Minimum Freeboard

At small angles of inclination, a vessel’s stability is indicated by her metacentric height (GM). But when a vessel encounters conditions that cause her to roll, heave, pitch, sway and yaw excessively, for example in heavy or rough seas, it is her reserve buoyancy that counters the effects of those forces. Reserve buoyancy is the volume of enclosed spaces above the waterline and is measured by freeboard.

Generally, more reserve buoyancy is required in stormy than in placid waters. In the relatively tranquil seas defined as the “Summer Zone,” less reserve buoyancy is needed to offset the environment than in waters described as “Winter” or “Winter North Atlantic,” when tumultuous seas are more common. Loadlines, designed to indicate the appropriate amount of freeboard, vary depending on the season or the service. If a vessel is loading in a tropical zone, passing through a summer zone and headed for a winter zone, she will have to meet the freeboard requirements for each of those zones upon entry. Before leaving the dock, officers must determine the initial freeboard necessary to meet the minimum requirements of each zone.

Another factor must be considered when loading in the fresh or “fresher” water of a port. Fresh water, which has a specific gravity of 1.00, is less buoyant than salt water (specific gravity 1.025 or 1.026). Dock water, while rarely fresh, is still less buoyant than the salt water for which the vessel is headed. Because of this density difference, a vessel may submerge her marks in port (have less freeboard) for as she enters salt water, buoyancy will have raised her higher in the water. Fresh Water Allowance (FWA) is the number of inches by which the mean draft changes when a ship passes from salt water to fresh water, or vice-versa, when the ship is loaded to the Summer displacement. The hydrometer measures the specific gravity of the water.

Here two typical USCG examination questions ask for the minimum freeboard required to meet the requirements of each zone. In solving these problems, it is easiest to calculate the minimum freeboard required at departure to meet the requirements of each zone. The largest freeboard will be the minimum required at departure.