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(Sailing Lessons Table Of Contents)(link)
We are fortunate that we live in a world of very reliable, very complex machines. We take it for granted that we can travel long distances quickly, talk to people who are far away, have hot food... or a cold drink... when it is convenient to us. Humans started out naked in the woods with nothing but sticks and rocks, now many thousands of years later we have all this, plus the
Mars Rover and iPods.
How did we get to this level of technology?
And what does it have to do with SAILING ?!?
Our technology depends on
simple machines. The classic
SIMPLE MACHINES from ancient times (link)
are the lever and the inclined plane... sometimes the WHEEL &
AXLE, the WEDGE, the PULLEY, and the SCREW are included but these are
all special applications of the lever and the inclined plane or a
combination of the two.
Although ROPE is not classified as a simple machine, it is certainly very useful. Here we have an assembly of wheels, along with some specially-shaped wedges, that enable us to control large amounts of force on a vessel's sail.
Ancient peoples had the technology of
sails; is this a simple machine?
In a way, it is. A sail is a FOIL which is very much like an inclined plane (or wedge) used in air, or water, or any other FLUID
(yes air is considered a fluid) (link). Other FOILs include wings, impellors, and turbine blades; foils are used in many types of complex machines.
The word "foil" can mean several things, from a type of light weight sword to very very thin metal used for wrapping up food. These things are useful too but they will not make a boat sail!
A "FOIL" is a simple machine... look Ma, no moving parts... which uses the energy in a moving fluid to generate power.
When we sail, we use two sets of FOILs. One set is the sails using the air to propel the boat, the other set is less obvious: the centerboard and rudder which control the the boat's direction. Both sets of foils are critical to sailing, and we are lucky to have centuries of development in making better foils which make sailing easier & more efficient.
Fluid... air, water, oil, nitrogen, whatever... has
MASS (link). This is a scientific way of saying it weighs more than nothing, although weight is not quite the same as mass.
So, the movement of fluid implies energy, and that energy means a fluid
will always exert force on any object in it's way.
Just because the movement of fluid generates force, this does not necessarily mean that we can gain useful work from that force.
It may help to think of the FORCE in these diagrams as rubber band, stretched, pulling in the direction of the arrow. The bigger the arrow, the harder it is stretched.
IMPORTANT CONCEPTS:
The energy present in a fluid in motion will always exert some force on an object. "Drag" may be considered as similar to friction.
"Lift" is always at right angles to the direction of fluid's movement.
"Drag" is always in the same direction as the fluid's movement.
Can "Lift" be downward? Sure, why not? Lift will be exerted in whatever direction the foil is oriented, it is up to us to decide how to use that force. When we are sailing, the force of LIFT is exerted horizontally by the sails and the centerboard & rudder.
Generally we think of "lift" as being upward, away from the ground, but when we use "lift" as a science or engineering term, it has a specific meaning that has no relation to up or down.
Review... when fluid is moving past an object, or an object is moving through a fluid, we will always have some force being exerted. We will always have drag, but we might or might not have LIFT.
The shape of that object makes a big difference. A FOIL can have many shapes, some are better than others. By "better" we mean generating more lift and less drag!
However, even a flat plate can work as a foil.
What kinds of foil work best? Like any type of machine with many many variations, the desired function has to determine what kind works best. Wind turbines will use foils that look different from steam turbines which look different from airplane wings. An airplane intended to fly slowly, or specialized to take off & land on short runways, will have very different wings than planes designed to fly high or fast.
A spinnaker, which is a sail used for going down wind
(broad reaching or running), looks very different from a genoa jib, which is a sail used to primarily to go upwind
(close-hauled).
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Let's take a quick look at how to define or describe the shape of a foil:
The Spitfire was the best fighter plane of it's time, largely due to the performance it's wing. This is a nice picture and gives you a good idea what the Spitfire looked like, but it doesn't really show important details of the wing.
The
CROSS-SECTION (link) (often simply called the "section" or "foil section") is what you would see if you cut the wing smoothly across the same direction as fluid flows over it. This is how the fluid "sees" the foil and this is the most important characteristic of the foil's shape.
It is easy to guess that a thick wing is for flying slow, and thin wing is for flying fast, and generally this is true. However as we will see, flying is
not that easy (link to STALL) and foil behavior is not that simple!
(link to Supersonic Foil wikipedia)
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When we say "thick" or "thin" foils, we don't mean a simple measurement in
feet or meters (link to wikipedia). Obviously, making the whole foil section bigger will make it more thick, in the normal meaning of the word. However, the top two foil sections in the diagram are identical, the second (middle) foil is NOT thicker. The third (bottom) foil is thicker.
"Thickness" is relative to the size of the foil... specifically, the length of it's cross-section.
When sailing (or carrying out any other complex process) we must be as exact & precise as possible when communicating. This means using correct terminology, whether it is a matter of Right-Of-Way or
foil geometry (link to NASA) ... One advantage we sailors have over other people using FOIL technology is that we can change the shape of our foil(s) relatively easily, using the vessel's rigging. This will be covered in more detail later.
What else is important about how a FOIL works? Lots of diagrams show the foil is at an angle to fluid flow, and obviously this angle can be changed. We sailors use the SHEETs to change the angle, a pilot must change the angle of the whole airplane.
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Here we see three boats, the 3 skippers have different ideas about how to trim their sails even though they are all on the POINT OF SAIL (in the diagram, a close reach on port tack).
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. . . . . . . . . . . The first boat has their sheet eased out and the sail has an ANGLE OF ATTACK of 15 degrees, although this is not any data that you would have (or could use) while actually sailing.
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. . . . . . . . . . .The second boat has pulled their sheets in so the sails are tighter, to an ANGLE OF ATTACK of 25 degrees.
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. . . . . . . . . . . The 3rd boat has pulled their sheets in even more, so the sail is pulled in tight to an angle of 35 degrees. The sail is set in a position approaching what would work if they were CLOSE-HAULED. As you can see by the air flow swirling (turbulence) around the sail, this is not an efficient way to trim the sails!
Both LIFT and DRAG will change as the ANGLE OF ATTACK changes. For any given FOIL SECTION and a given fluid flow past it, there will be one ANGLE OF ATTACK which gives the greatest amount of LIFT, and one specific angle which may not provide maximum lift but will provide the most efficient ratio between LIFT and DRAG. The section shape will affect how large of an angle will be effective, and whether the balance between lift and drag changes smoothly or dramatically, and whether the foil will be prone to stalling.
You can see this is a complex subject and we will clarify some of these concepts later. More specifically, we will show how to apply the relationships between angle, flow, camber, lift, and drag, in order to sail better!
For now, let's review-
- moving fluid exerts FORCE on objects in it's path
- the force of LIFT is always considered as perpendicular to flow, DRAG is in the same direction
- the ANGLE OF ATTACK is very important to making a foil work efficiently.
- the SECTION is one of the most important ways to define/describe the shape of a foil.
- THICKNESS of the FOIL SECTION is relative to the length of it's CHORD, not a measurement in inches or centimeters
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There is a great deal more to study on this topic: pressure and flow relationships, how the angle and shape of the foil affect it's performance, and specifically how all these things relate to sailboats and other vessels. For example, a ship's propellor has blades which function as foils. We will soon have a 2nd lesson available on FOILs to go into more detail.
For now, we will show a diagram of an FJ sailing, with the forces of
lift and
drag shown on the sails and the centerboard.
F.F
(Sailing Lessons Table Of Contents)(link)
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