The lower middle-line girder which, in conjunction with a flat plate keel on the bottom and a rider plate on top, forms the principal fore-and-aft strength member in the bottom of a ship. In addition to its importance as a "backbone" or longitudinal strength member, it serves to distribute and equalize the pressure on the transverse frames and bottom of the ship when grounding or docking occurs. In steel ships this keelson usually consists of a vertical plate with two angles running along the top and two along the bottom. The girder, however, may be made up of various combinations of plates and shapes. This member should continue as far forward and aft as possible. Usually called the Vertical Keel.
Umgca osembindini osisixhasi ophantsi odityaniswa necangci elisicaba lesiseko senqanawe emazantsi kunye necangci lomkhweli ngaphezulu, zenza intloko yomphambili kunye nowesiqu senqanawe amandla elungu kumazantsi enqanawe. Ukongeza ekubalulekeni kwayo "njengomxhasi" okanye ubude amandla elungu, isebenza ukusasaza nokwenza kulingane unyanzelo lwefreyimu enqamlezayo namazantsi enqanawe xa isemhlabeni. Kwiinqanawe zentsimbi lo mgca weplanga wenziwe waba necangci elithe nkqo neembombo ezimbini ezibaleka ngaphezulu ezimbini emazantsi. Isixhasi sinokwenziwa zindibaniso ezohlukeneyo zamacangci nezakhelo. Eli lungu liza kuqhubeka phambili nasesiqwini senqanawe kangangoko, idla ngokubizwa isiseko senqanawe esithe nkqo.