How -do- boats sail?
To many people, it makes no sense that a sailboat can go upwind. They will say things like "It's against the laws of physics" or maybe "That would be like a ball rolling uphill." And in some ways, it does seem to defy common sense that a boat can use the power of the wind to make progress in the opposite direction the wind is blowing.
But it works, sailors have been doing it for many centuries.
BASIC POINTS of SAIL
In order for the boat to move forward, the sails must produce some power... remember the wind blowing on the sails will -always- produce DRAG, which is a force affecting the boat, but it is not useful power to drive the boat... unless the boat is going downwind anyway ("broad reaching" or "running")...
How do sails produce power? A sail is a simple machine called a FOIL. This is one of the basic machines of our whole system of technology, like the lever or the wheel.
A FOIL is a like a wedge, except that it works in fluid (such as air or water) rather than sitting on solid ground. The fluid has MASS and thus when it is moving, that is a form of energy. The FOIL takes that energy to create a useful force called LIFT.
An airplane wing is the most obvious example of a FOIL but sails have been around a lot longer than wings. This technology is also used in windmills, pumps, hydraulics, turbines, propellors, fans, and many other applications.
But let's look primarily at SAILS!!
We know that sails can be hoisted up and lowered down (key word: "halyards"). We also know that sails can be pulled in or eased out, thus changing the angle of the sail (key word: "sheets"). When you look at the first diagram on POINTS of SAIL, you see that the narrower the angle of the boat to the wind, the tighter the sails are pulled in.
What you might not notice is that the sail stays almost a constant angle to the wind!
Pilots worry about the 'angle of attack' especially when taking off, but sailors don't use the term as much. It's still an important concept. The sails of a boat "in irons" have an angle of attack of zero. When the boat is steered on a course anywhere from 45 degrees to 180 degrees relative to the wind, the sails are adjusted (sailors use the word "trimmed") to get the most efficient angle of attack.
Is this making any sense? Does it have any relevance to how a sailboat goes upwind, aganst the very force that is driving it, like a ball rolling up hill ??
It's time to use some pictures of real boats sailing. Diagrams are great but they get boring. In fact, I love to see pictures of -my- boat, so look here...
The start of the 2009 Wilkerson Cup. Wind direction is almost straight from right, but angled slightly towards us (ie out of the picture). You can see that each skipper has chosen a slightly different angle as his boat's best course, and sails trimmed to slightly differing 'angles of attack' for going CLOSE-HAULED. These boats are racing to a point about 1 1/4 miles to the right... in other words, upwind...
BTW Coach Mark Hittner won this race, sailing his Morgan 25 PESTY (furthest to left). There are also two Ensigns and Coach King's Santana 23 BLUE YONDER in this picture... ...Photo taken by John Jackson.
So, what have we learned? That a sail is like a wing, and the direction or angle at which the wind hits it can make a big difference in the boat's moving forward.
The sailor will adjust the 'angle of attack' of the sail (this is what 'to trim a sail' means) so that the forces of lift & drag will propel the boat.
At a low angle (top), not much LIFT but then there is not much drag either. In the middle, we see approximately the "most efficient" angle. At the bottom, we see a foil that is stalled. Fluid flow does not like to go around corners. Too high an angle of attack seperates the flow from the surface and it just swirls at random (technical term: "turbulence"). The FOIL is producing not much lift, and large amounts of drag.
So let's apply this new knowledge to HOW A BOAT SAILS... the point of this lesson!
Here are 3 FJs with their mainsails set at 3 different angles. All 3 boats are sailing at the exact same angle to the wind, but their sails are trimmed differently.
The top boat has pulled in the mainsheet just enough to stop the sail from flapping. The sail is just beginning to work as a FOIL and the LIFT it generates will move the boat forward.
The middle boat has pulled it's mainsheet in a bit further. The sail is at a tighter angle to the boat, which is what everybody sees, and a wider angle to the wind. The sail is producing more LIFT and the boat should move faster.
The bottom boat has pulled it's mainsheet tighter yet, the sail is producing lift but also much more DRAG due to the turbulent flow. This boat may have as much driving force as the middle boat (but probably doesn't) however it will feel a lot of heeling force.
If these boats were racing, which one would you bet on? I'd take the middle one!
Let's review- a boat's Point Of Sail refers to the boat's angle to the wind.
The sailor trims the sail to keep that sail at an effective angle to the wind, which means adjusting it's angle to the boat. So the sails will be trimmed differently for different points of sail; in fact the sails may be trimmed differently according to wind or sea conditions... such as when the wind is strong and heels the boat too far over...
The sail is a machine called a foil which uses the energy of the wind to produce power.
A sail, a wing, a turbine blade, or any other FOIL always has the force of "drag" which can be thought of as simply the friction of the wind (or other fluid) in motion along it's surface. A FOIL can also generate a force called LIFT, and this varies in relation to the foil's angle to the wind.
The sailor adjusts the angle of the sail to the wind to use that power to propel the boat.
It may be more clear to look at it from the point of view of the boat remaining at a constant angle. The top boat here is close-hauled, the sail is at approximately 25 degrees to the wind, the boat is at approximately a 40 degree angle to the wind.
The LIFT force of the sail is pulling the boat almost sideways, which will tend to make it heel over and also to slide sideways through the water. This side-slipping (key word "leeway") is counteracted by the centerboard, which is also a FOIL.
Next we see a boat at a different angle to the wind (although it is the same angle on the screen). However the sailor has adjusted the sail so that it is still at an efficient angle and is propelling the boat forward. This is a Close Reach point of sail.
Now the wind is at yet another angle to the boat. The sailor has again adjusted his mainsheet, and the boat is being propelled efficiently. This point of sail is called a Broad Reach.
Since the boat has Standing Rigging, this is about as far as the mainsail can be eased out. The boom will be against the side-stay, but the sail need not be eased out further because the DRAG of the sail will propel the boat downwind quite well.
For basic sailing, we don't need to know a lot about aerodynamics. In fact you could sail a boat simply by looking up and adjusting the sail so that it was neither flapping freely nor heeling the boat too much. In the NJROTC sailing course, we teach enough about the laws of physics which govern sailing (and vessels in general) that the student will be able to go on to bigger & better ambitions with a solid background. The goal is not -just- to sail a boat... or even to sail a boat well... but to understand why boats sail, and to be able to apply those principles elsewhere in life.
For those with a thirst for more knowledge, here is a link to NASA's web site "Beginner's Guide To Aerodynamics"
...posted by Assistant Coach Douglas King
One of the things beginning sailors must learn is the "POINTS of SAIL." This refers to the angle that the boat is sailing, relative to the wind direction. This is important, and entirely seperate from which direction the boat is sailing relative to the earth, or the compass.
When a boat is headed straight into the wind, it is "IN IRONS" and if it is moving forward, it will soon stop. The sails are flapping, not producing any power!
When a boat is headed straight into the wind, it is "IN IRONS" and if it is moving forward, it will soon stop. The sails are flapping, not producing any power!
In order for the boat to move forward, the sails must produce some power... remember the wind blowing on the sails will -always- produce DRAG, which is a force affecting the boat, but it is not useful power to drive the boat... unless the boat is going downwind anyway ("broad reaching" or "running")...
How do sails produce power? A sail is a simple machine called a FOIL. This is one of the basic machines of our whole system of technology, like the lever or the wheel.
A FOIL is a like a wedge, except that it works in fluid (such as air or water) rather than sitting on solid ground. The fluid has MASS and thus when it is moving, that is a form of energy. The FOIL takes that energy to create a useful force called LIFT.
An airplane wing is the most obvious example of a FOIL but sails have been around a lot longer than wings. This technology is also used in windmills, pumps, hydraulics, turbines, propellors, fans, and many other applications.
But let's look primarily at SAILS!!
We know that sails can be hoisted up and lowered down (key word: "halyards"). We also know that sails can be pulled in or eased out, thus changing the angle of the sail (key word: "sheets"). When you look at the first diagram on POINTS of SAIL, you see that the narrower the angle of the boat to the wind, the tighter the sails are pulled in.
What you might not notice is that the sail stays almost a constant angle to the wind!
Pilots worry about the 'angle of attack' especially when taking off, but sailors don't use the term as much. It's still an important concept. The sails of a boat "in irons" have an angle of attack of zero. When the boat is steered on a course anywhere from 45 degrees to 180 degrees relative to the wind, the sails are adjusted (sailors use the word "trimmed") to get the most efficient angle of attack.Is this making any sense? Does it have any relevance to how a sailboat goes upwind, aganst the very force that is driving it, like a ball rolling up hill ??
It's time to use some pictures of real boats sailing. Diagrams are great but they get boring. In fact, I love to see pictures of -my- boat, so look here...The start of the 2009 Wilkerson Cup. Wind direction is almost straight from right, but angled slightly towards us (ie out of the picture). You can see that each skipper has chosen a slightly different angle as his boat's best course, and sails trimmed to slightly differing 'angles of attack' for going CLOSE-HAULED. These boats are racing to a point about 1 1/4 miles to the right... in other words, upwind...
BTW Coach Mark Hittner won this race, sailing his Morgan 25 PESTY (furthest to left). There are also two Ensigns and Coach King's Santana 23 BLUE YONDER in this picture... ...Photo taken by John Jackson.
So, what have we learned? That a sail is like a wing, and the direction or angle at which the wind hits it can make a big difference in the boat's moving forward.
The sailor will adjust the 'angle of attack' of the sail (this is what 'to trim a sail' means) so that the forces of lift & drag will propel the boat.
At a low angle (top), not much LIFT but then there is not much drag either. In the middle, we see approximately the "most efficient" angle. At the bottom, we see a foil that is stalled. Fluid flow does not like to go around corners. Too high an angle of attack seperates the flow from the surface and it just swirls at random (technical term: "turbulence"). The FOIL is producing not much lift, and large amounts of drag.
both links above to Wikipedia
So let's apply this new knowledge to HOW A BOAT SAILS... the point of this lesson!
Here are 3 FJs with their mainsails set at 3 different angles. All 3 boats are sailing at the exact same angle to the wind, but their sails are trimmed differently.
The top boat has pulled in the mainsheet just enough to stop the sail from flapping. The sail is just beginning to work as a FOIL and the LIFT it generates will move the boat forward.
The middle boat has pulled it's mainsheet in a bit further. The sail is at a tighter angle to the boat, which is what everybody sees, and a wider angle to the wind. The sail is producing more LIFT and the boat should move faster.
The bottom boat has pulled it's mainsheet tighter yet, the sail is producing lift but also much more DRAG due to the turbulent flow. This boat may have as much driving force as the middle boat (but probably doesn't) however it will feel a lot of heeling force.
If these boats were racing, which one would you bet on? I'd take the middle one!
Let's review- a boat's Point Of Sail refers to the boat's angle to the wind.
The sailor trims the sail to keep that sail at an effective angle to the wind, which means adjusting it's angle to the boat. So the sails will be trimmed differently for different points of sail; in fact the sails may be trimmed differently according to wind or sea conditions... such as when the wind is strong and heels the boat too far over...
The sail is a machine called a foil which uses the energy of the wind to produce power.
A sail, a wing, a turbine blade, or any other FOIL always has the force of "drag" which can be thought of as simply the friction of the wind (or other fluid) in motion along it's surface. A FOIL can also generate a force called LIFT, and this varies in relation to the foil's angle to the wind.
The sailor adjusts the angle of the sail to the wind to use that power to propel the boat.
It may be more clear to look at it from the point of view of the boat remaining at a constant angle. The top boat here is close-hauled, the sail is at approximately 25 degrees to the wind, the boat is at approximately a 40 degree angle to the wind.The LIFT force of the sail is pulling the boat almost sideways, which will tend to make it heel over and also to slide sideways through the water. This side-slipping (key word "leeway") is counteracted by the centerboard, which is also a FOIL.
Next we see a boat at a different angle to the wind (although it is the same angle on the screen). However the sailor has adjusted the sail so that it is still at an efficient angle and is propelling the boat forward. This is a Close Reach point of sail.
Now the wind is at yet another angle to the boat. The sailor has again adjusted his mainsheet, and the boat is being propelled efficiently. This point of sail is called a Broad Reach.
Since the boat has Standing Rigging, this is about as far as the mainsail can be eased out. The boom will be against the side-stay, but the sail need not be eased out further because the DRAG of the sail will propel the boat downwind quite well.
For basic sailing, we don't need to know a lot about aerodynamics. In fact you could sail a boat simply by looking up and adjusting the sail so that it was neither flapping freely nor heeling the boat too much. In the NJROTC sailing course, we teach enough about the laws of physics which govern sailing (and vessels in general) that the student will be able to go on to bigger & better ambitions with a solid background. The goal is not -just- to sail a boat... or even to sail a boat well... but to understand why boats sail, and to be able to apply those principles elsewhere in life.
For those with a thirst for more knowledge, here is a link to NASA's web site "Beginner's Guide To Aerodynamics"
...posted by Assistant Coach Douglas King
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