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We had problems with the main outhaul line slipping - it was a polyester line.
We replaced it with 10mm spectra and had the same problems.
Before the 2400 mile trip to French Polynesia on riggers advice, we replaced it with 12mm Vectran and it has never slipped, not even in 50+ knots with 3.5 reefs.
For me this is the solution.
Maybe this can assist others with the same problem that we had.
SV Elyse SM437, currently Taina marina, Papeete.