The maximum permissible draft when loading in a Summer load line zone, shown on the ship's side as the Summer load line mark.
In practice
Before departure, the chief mate calculates the maximum cargo quantity that can be loaded without causing the vessel to exceed its summer draught allowance. The calculation accounts for bunkers and fresh water on board and the density of the port water—a vessel loading in brackish river water may have a deeper draught than in salt water. The OOW confirms the departure draught by reading the draught marks and verifying compliance before the load line certificate is presented to port authorities. Overloading beyond the summer draft is a serious statutory offence, can void the vessel's insurance, and increases the risk of structural failure and loss of reserve buoyancy in heavy seas.
Regulatory detail & full definition
The summer draft is the maximum permissible draught to which a vessel may be loaded when operating in a Summer load line zone, assigned under the International Load Line Convention 1966 and its 1988 Protocol. The summer draught corresponds to the Summer (S) mark on the Plimsoll line painted on the vessel's side; a disc with a horizontal bar through its centre indicates the summer load line. Different marks—Tropical (T), Winter (W), Winter North Atlantic (WNA), Fresh (F), and Tropical Fresh (TF)—correspond to additional load line zones and densities, each specifying a different maximum draught. The Load Line Certificate issued by the flag state or classification society states the permitted freeboards for each zone.
Before departure, the chief mate calculates the maximum cargo quantity that can be loaded without causing the vessel to exceed its summer draught allowance. The calculation accounts for bunkers and fresh water on board and the density of the port water—a vessel loading in brackish river water may have a deeper draught than in salt water. The OOW confirms the departure draught by reading the draught marks and verifying compliance before the load line certificate is presented to port authorities. Overloading beyond the summer draft is a serious statutory offence, can void the vessel's insurance, and increases the risk of structural failure and loss of reserve buoyancy in heavy seas.