A removable sail for a paddle-style boat, such as a kayak preferably takes the form of a fast deploying down-wind sail attachable to a deck of the kayak. The sail includes a curved, but non-circular shaped flexible material coupled to a collapsible, continuous frame member, for example the frame member may be sewn into a channel made by the material. When mounted to the deck of the boat, the sail may be arranged in a deployed or a stowed configuration, the latter being deployable on the fly in that the sail can be quickly sprung into the deployable configuration. The non-circular shape of the sail cooperates with a hull shape of the boat while providing a sufficient surface area for capturing wind energy. The sail may also include a transparent region that helps prevent excessive amounts of spray from striking the paddler and yet still provides an adequate viewing range.