Black Keel/Self centering roller 
Length 197mm, Flange 67mm, Diameter 40mm, bore 21mm
20mm shaft required - sold sepratley 
 
Durable construction - made from hi-impact HDPE Polyethylene
Non-abrasive
Allows your boat roll across the trailer without damage
Better grip and less friction while rolling
Non abrasive 
 
Non abrasive