 The two caissons are 6 @.@ 5 metres ( 21 ft ) wide , and can hold up to four 20 @-@ metre @-@ long ( 66 ft ) canal boats . Watertight doors at each end match doors located on the upper structure and lower dock pit . Due to space concerns , where a normal hinged door would dramatically reduce the useful length of the caisson , vertically rising doors were chosen . The doors are raised from a recess in the base of the caisson and powered by a hydraulic lance when docked . 
