The kelson or keelson is the member which, particularly in a wooden vessel, lies parallel with its keel but above the transverse members such as timbers, frames or in a larger vessel, floors. It is fastened to the keel partly to impart additional longitudinal stiffness to it but principally to bind the longitudinal members (keel and hog) to the transverse members (frames and floors).
In an open boat it is often fastened to the keel and hog in such a way that it can be removed for maintenance. Again, it lies above the boat's frames or timbers as they cross the hog but in this instance, its main function is frequently to provide a means of holding down the bottom boards in such a way that they can easily be removed for maintenance. The keelson of an open boat is normally arranged to be flush with the bottom boards so as to reduce the chance of the crew's tripping over it.
In an open boat or in a larger vessel, the hog is the structural member which lies immediately above the keel to which it is permanently and securely fastened so that the two form one member to which the lowest strakes (the garboard strakes) are fastened.