rgranger has it right. I had at least 3 of different weights on my first boat. The sail maker (Peter Sutter) told me that one was for 6 knots of wind or less
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As anyone who sails the Bay suspects, there are the occasional days when the wind is very light. No kidding. So sailing around the north side of Angel Island all was perfect but when we stuck her bow out into the east bay, the wind increased above six knots (maybe 6.5 or possibly 7) and that drifter tore in half as if it had a zipper.
I would advise that one use much lighter sheets with a drifter to take full advantage of the sail.