Keel

A keel is the bottom-most longitudinal structural element of a watercraft. It runs along the centerline of a ship from the bow to the stern. This component is vital because it provides essential stability for the vessel. In many shipbuilding traditions, the process of "laying the keel" marks the official start of construction. This event signifies the moment the ship's foundation is placed in its cradle.

The way a keel is used depends on the construction style. In carvel-built hulls, builders start by laying the keel, then the stern and stem. They then set up frames at key points along the keel. A keelson is later attached to the keel using bolts or treenails. Other methods, like clinker construction, use overlapping planks to shape the hull. In the classical Mediterranean period, builders used mortice and tenon joints to connect planks edge-to-edge. In those older systems, the planking itself provided much of the hull's strength.

There are different types of structural keels used in modern maritime engineering. The most common version is the flat plate keel. This type is typically fitted to ocean-going ships and most large vessels. Smaller boats, such as tugs, ferries, and trawlers, often use a bar keel. A bar keel has massive scantlings, which refers to its thick and heavy dimensions. While this makes it strong for grounding, it increases the vessel's draft. A draft is the depth of the ship below the water line.

Modern shipbuilding has changed how these foundations are handled. Large ships are no longer always built around a single, central keel. Instead, many are constructed using pre-fabricated, complete hull sections. In these cases, the construction process begins with the cutting of the first sheet of steel. This differs from the traditional method where the keel is the very first piece placed. Regardless of the method, the keel remains the central structural member of the hull.

History shows that keels have been used since antiquity. The Uluburun shipwreck, dating to 1325 BC, had a rudimentary keel. This may have been a simple center plank rather than a true keel. Evidence of a more developed keel appears in the Kyrenia ship, a Greek merchant vessel from 315 BC. The word "keel" itself has a very long history. Some scholars believe it is the first word in the English language recorded in writing. It was recorded by Gildas in his 6th-century Latin work, *De Excidio et Conquestu Britanniae*.

In sailboats, keels often serve a hydrodynamic purpose. This means they are designed to interact with the water to improve movement. A fixed hydrodynamic keel provides the structural strength to support the boat's weight. It also acts as an underwater foil. This foil uses the boat's forward motion to generate lift. This lift counteracts the leeward force of the wind, which prevents the boat from sliding sideways.

Beyond movement, these keels act as a vital counterweight. When wind hits the sails, it creates a lateral force that causes the boat to heel, or tilt. The heavy keel offsets this heeling moment to keep the boat upright.

Some sailboats use specialized parts like centreplates or daggerboards. While these can act as counterweights, they are lighter than a full keel. Some advanced systems even use canting keels. These can swing sideways in the water to move the ballasting effect to one side. This allows the boat to maintain a more upright position while sailing.

The term keel is also linked to the Latin word *carina*. This is the origin of the term "careen," which means to clean a keel and hull. This is often done by rolling the ship on its side. An example of this history is Careening Cove, a suburb in Sydney, Australia. In the early colonial days, this area was used for the careening process. This shows how the physical needs of a ship connect to the names of places around the world.