Boats sailing faster than the wind

Cecil's Columns/Staff Reports
21

Yes, the standard diagram for a powered aircraft in level flight has lift equal to weight and thrust equal to drag. But this doesn’t change the fact that in order to produce lift, the wing must deflect air downward. The best familiar demonstration of this is seen when a helicopter (whose rotor consists of rotating wing sections) hovers over water: vast quantities of air are quite obviously being flung downward.

A wing producing lift without pushing air downward would be like a swimmer trying to move without pushing water in the opposite direction.

No, but there would be spanwise flow and wingtip vortices (which include regions of lowered pressure) - these are inescapable when a real wing is actually producing lift in free air.

But in no real case does the air ever “match up” - so explanations that imply it does, though admirably simple and easily comprehended, suffer from the drawback that they are inherently false.

No - the problem is predominately the wingtip vortices, which can in some cases persist for a surprising time (minutes).

22

Try the simple experiment - hold a strip of paper under your lip, blow across the top, and it rises up. You’re not blowing under it. Air pressure differential is forcing it up. side pressure of a moving air stream is less than a static one. Blow across a straw and the liquid comes up the straw; same principle as carburetor or an airbrush. (Many pull their paint up from below…)

That’s Bernoulli principle.

Run a wind across a curved surface and you get lift; The air pressure on the bottom of the wing is tronger than the top. The flow, with the curve, creates a bit of a lower pressure area on top of the wing, behind the curve.

Yes, if you are slowing down, you can also get a lift from the “water-ski” effect of tilting the wing up. Tilt it too far and you get separation of flow, the area behind the curve onto burbles instead of smooth flow - and the plane drops out of the sky. It stalls. No more lift from airfoil flow, and the drag from turbulence is too much to overcome.

23

Stranger On A Train said:

This is a conceptual issue. How is the air gripping the sail to pull it? It doesn’t have hooks or claws.

What is the difference between blowing and sucking? You have a high pressure area and a low pressure area. One side is being pushed harder than the other. The force differential drives the object toward the area of low pressure.

I’m trying to follow the rest of your explanation, but I’m not well-versed in sailing terminology.

I can see how turning the sail can convert a north/south wind to an east/west force. I can sort of grasp how the keel and rudder can convert an east/west force into a force running back against the original wind at an angle. But I’m still a bit puzzled by how that force can be higher than the original north/south force.

md2000 said:

This explanation is based upon the faulty assumption that the flow lines on both sides of the wing must reach the tail of the wing at the same time. This is patently false. Wind tunnel pictures confirm that it isn’t true. The truth is the flow over the top actually flows faster than the air under the bottom of the wing, and gets there first.
UncleRojelio said:

You are misunderstanding. Although the lift is caused by lowering pressure above the wing rather than increasing pressure below the wing, the net effect still requires that the air get turned and moved downward. If air wasn’t moved downward, there would be no way for the plane to move upward. Conservation of momentum - something has to go down for the plane to stay up.
Jinx said:

No, you misunderstand Newton. The Free Body Diagram you are drawing are the forces affecting the plane, that keep the plane neutral in the air. Weight wants to make it fall, lift wants to make it go up, the balance is where the plane flies level. But a Free Body Diagram is not what Newton’s Third Law addresses.

Newton’s Third Law is about the push between the plane and the air. For something to push the plane upward, that something must be pushed downward. That is the equal and opposite. Just like if you push on a door to open it, the door also pushes on you. Your weight is a downward force on the floor, the floor has an equal and opposite upward push on your feet. Otherwise, you’d go through the floor.

24

One thing I always like to point out when the question of “faster than the wind” comes up is that the idea is ill posed. Non-sailors tend to think about a sailing boat running before the wind, and - quite rightly - can’t understand how the speed of the boat can be greater than that which pushes it.

So then you get into the discussion about how a sailing boat can travel at other angles to the wind. All the way round to close hauled - where the apparent wind can get close to 20% and still make good speed. But somewhere in here the notion of faster than wind speed got lost. There was actually notion of velocity, not speed. That was implicit in the idea that the boat was running before the wind, and the boat and wind were in the same direction. Now we are talking about quite significant differences in direction, and the notion of faster than the wind needs redefining. Can the speed of a boat sailing on a reach (wind at 90 degrees) be faster than the speed of the wind? Why not? Two additional points to be made.

Apparent wind. As the boat gets faster the apparent direction of the wind moves forward. If you sail with the wind at 90 degrees to the direction of travel, the vector sum of the boat velocity and wind velocity quickly brings the wind angle around to the front of the boat. High performance boats spend most of their time with apparent wind angles less than 90 degress and often closer to 45 degrees. The apparent wind speed also increases for the same reasons. So you can get a situation where the apparent wind is significantly higher than the real windspeed.
Counter-intuitively the slowest direction of sailing is where the wind is exactly behind the boat.

The bottom line I like to point out is that it is all about energy input. The boat is simply harvesting energy from the wind and turning it into motion. The sail plans are most efficient at this task when the apparent wind is quite some way forward. At this point there is really no special meaning to the idea of “faster than the wind” at all. It is a nice notion, and a touchstone of a high performance boat, but has no physical significance.

Another way of thinking about the physics is that it is a conservation of momentum problem. The airflow around the sails results in the airflow being redirected. This involves a change in momentum for the air. This momentum goes into the boat.

The actual motion of the air is, well, complex. A substantial part of the problem of making a sail work well is to manage the vorticies. The air on the windward side slows down and pressure increases, on the leward side it accelerates and pressure drops. When the air gets past the sail the recombination is of different velocity streams, and you get some giant vorticies. The triangular shape of a modern sail plan is partly in response to managing this.

There is also the question of velocity made good. A sailing boat cannot sail directly into the wind, and when very close hauled, the speed drops off again. Eventually you stop, even though the sails are filled. So, there is clearly a sweet spot, zig zag towards the target (tacking), sail as directly to the target as you can, balancing the speed of the boat against the additional distance travelled as you sail a wider angle from the wind (to get more speed.) Same downwind. Rather than sail directly downwind, sail with the wind off the quater, zig zaging (gybing) toward the target, again balancing further distance travelled against higher boat speed. The answer differs for every boat out there, but every boat has a sweet spot (although it changes depending upon wind strength too.) Very high performance boats may well be optimised for a specific set of angles.

Perhaps a better question than “can a sailing boat sail faster than the wind?” is to ask:
Can a sailing boat get to a target that lies at an arbitary angle from it, faster than the wind can?

That is harder. If the target was dead into the wind, that answer is generally no. Velocity made good (VMG) of even very high performance boats to windward is less than the windspeed. But once you get off the wind things pick up. Many higher performance boats could manage targets around 270 degrees of the circle. Wind speed matters too. Too high or too low and things get hard. But in the design range of the boat. Not that hard.

What about sailing on land? A sailing boat has a lot of work to do moving through the water. How much better is a land yacht? All the physics is the same, you just have a set of wheels for lateral resistance instead of a keel or centerboard. How fast can these be? How about 4 times windspeed on a reach, and twice windspeed on most other angles?

25

The idea of a boat sailing faster than the wind is so counter-intuitive that scientific explanations might always seem murky. Think of it this way: a large jetliner, weighing many tons, takes off from the runway at an airspeed of around 150 to 160 knots. Now imagine the same jetliner sitting, stationary, on top of a huge vertical wind tunnel. Do we think that a wind speed of 150 to 160 knots would lift the aircraft straight upward? I don’t think so.

This doesn’t explain the aerodynamics involved, obviously, but it might get us past that counter-intuitive hurdle and give us something we can relate to.

26

Showed this column to a couple of people involved in yacht racing in Thailand, and one of them responded: “The speed record for a sail boat in Thai waters is about 27 knots, which was done by a racing catamaran between Phi Phi island and Phuket a couple of years ago.”

27

No, I don’t mean the bit of air that splits and goes up over the wing ends up glued back to it’s uncer-the-wing counterpart. Obviously, the air 20 feet above, does. Ditto 20 feet below. There is an air resistance force slowing the wing(The airfoil is rather blunt).

There is a bit of a lower pressure zone at the backside of the airfoil, because the air does flow along the topside further than along the bottom. The air is squashed at the apex of the top curve, then it flows back down to fill in so that there is no trailing vaccum behind the wing. This creates lower pressure.

In fact, the air on top also flows outward and away from the wing on big jets. IIRC, because of the swept wing, the area of lower pressure is further back as you move away from the fuselage, the top-side air is drawn outward as well as back.

The result is large spirals at each end of a jet wing. The air flows outward and down and then spirals around. This is the turbulence (from big jets, especially) that is dangerous to smaller jets. Sufficiently so that it could flip a 737 that flies into the wake of a jumbo jet. Hence, 1 to 2 minute separation between jets.

That is why those little turned up wingtips on newer reduce drag and save fuel and wake turbulence, they control the wake spirals.

If the angle of attack increases too much, and the airfoil slows too much, the air can burble back into that low-pressure zone (from the top or the underside) instead of flowing neatly over the airfoil. This is stalling. The lift disappears because burbling turbulent air is not smooth, fast moving air that gives the pressure differential that produces lift.

28

Here’s a link to an article about the French boat L’Hydroptere, which has been setting numerous world speed records. The article quotes a claim (apparently verified) of 55 knots in a 28-kt wind.

Google the name of the boat for many links, including videos.

29

This seems to be describing a vortex rotating in the opposite direction to what is actually the case.

The higher-pressure air beneath the wing flows outward toward the wingtip and up toward the lower-pressure area above the wing. The resulting vortex, viewed from behind the plane and looking forward, rotates clockwise off the left wingtip and counter-clockwise off the right.

Here’s a photo that shows a right-wing vortex.

30

A stationary aircraft doesn’t provide much in the way of useful illustration.

If instead we consider a modern jetliner doing around 240 knots, we’ll see something rather interesting. At that speed, it will have a glide ratio around 16:1. This means that a vertical air current of 15 knots would allow the plane to glide in level flight with engines off. 20 knots would allow it to climb at around 500 feet per minute, again with engines off.

31

I think you had best go back and review all the threads on this Board dealing with airplane lift; it’s quite a contentious topic in its own right, but appears applicable here. Maybe you’re aware of that, but this discussion seems to be headed down the same pathways… :eek:

32

Just don’t put that airplane on a treadmill! :smack:

33

We are not talking about force but about speed: the two are not directly related. A large slow moving force can cause something small and light to move much faster than itself, through leverage for example.

Think about a small hard ball between two planks set on edge at a slight angle to one another. Now imagine a large slow moving force squeezing the planks together: the ball might well shoot out the end from between the planks even as the large force just slowly squeezes the gap closed.

34

Yes, I know the history, but the point here is that unlike an airplane wing, air is not being forced across the wing; it is instead, the momentum of the moving wind that draws the sailboat forward (in an upwind orientation). So whereas pressure differential from the aerofoil shape is only typically a small contributor to lift in airplanes (and with enough thrust, can be dispensed with almost entirely, just using gross redirection of air to account for lift), in the case of a sailboat this is the dominant effect.

This is not, as implied by posts above, any kind of violation of Newtonian mechanics. The conservation of momentum is maintained by the change in inertial state by the air that jets across of the front side of the sail (or in between the mains’l and the jib) as it provides a forward impulse via partial vacuum. Nothing is forcing air into the back side of the sail to create forward impulse except ambient pressure.

Stranger

35

You’re not mentioning an aspect of this situation that can reconcile the apparent conflict between the “Newtonian” and “Bernoullian” explanations of lift:

The lowered pressure on top of a wing is accompanied - and in a real sense caused - by the acceleration of air along the upper surface of the wing. The air’s boosted velocity and the downward inclination of its path represents a net deflection of air downward, which is exactly what Newton’s laws of motion require if the plane is to stay aloft (or the sailboat is to move forward).

The Bernoulli “faster means lower pressure” explanation is correct. But if the fact that it also involves deflection of air is omitted or glossed over (as is all too common) it can come across as a form of hand-waving that gives a puzzling and unnecessary appearance of conflict with Newton.

There’s also the point that the Bernoulli explanation of lift usually amount to explaining the unknown by use of the unfamiliar. By contrast, Newtonian action-reaction tends to be familiar - indeed part of daily human experience

36

True, and you offered up a very lucid explanation, Xema. The problem with referring to Bernoulli’s principle is that it really applies to a fixed mass of air, implying a bounded system, which confuses people. The basis for the Bernoulli principle is, of course, conservation of energy (that the energy level at any given point in the equivalent streamflow is the same, plus or minus any work done on or by the flow, which is sometimes conflated with the incorrectly “flows meet up” notion), so it is exactly an extension of Newtonian principles rather than at odds with them.

Stranger

37

So, if I may paraphrase:

In the case of a sail, what we have is roughly the equivalent of an airplane designed with its airfoil parallel to the direction of flight, which would result in the only lift mechanism being the pressure differential. For an airplane this doesn’t work well, but for the sailboat, it does, becaue the airplane is attempting to counteract gravity, whereas the sailboat is simply counteracting drag from the hull in the water.

Or to put it another way, if we could create the same pressure differential from one side of the sail to the other without the moving air, the boat would still move forward, solely from the pressure differential, not from moving air striking the “back” side of the sail.

Is this essentiall correct?

38

For the beam reach and higher direction, yes. The vacuum of air pulls the boat forward the same way a vacuum cleaner pulls dirt into its maw.

Stranger

39

Analogous to the thing about it being possible to sail faster than the wind: squeeze a melon pip - it will shoot out from between your fingers at a speed greater than that at which your fingers close upon it.

This happens because the pip is approximately wedge-shaped. The angle of the sail vs the keel is similar, and the two forces of the wind’s acting on the sail, plus the sea acting on (resisting) the keel are like the squeezing fingers.

40

A small correction to Cecil’s article: BMW Oracle is no longer using a traditional mainsail. It has been replaced by a rigid foil.

This may in fact make the comparison to an airplane wing even more valid…