Tacking: Newsletter Challenge (08/01/06)

Why? Because, land lubber, sailing into the wind does not work!

Tacking is simply sailing at a vector that goes across the wind. While not quite as effective as sailing with the wind, you can still use a vector portion of the wind's energy to sail "upwind" and "crosswind" at the same time.

At some point you partially reverse direction and zig the other direction. Over time your sail boat moves upwind, wich represents the average trend for the boat.

Don't you actually use the wind to create an airfoil with the sail and use the pressure difference to "fly" up-wind? Like a vertical wing?

Well, it has been awhile since I last sailed, but the wind, as I remembered, folded the sail so it bulged away from the direction the wind was coming from. The force generated on the sail by the wind would essentially push the boat. However, the hull of the boat (positioned by the rudder) would cleave its way through the water based on the force applied to the sail by the wind. Yes, I think there would be a pressure gradient between the two sides of the sail, which generates the force to move the boat.

As far as aircraft goes, I can speak with a little more authority. The airfoil does not cause the aircraft to lift off the ground. Although an airfoil will generate some lift, contrary to popular belief, it is angle of attack of the wing that provides lift! Go look at the wing of an aerobatic aircraft. It has no airfoil at all! Both sides of the wing are curved the same.

Tacking also works due to the keel of the boat stopping sidways movement and all force produced by the sail results in the boat moving forward which in the case of tacking is up wind

You're probably the only one with the right answer! A sail is not an airfoil. It is only paper thick and if the wind followed a laminar path on both sides there would be an infintesimal difference in the path length, velocity and pressure. It is the angle of attack and the subsequent change in the direction of the air striking the sail that imparts a driving force to the sail. How else could an airplane possibly fly upside down?

Well, I think you put it correctly. A sail could be a flat piece of wood and it would still propel the boat forward.

Exactly!I wish everyone would get off of mathematical "theory"of lift.We all no from expierience,remember the balsa gliders we used to play with as kids?Perfectly flat wings!It's the "angle of attack"Buy the book "Stick and Rudder"

You are correct that the sail is an airfoil. Typically, when sailing upwind, the boat is pointed as close as 45 degrees from direction of the wind and the sail is positioned as close to the center line of the boat as possible (refer to as "close hauled"). Air flow curves the sail into a airfoil with the difference in air velocity on the front and back of the sail creating lift. The keel functions to translate the sideway vectors of the lift components into forward motion. There are boats that use wings as sails to produce the most effecient airfoil for max lift and max speed. Regular cloth sail continue to be the norm for most sailing crafts as there are a number of practical reason, such as cost, simiplicity of rig, storage, etc. where wings are at a disadvantage.

Getting back the basic question: most of us appear to get the basic idea of the sail producing lift so we can sail toward the wind. The limit as to how close we can sail to the wind is typically at 45 degree to either side of the direction of wind. So if wind is blowing straight from the north (0 degrees on the compass) we can sail a course of 45 degrees or 315 degrees. By alternating back and fore time spend sailing a course heading of 45 degrees and 315 degrees, we can get to a point directly north (0 degrees) of where we started.

Tacking works because the price of fuel is so high..

Tracking is only possible with a triangler sail. Back when rectangler sails were being used you could not sail against the wind.

thats not exactly true. yes the sails were shaped like rectangles but they couldn't sail against the wind because the mast was in the middle of the sail. some of the most advanced sails today are rectangular in shape simply because it provides more power to the rig (mast set up)and boat than a trianglular sail would, and the more power that you have the easier it is to get through waves without slowing the boat down.

Not true The old "square rigged" clippers could tack fairly well. Matthew Fontaine Maury figured out courses that required less tacking, but whan tacking was necessary, they could do it. How do you suppose they got to the west around Cape Horn against the westerly "Roaring Forties"?

More landlubbers than sailors on this site. Gaff rigged cutters used to ply there trade up and down the Thames, these boats had square sails. Chinese junks also have square sails and they can beat to windward with great efficiency

An interesting note is that most people would guess that the fastest that a boat can travel is when it is on a 'run'; ie. when it is travelling in the direction of the wind with the wind blowing it from behind. This is not true, and in fact, many sail boats can travel much faster than the speed of the wind that they are travelling in. Why?

(Should this be posted as a new 'challenge' and not just a reply?)

Going directly downwind your speed is restricted by your waterline. Hull speed velocity in knots = 1.35 x (squareroot) of waterline length. As you heel the boat (which happens at any sail angle but a run) your waterline increases.

This is accomplished with a bit of vector mathematics. The wind is the large force vector in the equation. As the wind pushes at approximate right angles to the boat, the boat's large keel (underwater wing shaped centerboard), poses a very large drag force against the boat being pushed in the direction of the wind. Since the keel is aligned with the length of the boat, the boat really wants to travel forward, and the resultant thrust vector is in that direction. The shape of the sail also provides forward thrust. As the triangular sail inflates with a wind it creates an airfoil shape. As subsequent wind passes around the sail (airfoil), negative pressure is induced out front of and on the leeward side of the sail. This in turn causes surrounding air to rush into the sail and propel the boat further. This sail/airfoil action is compounded, as the boat travels faster, the wind around the sail creates more negative pressure, causing the boat to travel faster causing more negative pressure and so forth.

So,

Here is my question as an avid and regular sailor and an engineer: Given the previous post and the earlier post about "attack angle", what is the true driving force of the sail?

I have recently read much about this "attack angle" in a book called Stick and Rudder (about the dynamics of flying) because I am in the process of building a wing sail for a small ice / land yacht. I know that when I am sailing, the angle of attack is critical to speed, but this discussion of creating a low pressure and high pressure side on the wing (or sail – it is the same thing regardless of the shape) that causes the force versus some other explanation that I can't seem to get a grip on. Anyone out there (especially you aerospace guys) care to explain the real way a wing uses the angle of attack to lift a plane (or move a boat)?

It doesn't seem logical to me that a simple pressure difference is enough to create the kind of forces that ice / land yachts use to generate their speed from a wing sail. These little boats can achieve 3 times the speed of the wind easily (often in excess of 100 mph).

Stephan

I submitted a long reply to this but it did not appear. The shorter version:

Review Marchaj's book, "The Aerohydrodynamics of Sailing" (second edition) for a complete explanation of the vector math involved. A complete vector diagram is on page 752, or therabouts.

Abbott and Doenhoff's book on "The Theory of Wing Sections" is a classic text on wing theory that any aero guy could lend you.

Keels, rudders, sails, wings are all fluid foils, and all work on the same principles. Google for a free or inexpensive airfoil analysis program (PANDA is one I have). Using such a program will enable you to see the pressure distribution across a foil (including one of your own design).

Having built a sailboat powered by a rigid wing (google Windrocket) I can assure you the the simple pressure difference you refer to is indeed enough to move a boat (and pretty fast at that).

Consider that a 1440 lb plane, with 100 ft of wing area (14,400 sq in) needs only a 1/10 psi pressure differential to fly.

Have fun, sail fast.

Another thing you may find interesting:

When sailing the Windrocket at high speed, the side force (that resisted by the centerboard's lift) was about equal to the entire weight of the boat and crew.

I am familiar with Windrocket! That is just an awesome vessel and a fine example of the possibilities here. I follow the MSN Wing Sailing group and since I am stuck in the middle of Montana (lots of wind, not much water) for the next few years I am building a wing to put on a land yacht with buggy tires for rougher terrain. I hope it works! I build composite boats all the time, so I figure the hard part will be to get the balance right.

With respect to the way an airfoil/wing/sail uses angle of attack to create lift, here is a (hopefully) brief description.

Effectively, when fluid has to flow around a convex curved surface, two things happen to it. First, it speeds up; and second, a pressure differential is created where there exists a lower pressure on the more tightly curving side and a higher pressure further out. Consider that a stream line (a path that is traced out by a particle in the flow) can only curve if it is forced out of a straight line by an external force (ie the pressure differential). These two traits are not cause-and-effect, but they both arise simultaneously from the fluid passing around a curved surface.

Now with a wing or airfoil: Basically, the greater the angle of attack, the greater the obstruction to the flow, and the larger effective shape for the air to flow around. This increases its effective curvature, and thus forces the air to move faster around the foil and decreases the pressure along the foil's surface. If the angle of attack is increased too much, the pressure differential required to keep the flow curved becomes too extreme and the flow seperates from the foil causing a stall and a loss of lift.

There is also an effect that is created near the ground where a wing at a high angle of attack actually compresses the air between it and the ground. The compressed air in turn pushes back up on the airfoil, but this doesnt really work too well in free-stream conditions.

If this doesn't seem like it should produce enough thrust, just consider how heavy jumbo jets are or how slow paragliders travel compared to their lift and it really isn't too far off-base.

For an interactive example of how AoA effects the pressure and lift of an airfoil, search for JavaFoil on google and you can create and play around with hundreds of airfoils.

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Actually...it IS rocket science...

Well done! This will actually help me in my wing sail powered land yacht work.

You keep referring to the "air foil"the curvature of the wing.Why then,does a pefectly flat wing,top and bottom,fly?

An airfoil shape is just a more efficient way of creating conditions under which lift can be achieved. A flat plate is a perfectly good, if very inefficient, 'airfoil' in that it does provide lift at non-zero angles of attack, however it also produces large drag forces too, which is why it isn't used to fly a plane. Replace 'airfoil' with 'flat plate' in much of my earlier post and the reasoning is still largely accurate.

Balsa planes rely on their light weight to achieve a semblance of flight, as lift can only be generated on a flat plate at a positive angle of attack (if the wing is angled downwards, naturally the 'lift' force will push further downwards, since the wing and thus the forces are symmetrical around 0 degree angle of attack). As the plane glides, it doesn't rely on the wing to provide lift, but uses it more like a well-placed, rigid parachute. Because of the wing, the plane can't just drop, and must take a long, sweeping path back to the ground.

Cheers

In some ways this restates what talentedfool has said. There is no one "airfoil". Anything that deflects an oncoming airstream works as an efficient or inefficient airfoil. Barn doors, sails on sailboats, soft wings on hangliders, wings on boeings, symetrical wings on aerobatic aircraft, a sheet of plywood held along one edge on a windy day, the roof of a house being lifted off by a storm, etc. The only difference is their efficiencies (lift over drag ratio).

The classic bernoulli effect explanation (that suggests that the the air must flow faster over the top surface, and which shows the streams rejoining at the trailing edge) is not really correct. Once circulation is established, the point at which the streams rejoin is actually forward of the trailing edge, on the top side of the wing (assuming it is mounted on an aircraft flying with the wheel side down). Some serious aerodymamicists will talk about Coanda effect, and other little known stuff, and among serious aero types there is not 100 agreement on the best way to explain lift to lay people.

I like to say simply that airfoils deflect air, and in so doing have to be forced away from that deflection by the old "opposite and equal" newtonian theories. Really efficient foils simply deflect the airstream without causing much turbulence and consequent drag. This relatively simple deflection approach also explains why a water foil produces many many times more lift than an airfoil of the same size -- in fact, the ratio is perfectly related to the mass densities of the fluids.

Some "thin" foils such as a piece of paper curved into an arc, or the sail of a sailboat, are actually reasonably efficient airfoils. The "classic" bernoulli efect does not (at least in a simple way) explain how they work.

Hold your hand out the window of a moving car and "fly" it up and down (like you did when you were a kid). That's how a wing works.

The Coanda effect has more impact on how a sail works than lift. The tell tails need to be horizontal on both sides of the sail for the sail to be working most efficiently. The best explanation on how a sailboat works can be found at http://www.sailtheory.com/index.html

A yacht sailing across the wind can move much faster than the windspeed, whereas a yacht sailing straight downwind cannot.

It's simple vector maths.

If you start off sailing at, say, 12 mph at 90° to a 16 mph wind, then the air will hit the boat at a combination of 12 mph from the front and 16 mph from the side. The actual wind across the sail will therefore be root(12^2 + 16^2) = 20 mph, at about 53° from the direction of travel.

This is still plenty of power left at that angle, so a streamlined yacht will actually accelerate - further increasing the windspeed but reducing the angle of attack.

At some point, the angle of the 'relative' wind will be too close to the direction of travel for the sails to produce enough forward thrust to overcome friction (mainly the resistance of the water, but also the increasing headwind). On ice or on wheels, friction is minimised so going three times the speed of the wind is quite possible.

For an ice yacht travelling at 3 times a windspeed of 16mph (with the wind coming from 90°), the air will hit the sail at root(16^2 + 48^2) = 51 mph, at an angle of asin(16/51) = 18° to the direction of travel. The sail would have to be very tight...

The keel is not needed to steer the boat. Many boats have a moveable keel that can be retracted. You can still sail that boat with the keel in the retracted position, however, it is much easier to tip the boat over.

The keel of a boat is a counter balance and part of the steering when you are closehaulled ie: sailing into the wind, without the keel, the boat would just slip sideways. Centerboards that are used on small boats are retractable but you only have them fully retracted on a run ie: with the wind coming from the rear of the boat and fully down when close haulled. They are adjusted as you change the direction you are sailing in, as they cause drag plus if they are fully down on a run they can make a sailing dingy unstable. The keel on a lot of larger racing boats can also have the angle adjusted using hydralics

The WEIGHT of a keels also serves to balance the boat against the force of the wind. Think of the little sailboats you've seen tipped over when the sail caught too much wind while trying to tack. The boat just rotated around it's center of gravity. Now, if you hang a nice, streamlined weight under the boat, it'll take a lot more force to displace that weight.

A major feature of a centerboard (or an adjustable keep) is to adjust the fore-and-aft position of the Center of Lateral Resistance of the vessel underbody to be very near and usually slightly forward of the Center of Effort of the sailing rig. This allows the steering gear to work with low effort and small angles (large angles have disproportionate braking effect). Usually CLR forward of CE is desirable because the boat will head into the wind rather than away if the helm is unmanned, lessening the likelihood of you treading water as your former steed bolts over the horizon. You'll likely still drown, but having the boat nearby at least gives you something to occupy your time while waiting.

Downwind in a light or planing dinghy is a special case where you hang the boat off the fin area way aft (i.e. the rudder) so that it can weathercock as needed and not capsize by tripping over the unneeded fin area forward. Downwind sailing in a light fore-and-aft-rigged boat tends to employ sail areas and forces that are much greater than the athwartships stability of the boat can manage, so it's important to keep them lined up fore-and-aft. The exact moment to shove the board down as you round up around the downwind mark is of concern; too early and you may be swimming, too late and everyone will see you sagging off helplessly sideways wondering why the bl**dy boat won't steer -- and if you're really lucky, fouling the six boats who *had* been behind you but who remembered to put the board down and are now using their spare energy to yell PROTEST at you.

AND the boat slips sideways vs. the boat heading with the keel removed. Without the keel, the hull is the main object resisting sideway movement as it attempt to sail close hauled, so you CAN still sail forward but with greater slippage sideways. While the weight of the keel counterbalances the force on the sail causing the boat to heel over, the OTHER function of the keel is to provide a large surface area under water perpendicular to any force from the side, making it difficult to push the boat sideways.

I have always concieved of tacking as the equivalent of squeezing a watermelon seed between your fingers. You are squeezing the seed and all of a sudden it goes zipping off in the direction of your sibling(or kid if you're just a grownup kid). I kind of figured tacking worked on the same principle the wind is your thumb and the water is your finger. The boat is the watermelon seed.

Tacking works because it is quicker than wearing away and gybing. Sail Bad the Sinner

The way a sailing craft extracts energy from the air/water interface reminds me that impedance mismatches in general are where free energy becomes available to be harnessed and/or floats around looking for mischief to do. And the more abrupt the step, the more intense the effect -- I well remember, for example, how the early Heath/Zenith terminals used to shed their CRT mounting posts (molded structural foam bezel with integral posts, no reinforcing fins) early in the shipping process so that the CRT could spend the rest of the trip bashing around inside the case. Retooling the mold to give a little fillet at the base of the posts cured it entirely.

My world changed when I started viewing the choice of hammer as an impedance-matching problem...