[Sailormon6]

6 oz. sailcloth sounds heavy for a 20 foot boat sailing in light air. 6 oz. sailcloth is stiffer and heavier than 4 oz. cloth. When the wind is light, it won''t have enough power to blow the sail smooth, and to hold the sail into the curved shape that drives the boat. It will tend to hang limp, like a sheet on a clothesline. 4 oz. sounds about right. You might be able to use 5 oz., but that seems a little heavy for your circumstances. If the sailcloth is too light, it will be less durable.