Their appearance then becomes more wing-like and less sail-like. Analyzing how a sail works as a wing is useful, not just for modern sails that look more like wings, but also for very traditional sails that, while they look like sails, operate very much like wings.
Flow accelerates over the top surface of an airfoil, either because it is at an angle to the flow, or because the top has more curvature than the bottom, or both. When a fluid like air or water is accelerated, the pressure that it imparts on an adjoining surface decreases.
This lower pressure pulling upward on the upper surface of a wing produces lift. If the thickness of an airfoil is ignored, it can be reduced to a thin curved line defining the camber. The shape of this camber line determines the amount of lift produced at a fixed angle of attack. Since a sail has essentially no thickness, it exists only as camber. The flow over the convex leeward side has reduced pressure through accelerated flow and the flow over the concave windward side has increased pressure through decelerated flow.
The difference in pressure across the sail holds the flexible sail into its cambered shape and produces force to pull the boat. An airfoil developing lift causes the flow approaching it to bend upward. This is because the lower pressure on top of the airfoil pulls air up toward it. This upward change in flow angle is called upwash. In addition to the upwash that an airfoil causes on itself due to the lower pressure on top influencing more air to flow over it, additional upwash occurs due to changes in the planform of the wing.
This is because, just as the low pressure on top of the wing influences the air some distance upstream to move upward toward it, that low pressure also influences air a similar distance away in the spanwise direction to alter its direction.
This causes variations in upwash along the span of the wing on adjacent sections. This is because an airfoil generates much more lift in its forward portion than it does aft, so using the quarter-chord line as a reference is a convenient manner to characterize the sweep of a wing.
Sweep has the effect of increasing the upwash on the outboard wing sections. As a wing is angled aft, flow over the outboard sections must pass by the low pressure on top of the wing sections immediately inboard and forward. The close proximity of that low pressure to the air just outboard causes the outboard flow to turn upward more, resulting in higher upwash on the outboard wing. Taper is defined as the ratio of the chordlength of the tip divided by the chordlength of the root.
For sails, where the head tapers to nearly a point, the taper is extreme zero , resulting in a triangular planform. A tapered wing has a much shorter tip section than root section. As the wing tapers, lift produced by the shorter outboard sections is less because they have less surface area to support lift. Since the outboard sections are smaller than the inboard sections, they are significantly influenced by the larger wing just inboard. Air approaching the outboard portion of the wing is deflected by the low pressure on top of the larger inboard wing that is still generating a large amount of lift only a short distance away.
The close proximity of that low pressure to the outboard wing causes the flow to be pulled upward additionally over the outboard wing. Hence, the smaller outboard sections operate with higher upwash.
This enhances the amount of lift that they produce but does not make up for their loss of area. Identifying the flow conditions that sails operate in is very useful for understanding how they work. The wind blows over the surface of the earth and, as with any fluid flowing over a surface, has friction with it.
This friction slows the air closest to the surface and through shear causes the air immediately above it to slow some, too. This effect continues upward until at some distance above the surface the air is all moving at a similar speed. This behavior is called the boundary layer. While it occurs at a very small scale in the water flowing along the surface of hulls and keels, it occurs at quite a large scale in the air flowing over the earth.
This means that the true wind speed is increasing up the entire height of a mast. Apparent wind is the wind velocity experienced by the sails on a moving boat. This is the wind speed and direction that can be directly measured felt from the boat while it is moving. It is a combination of the true wind and the wind generated by the motion of the boat. The figure shows how these two wind components are added to create the apparent wind. Notice that the apparent wind vector at the bottom of the rig, where the true wind speed is slower, is shorter slower and angled from a more forward direction, than the apparent wind vector at the top of the rig, where the true wind speed is faster.
This variation in true wind speed not only causes the variation of apparent wind speed with height, but also its variation in angle. This is because all of the mast and sail are moving at the same speed and in the same direction as the boat across the moving air. Since the wind solely due to the movement of the boat is identical at all heights, the apparent wind speed and direction resulting from its addition to different true wind speeds at various heights is different.
While in this example the true wind velocity only varied in strength with height, it is possible that a variation in true wind direction can occur with height. In that situation, each tack will experience different apparent wind twist than the other. The increase of apparent wind angle with height is a factor that influences a sail to fly in a twisted manner, where the top is angled more offcenter from the boat than the bottom.
Other factors affecting how much twist is appropriate are sweep and taper as they alter the amount of upwash along the span of the sail. A mainsail by itself cat rig is tapered, but if the mast is close to vertical is actually swept forward. In this case, the forward sweep would have somewhat of a canceling effect on the increased upwash due to taper.
Raking the mast back increases sweep and will cause additional upwash on the top of the sail, necessitating more twist to the sail. Genoas and jibs are very tapered and swept. Those two features, combined with the already twisted apparent wind, cause significant upwash toward the head of the sail. Each sail by itself is much simpler than the combination of a foresail and mainsail as in the sloop rig. The sails are operating so close to each other that they both have significant interaction with the other.
The most interesting feature of this is that the two sails together produce more force to pull the boat than the sum of their forces if they were each alone. Earlier, upwash was identified as the increase in flow angle immediately upstream of a wing. There is also a corresponding change in angle, called downwash, just behind a wing, where the flow leaving the wing has been turned to an angle lower than the original flow.
The mainsail of a sloop rig operates in the downwash of the forward sail, causing the flow angle approaching the mainsail to be significantly reduced from what it would be otherwise. This decreases the amount of force that the mainsail produces. The foresail of a sloop rig operates in the upwash of the mainsail. The wind as far upstream as the luff of a genoa is influenced by the upwash created by the mainsail. Hence, a jib or genoa in front of a mainsail has a higher flow angle than it otherwise would have by itself, causing an increase in the amount of force that the forward sail produces.
So, while the mainsail is experiencing detrimental interference from the foresail, the foresail benefits from the interference of the mainsail.
Notice that more air is directed around the curved leeward side of the foresail. This causes higher velocity lower pressure and more force. The net result is that the total force of the two-sail system is increased, with the foresail gaining more than the mainsail loses.
This is the same phenomenon from which a foresail of a sloop rig benefits. This angle represents the difference in upwash on the foresail and downwash on the mainsail due to each other. On a masthead rig, where the forestay is attached to the top of the mast and both sails taper to basically zero chordlength at their heads in a similar fashion, the interference effects of the sails on each other are similar along the entire height of the mast.
A fractional rig has the more complicated characteristic that the top of foresail is not as high as the top of the mainsail. So now back to our question:. Lots of boats can — especially the eighteen footer skiffs on Sydney Harbour.
Ask a sailor how, and he'll say "These boats are so fast that they make their own wind", which is actually true. Ask a physicist, and she'll say that it's just a question of vectors and relative velocities. Downwind diagram at left is easy. If the wind is 10 kt, and the boat makes 6 kt in the same direction, then the crew feels a wind of 4 kt coming over the stern of the boat. The true wind v w equals the speed of the boat v b plus the relative wind v r. So you can't go faster than the wind.
When the wind is at an angle, we have to add the arrows representing these velocities vector addition.
The faster that the boat goes, the greater the relative wind, the more force there is on the sails, so the greater the force dragging the boat forwards. So the boat accelerates until the drag from the water balances the forward component of the force from the sails. Why are eighteen footers always sailing upwind? In a fast boat, there's no point going straight downwind: you can never go faster than the wind.
So you travel at an angle. But if your boat is fast enough, then the relative wind always seems to be coming mainly from ahead of you, as these arrows show. So the eighteen footers never set ordinary spinnakers: they have asymmetrical sails that they can set even when they are travelling at small angles to the apparent wind.
A good list of links to technical material , courtesy of Sailboat Technology. How can you trim the mainsail using blocks and pulleys to multiply your force? More about hull shapes, bouyancy and sails. Australian Marine Services Directory has links to weather services, marine services and other information. Coriolis forces and the reasons behind the major ocean currents and winds. Another puzzle involving relative motion of the air: the plane on the conveyor belt. Did you know that both the special and general theories of relativity are important in the Global Positioning System?
See this link from Univ. See where the satellites are at the moment in this animation from J-Track. Details at Science Outreach Centre news and Activities for students and teachers.
Answer to puzzle. The faster heat is the one with no wind. When the wind and the water both move W to E at 10 kt, the boats drift down the river at 10 kt, with their sails hanging limp. Diagram credit: Tamela Maciel Now let's say we're trying to sail upwind with the wind coming from the left or "port" over the front of the boat. This sketch shows the sailboat as if we were looking down on the boat as it moves towards the top of the sketch.
Looking down on a sailboat, showing the equal and opposite forces on the wind and sail. Credit: Tamela Maciel The wind fills the sail into the shape of a wing, but because the sail is held fast at both ends, the wind can't push it out of the way. Instead the wind must change direction to flow parallel to the sail. The taut sail has created a force on the wind that causes it to change direction and Newton's third law tells us that there is an equal and opposite force on the sail by the wind, as shown by the red arrows in the diagram above.
If this was the only force acting on the boat, then we would be in trouble: the boat would move forward but also to the right.
But sailboats have a secret weapon hidden below decks: the keel. Keel and rudder below a sailboat. Credit: Paul Schultz In addition to the force on the sail, the large area of the keel resists being dragged sideways through water. You can feel this resistance if you drag your hand palm-first through water compared to edge-on. The water applies a force to your hand that increases with greater surface area. Forces acting on a sailboat cancel each other such that the total force moves the sailboat forward.
The downward pointing keel is outlined by the dashed rectangle. Credit: Tamela Maciel This force on the keel is shown by the purple arrow in the diagram above. By combining the force on the sail and the force on the keel triangle diagram , we see that the sideways forces are cancelled out and the total force on the sailboat is only in the forward direction green arrow.
The result is that the boat moves forward! Some sailboats can even move faster than the wind itself. When sailing upwind, the relative speed of the wind on the sails is greater than the actual speed of the wind and this relative wind creates a larger force on the sails that can push sailboats faster than the actual wind speed. There is a limit to how fast sailboats can move forward, of course. I have ignored boat drag in this example, but the boat also has an inherent friction as it moves forward through the water.
The boat will accelerate until the force pushing the boat forward is balanced by the drag force pulling the boat back, and then the boat will travel at a constant speed. Labels Fluid dynamics Force and Motion sailing. Labels: Fluid dynamics Force and Motion sailing. Anonymous May 13, at AM. Anonymous May 14, at AM. Anonymous May 19, at PM. Anonymous March 20, at AM. Anonymous September 8, at PM.
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