The original International Load Lines Convention, adopted in 1930, was founded on the notion of reserve buoyancy, while it was known at the time that freeboard should also maintain proper stability and avoid undue stress on the ship's hull as a result of overloading. Today, these two concepts are seen as two sides of the same coin: load line management.
In simple terms, the Load Line Convention states that ships should be loaded no more than 15% below their maximum safe load. This allows for enough reserve buoyancy to keep the ship stable even when fully laden. Any more than this and the risk of sinking increases.
Furthermore, the convention states that ships should not be overloaded by more than 10% without first having appropriate safety measures in place to prevent damage or loss of life. This gives sufficient room for error before things go seriously wrong.
Finally, ships should be unloaded no more than 10% above their safe working load. Again, this allows for enough reserve buoyancy to keep the ship stable even when heavily laden. Too much off-loadings can cause the ship to sink due to lack of support from beneath her weight.
These are only the general guidelines. Ships must also take into account their specific design parameters when determining how close they want to load/unload to stay within recommended limits.
The load line's aim is to guarantee that a ship has enough freeboard (the height from the waterline to the main deck) and consequently enough reserve buoyancy (the volume of the ship above the waterline). It should also provide enough stability and minimize undue stress on the ship's hull due to overloading. Finally, it provides guidance for safe navigation.
Load lines can be either painted or marked with tape. The type of load line used depends on how much information is needed by the pilot. For example, if the ship is expected to travel through heavy seas then painted load lines are best because they can be easily changed if the need arises. On the other hand, if the load line needs to be kept visible during daylight hours but not at night then taped load lines will do the job nicely.
In general, painted load lines are used on large vessels while taped ones are preferred on small craft because they are easier to see and avoid. However, both types of load lines are used extensively around coastal waters where visibility may be limited.
Painted load lines are made out of wood or metal and are attached to the side of the vessel with stainless steel bolts called "dolly pins". They usually extend between one-and-a-half times and three-quarters of the way up the side of the vessel. The top edge of the load line should be sharp to allow easy identification of when the load line is close to being reached.
A load line certificate attests to the vessel's adherence to the loadline protocol. The loadline concept essentially limits ships to the minimal freeboard required. Any more than this and the ship becomes unstable and likely to found.
There are three required documents for the loadline certificate: a copy of the ship's plan, a list of the vessel's machinery spaces, and a list of its stores rooms.
The loadline certificate is issued by flag states that have signed the MARPOL 73/78 agreement. It is necessary for vessels flying a foreign flag if they call at U.S. ports to request a load line certificate from the National Marine Fisheries Service.
How does the loadline affect the stability of a ship? Loads are distributed over as much area as possible which in most cases will be along the main body of the ship. As long as these areas are not larger than 15% of the total breadth of the ship then it will be stable.
If the main body is too wide then it will be difficult to balance the ship which will cause it to be unstable. If the cargo holds are too full then it will also be difficult to balance the ship which will cause it to be unstable.