This is the strangest folk non-explanation of lift I've ever encountered. > So the wind is actually pulling the sail, not pushing it. A handy distillation of

This is the strangest folk non-explanation of lift I've ever encountered. > So the wind is actually pulling the sail, not pushing it. A handy distillation of fluid dynamics is, "Just like you can't push string, you can't suck fluids." That is, fluids don't resist in tension except in very particular very dynamic conditions that are wholly outside of the realms where our intuitions are developed. It looks like you've inadvertently modified the popular "Bernoulli's Principle" theory of lift, which was wrong even before you modified it. There's no real point in doing anything like a rebuttal, so let me do the best I can, instead, to explain how lift is actually developed and how that applies to the case of a sailing vessel. The fundamental thing that makes lift possible is the Kutta condition. This states that a real fluid has to leave a cusp in the direction that the cusp faces. There are complicated reasons for this having to do with velocity gradients, viscosity, radius of curvature of the flow, and even relativity, but the layperson can observe, feel, or take on faith that a flow leaves an edge in a direction parallel to the edge. This fact literally turns the flow and results in lift. (Historically and physically, we say that the Kutta condition resolves D'Lambert's Paradox via the Kutta-Jukowski theory of lift, if you want some keywords to look up more information.) So, the direction of the trailing edge of the flow turns the flow. There are two ways that we can examine the result of this: momentum or pressure. It turns out that they give the exact same result for the lift of an airfoil, but are "intuitive" in different ways. The momentum theory just says, "Lift is just a force that is accounted for by the mass of the air that you're pushing aside. This is what you typically might call 'downwash'," but a bit more mathematically. The pressure theory says, "Look, that's all fine, but that force is experienced as a pressure distribution on a surface, because the only part of the wing that's in the flow is the surface. Let's keep track of (integrate) the pressure over the surface and see what the resulting forces are," but also a bit more mathematically. So, take your pick of intuitions: Imagine a wing throwing air or fluid aside, or imagine it experiencing a pressure difference from top to bottom. They're both true, and are just different ways of accounting for the same phenomenon, different procedures for accounting for forces and masses as they interact with "control volumes," as engineers and physicists like to say. And remember that both intuitive explanations are driven by the Kutta condition, which is far more interesting, I think, than the erroneous Bernoulli explanation. So, that's lift. I turn a flow and I get a reaction force. Peachy. How does it apply to sailboats? Other than the buoyancy force and gravity, every force that we care about on a sailboat is due to the operation of a foil (hydrofoil or airfoil) within a fluid (water or air). The hull of the boat, the rudder, and the keel act like wings of (typically very low) efficiency. Their total lift acts to resist the downwind drift of the boat in the water. The sail acts as a wing. Depending on the point of sail, the lift of the wing acts more or less, literally, in the direction of travel of the boat. (Close to the wind, the majority of the lift is actually wasted in heeling the boat over and trying to slide it downwind.) I know I said that a rebuttal would be unproductive, but there are a few things I suddenly can't resist. > However, the flat keel underwater prevents the boat from sliding downwind, with the result that the boat is forced forward. The "flat keel" is a wing. It prevents this "slide" via lift. It's also not helpful to split the analysis into 'forces' and 'constraints'. The keel produces lift that is a fore that contributes to the balance of forces on the vessel, resulting in motion. > [The wind] is split into two turbulent streams, one on each side of the sail. Turbulence is a complete red herring, here. Turbulence has nothing to do with lift (other than causing stalls and complicating the analysis). > Some of the lifting force is counterbalanced by the keel underwater keeping the boat upright. Well...the center of mass of the entire boat levered, as it is, upwind of the center of buoyancy, produces a moment that counteracts the moment of the sail's lift. > For instance, a sailboat is not powered by the wind pushing on the sail. It kind of really is. It's just more complicated because analyzing fluids requires that we analyze continua, and not just discrete particles. In the end, though, sailboats sail because they exchange momentum with the atmosphere and the water through distributed forces, or pressures.