How Does a Sailboat Work?

[crosscr]

Originally Posted by Dick Pluta

Second, the reason you can't sail directly into the wind is that the sail (wing, airfoil) must have an "angle of attack". It can't point right into the wind. If it points directly into the wind it flaps like a flag (luffs). The sail must be presented to the wind at an angle to work. This is why your boat will only point at some angle to the wind. The wing stops working.

Now, that's just the sail or the "motor" of the boat. You have a similar effect with the keel. The keel does more than just preventing the boat from sliding sideways. Just like the sail, it "lifts" the boat to windward.

It has an angle of attack because it is not a wing. A wing has a structure underneath its' skin to keep it shaped like a wing. A sail does not,thus a sail is not wing.

[Sandero]

A sail can act as a lifting surface - a wing when properly shaped and positioned relative to the wind.

A fin keel does not lift the boat as it is symmetrical with respect to the boat's motion. I heard other make this claim, but I don't see how the keel with the water coming directly on the leading edge can provide lift.

I can see how it counteracts the heeling caused by the low pressure on the lee side of the sail. And this IS the main purpose of a keel.

If someone can explain how a keel "lifts" or pulls the boat to windward... please do.

[cal40john]

For the OP, try this. Break it all down into simple parts. Mount a sail on a a board and put it on ice. Attach springs in all directions. Mark the position of the board with no load. Change the angle of the sail relative to the wind (sheet in and out) and find where the board has moved the most and note which direction it has moved. This is where the sail develops the most power, and shows you which direction it is pulling the boat. You should find that the direction it is pulling is slightly forward of perpendicular to the boom. Within the limitations of the boat this is the angle that you want to keep the sail relative to the wind no matter which way you are going. Picture this by placing an outline of the boat around the sail, now rotate the boat to demonstrate sailing on different courses, the sail does not move its angle relative to the wind. As you rotate the outline of the boat to point higher, you will eventually reach where the direction the sail is pulling is at right angles to the boat, this would be the theoretical limit as to how high you can point as the total force of the sail is all pushing the boat sideways and none forward. In reality you have to point lower because the keel can only accept so much side load to forward pull before it stalls out. A poor analogy since lift doesn't come in to this at all is to get a toy car, tape a string on it and pull the string at an angle to the direction that the car is traveling. The car will go forward until the angle of the string is perpendicular to the direction of travel. The car almost entirely goes forward and not sideways because the friction sideways is enormous compared to that of it going forward. This is similar to the keel on the boat, the boat easily moves forward, but the keel makes it hard for the boat to move sideways.

When you turn far enough downwind that the shrouds don't allow the sail to be at a constant angle to the wind, the sail will stall and produce less force. Even a sailboat without shrouds won't let the sail out much past 90 degrees, try sailing a Laser with an extra long mainsheet and try to keep it from death rolling as the sail pulls to the (nominally) windward side of the boat.

John

[cal40john]

This was in another thread. Sails and keels have lift. Even a flat plate creates lift. Planes fly upside down and the wings make lift in the opposite direction that they were designed to, some stunt planes have symmetrical wings. Airplane wings are attached onto the plane at an angle to provide an angle of attack to make lift so the plane travels forward rather than at an angle creating extra drag.

Whidden in The Art and Science of Sails did not say in his book that sails don't make lift, he said the old simple explanations like Bernoulli aren't correct. In his description as to how lift is generated he gives credit to Arvel Gentry. C.A. Marchaj in Aero-Hydrodynamics of Sailing credits Arvel Gentry with correctly explaining lift. Gentry is a Boeing engineer.

Here Gentry explains how even a flat plate generates lift:
http://www.arvelgentry.com/magaz/How...Gives_Lift.pdf

There are many interesting articles on his website, browse around.

Try this experiment at home. Sheet your sails in for close hauled, sail close hauled with all your telltales flying, note your angle of heel, now bear off to a beam reach. The wind is now perpendicular to your sails. Those who say there is no lift should find that their boats heel more. Mine heels less because the sails are stalled and are not creating lift, so there is less force generated.

Try short board sailing. The keel is somewhere around a 14" long piece of symmetrical plastic called a skeg. You're leaning out pushing with your foot to leeward, you can feel the board solidly pushing back and the windsurfer is traveling forward with very little leeway. When I was working on sailing upwind better, I worked on pointing higher and higher, then suddenly the board was now skipping completely sideways, I can't feel anything pushing back, as I crash I think I've broken off my skeg. I flip the board over and there is the skeg. I do this at least two more times before I realize when I have pointed high enough to stall the skeg out so it is no longer making lift, the force of just the area of the skeg pushing sideways through the water pushing back at me is indetectable. The force provided with lift makes the board feel like you're mounted on a track. An epiphany for me as to how important lift is.

John

[Dick Pluta]

Originally Posted by crosscr

It has an angle of attack because it is not a wing. A wing has a structure underneath its' skin to keep it shaped like a wing. A sail does not,thus a sail is not wing.

You better go tell all those guys flying a Rogallo wing. No structure. Just struts, not unlike a mast and spar. I hope they don't get hurt.

Dick Pluta
AEGEA

[Ex-Calif]

Sail vs. Wing - Technically a sail is a sail and a wing is a wing. They are both airfoils.

Angle of Attack - The angle of attack is the angle between the relative wind and the chord line of the wing.

Angle of Incidence - Is the "fixed" angle between the longitudinal axis of the fudelage and the chord line of the wing.

Lift - Is roughly perpendicular to the angle of attack.

The angle of attack changes in flight and except for a few strange experiments the angle of incidence does not.

A plane flying slowly and maintaining altitude will have a higher angle of attack than the same plane flying faster. The generation of lift is due to the pressure change. When the plane slows sufficiently the angle off attack will exceed the ability of the flow to stay laminar and the flow will separate. Stall will occur.

A sail is an airfoil and it definitely pulls the sail perpendicular to the angle of attack.

The lift of the sail is also perpendicular to the angle of attack. Because even close hauled the wind is not directly in "front" of the boat the lift vector is slightly forward. The keel resists lateral mevement of the boat and directs energy rearward through interaction with the water. The keel is also an airfoil shape and provides its own lift.

[Sandero]

I don't see how a keel "lifts" the boat to windward... or lifts at all. The medium which it moves in is basically fixed aside from current. When you motor the keel is moving through the water and the "angle of attack" is 0°.

Now some true wind appears on one side of the boat and you deploy and trim the sail so that sail causes the boat to both heel and provide a forward component - "lift". The keel is still moving through a fixed medium but with a slight direction to leeward (making leeway) and now tilted (heeling). If anything, the water moving at the keel is hitting the windward side pushing it to leeward and aft... like dragging a rudder.

Regardless of the cross sectional shape of the rudder, I don't see how it provides lift. Please explain. (again)

[Robertcateran]

Originally Posted by defjef

A sail can act as a lifting surface - a wing when properly shaped and positioned relative to the wind.

A fin keel does not lift the boat as it is symmetrical with respect to the boat's motion. I heard other make this claim, but I don't see how the keel with the water coming directly on the leading edge can provide lift.

I can see how it counteracts the heeling caused by the low pressure on the lee side of the sail. And this IS the main purpose of a keel.

If someone can explain how a keel "lifts" or pulls the boat to windward... please do.

The keel has an angle of attack so that it generates lift. There is some leeway

[GordMay]

Arvil Gentry < ArvelGentry.com > has some excellent Sailing Technical Papers
➥ Technical Papers

Including:

“The Origins of Lift”
➥ http://www.arvelgentry.com/techs/Origins_of_Lift.pdf

“A Review of Modern Sail Theory”
➥ http://www.arvelgentry.com/techs/A%2...l%20Theory.pdf

The Angle of Attack (AOA) of the keel, which generates "lift", is due to the difference between the direction the boat is pointed and the actual direction of travel - or leeway.

[cal40john]

Originally Posted by defjef

I don't see how a keel "lifts" the boat to windward... or lifts at all. The medium which it moves in is basically fixed aside from current. When you motor the keel is moving through the water and the "angle of attack" is 0°.

Now some true wind appears on one side of the boat and you deploy and trim the sail so that sail causes the boat to both heel and provide a forward component - "lift". The keel is still moving through a fixed medium but with a slight direction to leeward (making leeway) and now tilted (heeling). If anything, the water moving at the keel is hitting the windward side pushing it to leeward and aft... like dragging a rudder.

Regardless of the cross sectional shape of the rudder, I don't see how it provides lift. Please explain. (again)

You have which side the water is hitting the keel wrong. Change your frame of reference. As you said the boat is moving to leeward. Stand on the boat and look at the water, which direction is it moving? It's moving to windward, so the water is hitting the leeward side of the keel, so lift generated is to windward. Not to the point that leeway entirely disappears. The rudder is the same, but in addition with some weather helm has an increased angle of attack and so more lift, unless you have too much weather helm, then it has at least extra drag, if not stalled out.

John

[GordMay]

like this?

[Sandero]

Right, the water encountered in on the lee side, but the forces on a heeled keel are hardly forward and considering it size quite small I would say.

Additionally, I am not convinced that a symmetrical foil, top and and bottom or in the case of a keel port and starboard, provides much lift especially with such a small angle of attack.

For example, in the GordMay attachment you can see how close "winded" the keel is. You can't sail that close to the wind and get any forward motion.

I suspect those vectors are not the least bit accurate and the diagram neglects that the whole bit is rotated because of heel angle.

[cal40john]

Originally Posted by defjef

Right, the water encountered in on the lee side, but the forces on a heeled keel are hardly forward and considering it size quite small I would say.

Additionally, I am not convinced that a symmetrical foil, top and and bottom or in the case of a keel port and starboard, provides much lift especially with such a small angle of attack.

For example, in the GordMay attachment you can see how close "winded" the keel is. You can't sail that close to the wind and get any forward motion.

I suspect those vectors are not the least bit accurate and the diagram neglects that the whole bit is rotated because of heel angle.

You're right the force is mostly sideways, that's the point, the sails are mostly pulling the boat sideways when you are close hauled. When the forces all balance out there is no more acceleration and the boat travels forward at a constant velocity with some leeway. Typical numbers for leeway on keelboats are 3-5 degrees.

If you can't relate to my windsurfer story, try it on a dinghy like a Laser. Grab the boom with your hand and pull the sail to weather and head up from sailing close hauled. When the daggerboard stalls out the boat slides surprisingly fast to leeward, bear off and sheet out until the daggerboard establishes lift again, and all the sudden you're only making a few degrees of leeway. If there were no lift involved the boat should slide sideways close to equally fast whether you're pointing too high or not.

John

[Christian Van H]

Here's a great video that hits it right on the head: The Physics of Sailing - KQED QUEST Television Story

[anotherT34C]

Remember that at sea level, and room temperature, the density of air is about 1kg/m^3, while the density of water is about 1000kg/m^3.

Three orders of magnitude is an awful lot.