Any frustration that shows in my reply is not particular to you. It's just that folk ideas about fluids insinuate themselves everywhere with sometimes irritatin

Any frustration that shows in my reply is not particular to you. It's just that folk ideas about fluids insinuate themselves everywhere with sometimes irritating confidence. (For example, the book you cite!) You're right about the sail turning the flow. However, the "curve" of the sail is not the only way to turn a flow. In the keel's case, it sees an angle of attack to the flow, and this causes lift. Imagine, for a moment, that the boat isn't heeled over, and we're looking down on the boat as it moves through the water. The bow of the boat, when pointing, points a few degrees upwind of the direction of travel of the boat relative to the water. We go back to the Kutta condition to see that the flow has to turn to point directly off the trailing edge of the keel and see that water is being "thrown" to leeward. It's a little more complicated than that, even, for keels, especially for full keels. They're still "throwing water," but they're solidly "low aspect ratio" and finite wing effects dominate: Lower lift, higher drag, higher stall angle, softer stalls, attached vortices, and so on. All sorts of fun. There are reasons that race boats tend to have fin keels and daggerboards! :)