Plastic open Fairleads are rubbish, when stressed they open all too easy. Get decent ALU ones instead that do not bend open when stressed. In any case you have to get the type with overlapping ends. The more overlap the better. Also note that Starboard types differ from Port types. If you use a Starboard type on Port the line will easily jump out. If correctly placed you have to bend the line over 90 degrees, relative to the normal pulling direction, to get it in or out of the Fairlead.