This ball bearing swivel base has two rows of Delrin ball bearings that allow for smooth swiveling, even under heavy loads. Each base comes with stand-up springs and a U-adaptor, compatible with a range of suitable blocks. The 144 is the standard model and has a tall arm, making it perfect for cockpit mounting or for use on larger keelboats and small offshore boats that utilize 3.00 in (76 mm) plastic blocks. Delrin is a registered trademark of E. I. du Pont de Nemours and Company or its affiliates.