In a wooden sailing vessel, the kelson — also called the keelson or apron — is a timber located above the keel and attached to it. The kelson strengthens and reinforces the keel, but also helps attach the beams which run laterally across the vessel to the keel, which runs fore and aft. The kelson is thus a vital part of the framing of a timber-framed sailing vessel.
Wooden ships rely on a complicated system of timber framing in order to make them both strong and hydrodynamic. The central part of this framing is the keel itself. The keel is a long timber which serves as the "spine" of the ship, running fore and aft most of the length of the vessel. Naval historians consider the building of a ship to begin when its keel is laid down; indeed, the keel is so important that the word "keel" comes from a word meaning "ship," and may be the first word of English ever written down. The keel is the largest and most important of what are known as the ship's longitudinal timbers.
Running across the keel are the vessel's lateral or transverse timbers. In larger vessels, these are called floors and are heavy supports which hold up the ribs. In smaller vessels, the ribs may rest directly on the keel itself. The ribs are curved timbers which support the long planks, called strakes, which make up the sides of the vessel's hull. The combination of longitudinal and lateral timbers makes up the basic skeleton of the vessel.
The kelson lies on top of the floors or ribs where they cross the keel. The kelson is fastened to the keel, compressing the transverse timbers between them. The resulting structure binds the timbers fast, creating a rigid frame for the rest of the vessel.
In addition to giving the keel more rigidity and holding the transverse timbers in place, the kelson has another important function. In many vessels, the masts are grounded, or "stepped," on the kelson. The base of the mast has a protrusion called a tongue which fits into a specially constructed wooden holder known as a step. In many vessels, the step rests on the kelson, which supports the weight of the mast. In some vessels, however, particularly in larger ships, not all masts were stepped on the kelson; some or all might be stepped on one of the decks above it.