By Foluke Akinmoladun
There are various statutory provisions in domestic laws and international conventions concerning the quantity and type of life-saving apparatus that is to be carried on board a vessel. Broadly, it is determined by the type of vessel, crew establishment and the passenger certificate (authorized number of passengers permitted to be carried). Life-saving apparatus includes lifeboats, lifebuoys, inflatable life rafts, and individual lifejackets.
Another important aspect of the safe use of the vessel is buoyancy. Buoyancy refers to the upward force exerted by sea water on the vessel when the vessel is afloat. It is the upward force that results from immersing the vessel in water and which increases the deeper the vessel is into the water. Therefore, the stability of the vessel is determined by the force of buoyancy provided by the underwater parts of a vessel to the sea, coupled with the combined weight of its hull, equipment, fuel, stores and load. These forces can be adversely affected by the prevailing weather conditions and sea-state.
Freeboard thus represents the safety margin showing to what depths a ship may be loaded under various service conditions depending on the type of cargo the vessel is carrying, the waters to be navigated, and the season of the year. Freeboard is determined by the design of the vessel, particularly the shape and dimensions of its watertight hull; by its structural strength; and, in the case of a passenger ship, by the subdivision of its watertight compartments.
Freeboard encapsulates the principle of reserve buoyancy, where freeboard allowance gives the vessel enough counterweight against the force of buoyancy of the sea. It is therefore recognized that the freeboard should ensure adequate stability and avoid excessive stress on the ship’s hull as a result of overloading.
While ships trade and move mainly in sea water, they may also travel on fresh water in rivers and brackish water (a mixture of sea water and fresh water) in estuaries. Sea water by its nature, is denser than fresh water due to the quantity of salt it contains. Cold water, on the other hand, is denser than warm water. Vessels float higher in denser water than in less dense water with the same load on board. This means that they need more weight in denser water to keep it from being unstable than it would need in less dense waters. The implication is that failure to have the ship with the right amount of weight could adversely affect the balance and buoyancy of the ship depending on the type of waters it is upon.
A vessel that is overloaded is at greater risk of being overwhelmed in hazardous sea conditions than if it is correctly loaded. Overloading can also result in catastrophic failure of a ship’s hull. The more cargo that is carried the greater the earnings of the ship operator and this informs the temptation to overload the ship. This has resulted in ship hulls being marked with a ’loadline’, to indicate a safe draught to which the ship can be loaded.
The IMO in 1966 adopted the 1966 Load Lines convention, whose provisions were made for determining the freeboard of ships by subdivision and damage stability calculations. The regulations take into account the potential hazards present in different zones and different seasons. In the 1966 Load Lines Convention, freeboard was adopted as the distance measured from the waterline to the main deck of the vessel. This is normally the uppermost continuous deck in a ship with one or more deck. Another important measurement to the stability of the ship is the draught of the ship. The draught of a vessel is the vertical distance from the keel to the waterline (height wise). The maximum permitted draught varies according to the seasons and waters in which the vessel moves on the sea.
The purpose of the load line is to ensure that a ship has sufficient freeboard (the height from the waterline to the main deck) and thus sufficient reserve buoyancy (volume of ship above the waterline). It should also ensure adequate stability and avoid excessive stress on the ship’s hull as a result of overloading. Ships intended for the carriage of timber deck cargo for example, are assigned a smaller freeboard as the deck cargo provides protection against the impact of waves.
The exact location of the load line markings is calculated and/or verified by a classification society, which then issues a load line certificate.
Foluke Akinmoladun is a lawyer, accountant, mediator and arbitrator. She is the Managing Solicitor of Trizon Law Chambers and can be reached at: Foluke.A@trizonlawchambers.com
