A number of factors need to be considered by personnel loading a vessel. Some of these factors are given below.
The amount and distribution of ballast contribute to the seaworthiness, control, and trim of a tanker. Ballast also controls stress vibration in the hull caused by poor weight distribution. The master of a vessel must select ballast tanks that will distribute weight evenly through the hull. At the same time, the tank cleaning and inspection schedules and the effects of tank corrosion must be considered. If different ballast tanks are used on each voyage, tank corrosion is slowed down and tank life is increased.
Loading above load lines is a safety hazard. Therefore, load weights must be calculated accurately. The steps for calculating maximum tonnage are as follows:
§ The load line to which a vessel can be safely loaded under summer, tropical, fresh water, or other conditions is determined.
§ If a vessel is being loaded in water that may be of a variable specific gravity, the gravity is checked with a hydrometer. The vessel's allowance is checked in tables provided with the vessel.
§ Cargo deadweight tonnage is determined by deducting the total weight of the fuel, water (excluding the water in the boilers), stores, and other items needed on the vessel from the vessel's deadweight tonnage. This quantity, in long tons, is the maximum allowable weight that can be carried. The weight of the scheduled cargo should not exceed this amount.
The space the scheduled cargo will use in proportion to the weight allowed is then calculated. This is done by referring to tables and using the gravity of the scheduled cargo and the number of barrels per long ton. Available cargo space then must be checked to make sure that the vessel can hold the scheduled cargo.
§ Space is then allowed for cargo expansion. This is done in case the vessel enters regions where the temperature is higher than that of the product at the loading port. Tables provided with the vessel are used to determine the cargo expansion allowance.
The master of a vessel and the shore petroleum inspector consider weight distribution factors when they plan the cargo layout so that the cargo is not contaminated by the mixing of one product with another. There is little chance of this problem on a tanker carrying only one product.