3 ms·

I appreciate your explanation. I'm a sailor, not a PhD in fluid mechanics. All I know about the physics of sailing I learned in the first couple of chapters i

I appreciate your explanation. I'm a sailor, not a PhD in fluid mechanics. All I know about the physics of sailing I learned in the first couple of chapters in the book I cited, which I think my post succinctly summarizes. So, if I understand you correctly, to put it into one sentence for my friends at the club, air flows along the curved sail which, through pressure differences or momentum, turns the direction of the wind, and the opposing force of the turn causes lift? My other question is, how does a symmetrical keel then cause lift? Thanks.

theothermkn 12y ago
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! :)

3 ms·

I appreciate your explanation. I'm a sailor, not a PhD in fluid mechanics. All I know about the physics of sailing I learned in the first couple of chapters i

by joshuaheard 12y ago

theothermkn 12y ago
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! :)