These Chain Stoppers help prevent damage to the windlass and reduce the risk of accidental free fall of the anchor while under way. Chain Stoppers mount alone or inside most bow rollers and work with your anchor roller to take tension off your windlass. Chain and rope should be secured at anchor when your windlass is not in use to avoid high loads the windlass was not designed for. 321820 fits chain sizes 3/16" - 1/4". 321825 fits chain sizes 5/16" - 3/8". Constructed of corrosion resistant formed 304 stainless steel.