8" Bilge Roller. 21mm ID. Suits 20mm spindle. Flat or Bilge rollers as they are called in some States, go down on the transom of some boats where a flat bottom demands this type of roller. Certainly not for deep Vee'd boats unless they are part of a spring loaded Retriever Mate type self centering system. Sometimes used as a Bow Stop on Power Cats twin hulled boats.