its weight while loaded with everything needed for a voyage. That includes the cargo, the ship's fuel and ballast, the crew and passengers and their life support -- food, water and supplies. The difference between the ship's displacement light and displacement loaded is its deadweight. This weight is expressed in 2,240-pound tons.
A deadweight survey is a study of a ship to determine the total amount of cargo and supplies that ship may carry. It can include information from the ship's design and original specifications, as well as direct inspections of its draft lines and other technical steps. Important to determining a ship's deadweight or cargo carrying capacity is the physical space inside the ship. The more efficiently a ship is loaded -- provided it is within safety margins -- the more of its total carrying space is taken up, increasing its deadweight. Deadweight surveys are important to ship owners and operators, as they determine how much in valuable cargo can be moved with each voyage.
There are several other measures to describe the size and capacity of ships. Cargo deadweight is the deadweight of the ship minus the weight of the crew, their stores, fuel and other items not considered cargo but necessary for the voyage. Gross tonnage is a function of the space inside a ship. For every 100 cubic feet, 1 ton is added to the tonnage. Net tonnage accounts for the loss of cargo-carrying space to the ship's engines, below-deck crew cabins, navigation functions and the like.
