A sail assembly having a single, closed-loop support batten that supports a flexible sail. The sail is a hollow body defining a substantially hemispherical shape when fully expanded by captured wind. The support batten is housed in a perimeter sleeve at the front end of the sail. The support batten is stiff enough to support the sail, yet elastic enough that it can be twisted and coiled as three contiguous coils for compact folding. The sail has a significantly smaller size in the folded and coiled orientation, and is capable of rapid self deployment due to the energy stored in the folded batten. The deployed sail can be fastened to the deck of a small watercraft, hand held, or fastened to the blade end of a paddle or oar for capturing the energy in the wind to propel a small watercraft.