Physics of Sailing

Physics of Sailing

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The Hull of the boat is the frame which keeps the boat and its crew floating in the water. The mast is the tall pole that sits vertically near the center of the hull, the mast is what the sails are attached to to keep them suspended and straight. The boom is a large pole attached to the bottom part of the mast which is able to rotate up, down and side to side. The sails attach to the boom and the mast then the person controlling the main sail moves the boom around to change the direction and tension in the mainsail.

The rudder is atached to the rear of the boat and is what essencially steers the boat (note that the rudder alone will not steer the boat exactly where you want to go in a sailboat unlike other motorboats).

The most difficult part of a sail boat for most beginners to understand is the keel or centerboard. The keel of the boat does many things, it acts as a pivital point for sharp turns and "jibbing", it helps keep the boat upright by lowering the center of gravity of the boat, it works to keep the boat moving in a line of motion close to that of the direction that the boat is facing. This is because the boat is usually being pushed heavily to the side by the wind. And the keel also works to pull the boat into the direction of the wind the same way the sails do. This is a concept that I will discuss later on.

The Sails:

The sails of the boat are what provide the forward thrust. There can be several sails but in every sail boat there is a main sail. That is the sail which is attached to the mast and is usually the tallest sail on the boat, it is controlled by the boom and has several attachments used for slight adjustments to get the most effeciency. There is usually a "jib", a smaller sail in the front of the boat attached to a guy wire which reaches about %75 of the height of the mast or main sail. The jib provides additional forward force but is just as usefull in providing manuverability in low speed or low wind situations or moving out of a docks area. In this site I will refer only to sailboats with a standard mainsail and jib, as in the picture to the right.

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The main parts of the sails that are important to remember are the head, the tack, the foot, the leech and the clew. These are in the following picture to the right.

Easiest Method of Sailing
Its probably quite straight forward how a boat can sail in the same direction of the wind. This is done by letting out the boom so that the sails are close to perpendicular to the direction of the wind. When doing this the sails deccelerate the wind, the wind accelerates the sails, and the sails push the boat forward. Not much to it. But you may realize that this method only works if you are going in the same general direction of the wind. So how do sailors navigate around this problem?
Sailing against the wind
There are two ways in which a sailboat can move in the opposite direction of the wind. One is by the the sails changing the direction of the wind to create a thrust. The other is by Bernoulis Principal.

In the first part, while the boat is facing the wind at aproximately a 45 degree angle, the sail is kept straight with the boat and the wind flows into and over the sail where its direction is changed as it follows the shape of the sail. There are three resulting forces: the drag caused by the wind moving over the sails, the lateral force exerted on the sail while the sail changes the direction of the wind, and the final direction and velocity of the air after being redirected. Now the drag between the air and sail is usually low and is usually not considered in the diagram. The two important forces are the final velocity and the change between the initial and final wind velocities, this is however resisted by the boats keel. The keel works against the slower moving water to resist any lateral forces. This is the key component in making a sail boat move against the force of the wind, without it the boat would drift as the wind pushed it.

Bernoulli's Principal
Probably the more important factor in sailing upwind is the effect best explained by Bernoulli's Principal. The sail acts just like an airplane wing. When the wind flows over one side it fills the sail while the air flowing on the other side is moving faster and cannot push as hard and thus the sail recieves a force that is perpindicular to the direction of the wind. This would normally not push the sailboat against the wind but the keel of the boat again resists much of the lateral movement so that the boat has only one direction it can move which is forward, providing that the combined forces that are pushing the boat perpendicular to the wind are greater than the force of the wind pushing the entire boat and sails backwards.

Also note on the diagram that the vectors here are considered forces not velocites. This is important for two reaasons. One is that the drag force will increase as the velocity of the boat increases, when it becomes equal to that of the forward movement it only means that the boat cannot accelerate any further and that is the top speed. Also these forces are relatively independent of speed. The forces do increase as the velocity of the wind increases however the forward movement can become greater than the speed of the wind that is causing the boat to move!

As mentioned earlier, to sail into the wind one must keep the sails quite close to the direction of the boat, as the direction of the boat aproaches the perpendicular to the wind the sails should be slightly further out till they are about 20 degrees from the centerline of the boat. Then as you start moving with the wind the sails are left further out to get the best performance.

Moving at an angle to the wind does not do much to get a person where they are going if they need to go straight into the wind, if this is the case then the sailor would need to perform "tacks", turns in the boat in order keep perpendicular to the wind and get where you need to go.

The picture on the right demostrates the use of tacks and the position of the main sail as your direction changes compared to that of the wind.

Apparent Wind vs True Wind
Apparent wind is something that confuses most sailors and is probably the most difficult part of sailing to master. Apparent wind is the wind that a person in the moving boat feels. It is caused by the movement of the boat and the addition of the wind created by nature or the true wind. The true wind is what a sailor needs to know in order to adjust the sails to navigate the boat. The problem is that you cannot simply measure the true wind from the moving boat nor can you tell from the difference in measured wind on the deck and the direction the boat is moving in the water, because the water is usually moving as well. In smaller boats it is fairly easy to "feel" for the best angle and sail adjustments as there are imediate responces in the movement of the boat.

The apparent wind is easy to diagram and explain. The pictures to the right are all basically vecter addition problems where the apparent wind is the sum of the true wind and the wind caused by the boat speed. This is pretty simple from this view but when you are on the boat you can only tell where the apparent wind is and you have to estimate what the exact heading of the boat is.

It is even worse when the boat is moving downwind. In these cases it is very dificult to estimate the heading and true wind since a slight change in the true wind can cause a huge change in the apparent wind. The following pictures can prove this.

Here a 16 degree shift in true wind can make a 28 degree change in apparent wind!

This is why it is more difficult to navigate while moving with the wind.

Ways to find true wind
There are a few ways to actually find the true wind. The most common is to just turn the boat until the sails begin to luff and steer themselves to the center of the boat. then you know the exact true direction of the wind. However this can be aa large waste of time sinse it will probably bring the boat to a near hault. The best way solve the problem is by experience. An experienced sailor should be familiar with the effects of the direction of the wind and boat to make adjustments based on how the sails feel.
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