Five Oceans Marine Aluminum Adjustable Seat Base Pedestal 18"" to 24"" 360&ordm; Swivel 18""  24"" Satin Anodized Post Seat raises or lowers, locks at desired height 4 prong spider swivels 360 and locks 2-7/8"" (73mm) diameter tube, 9  (23cm) diameter base plate