thanks, i replaced shackel and tied a bowline when i replaced line. im guessing donut prevents wearage of shackel and metal corner of mainsail?previous owner has donut on upper line of furler and it does nothing. thanks alanthe donut goes between the shackle on the outhaul car slide and the end of the mainsail. It should not travel freely, it gets sandwiched between the end of the mainsail and the outhaul car.