Join Date: May 2002
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Generally a fluttering leech means the airflow is disturbed or separated from the sail. The reason is probably because the sailcloth is too thin and flexible to hold its shape by itself. The leech lines put just enough tension on the leech to help it hold it's shape.
I have always believed that a fluttering leech is more detrimental than a cupped leech, so I put whatever tension on the leech lines is necessary to stop the fluttering.