Freeboard explained ,and why it is given.

In sailing and boating, free board means the distance from the waterline to the upper deck level, measured at the lowest point of sheer where water can enter the boat or ship.

Freeboard defines the reserve bouyancy of a vessel. It is to be there to satisfy the reserve bouyancy regulations by IMO and other conventions. Reserve buoyancy is important for the vessel’s safe operation at sea.

Sufficient free board is required at all times to prevent the vessel being swamped and overwhelmed. For ocean going vessels, it is important to note that those structures above the waterline that are not watertight will not contribute to the reserve buoyancy of the vessel. Again, this makes sense – an open wheelhouse, or a cabin with the doors left open will not offer much protection if the vessel begins to ship water over the side.

Another factor affecting the freeboard to take into account is the trim of the vessel. Trim is defined as the difference between the draft forward and the draft aft. Draft is the depth of the hull below the water. If the aft draft is greater, the vessel is described as being trimmed by the stern, if the forward draft is greater, she is trimmed by the bow.