Freeboard: What It Means and Why It's Important

oil tanker ship
The crude oil tanker Heather Knutsen in Halifax Harbour. (Dennis Jarvis/Wikimedia Commons/CC BY-SA 2.0)

Freeboard in the simplest terms is the distance from the waterline to the top of a vessel’s hull.

Freeboard is always a measurement of vertical distance but in most vessels it is not a single measurement unless the top of the hull is completely flat and parallel to the water along the entire length.

Minimum Freeboard

One way of expressing freeboard is to refer to the minimum freeboard of a boat or ship. This is an important measurement since it determines how much weight a vessel can carry or how it will perform in wind and waves.

If minimum freeboard ever reaches zero it is possible that water could run over the side of the hull and into the boat causing it to sink if enough water accumulates. Some boats have a very low freeboard design that allows easy access to the surface of the water. Examples of this are buoy tenders and research boats which must have easy access to the water to go about their business.

By Design

Naval architects design these ships with sealed decks so if water does reach the top of the hull it drains off back into the water and does not impact the buoyancy of the ship.

Most vessels, large and small, do not have a simple freeboard that is a straight line. Instead, the freeboard is higher at the bow, or front of the vessel, and slopes down to the stern at the rear.

The designers shape the hull like this because as a boat moves through the water it might meet waves which are higher than the surface of the water. The higher bow allows a boat to ride up the surface of a wave and keeps water out.

Deadrise

The method that is used to describe the shape of a hull in naval architecture is called Deadrise.

Deadrise is used in all forms shipbuilding since it is an ancient solution to keep unwanted water out of your ship.

Cross Section

The ideas of freeboard and deadrise come together when we consider a cross section of a hull.

If we cut a slice across the hull we see that the profile of the hull rises from the keel at the bottom up to the waterline and then to the top of the hull. The area between the water and the top of the hull is the area where freeboard is measured.

If we look at other slices of the hull the freeboard may change from higher in the area of the bow to lower near the stern.

Freeboard Is Not Fixed

The amount of freeboard is not a fixed number unless a boat always carries exactly the same load. If you load any vessel with more weight the freeboard will decrease and the draft will increase. That is the main reason any vessel must operate within the load capacity calculated by the designers.

Compared to old-style pencil and paper drafting techniques that resulted in blueprints which were interpreted by each foreman, new building techniques offer the potential for much more complex and efficient designs.

State of the Art

Software drafting programs now allow naval architects to design precisely and cnc machines allow builders to stay within a few millimeters of the planned dimensions, even on a 300-meter vessel. The key to this accuracy is the number of "stations" found along the length of the hull.

In the old days, maybe three meters of the hull were described in detailed drawings. Today, the number of stations is only limited to the size of the plan. A taper of one centimeter over 100 meters is possible today, which lets designers make complex shapes and also allows for modular construction and float out before final assembly.