The Physics of Sailing

Sailing is a skill and an art which requires much time, practice and understanding to master. One may not understand the physics behind driving a car, however one can understand the basic principals behind using a car: press on the accelerator and the car will go faster; press on the breaks and the car will stop; turn the steering wheel left and the car will turn left. The same mentality applies in sailing, although in order to understand the physics behind sailing one must have a general knowledge of sailing terminology and how a sailboat is made.

Let's Speak "Sailing"!

Let us start by learning the different parts of the boat. The shell of the boat is called the hull; the large, vertical pole is known as the mast. The mast is held in place with two shrouds and a forestay (jib stay). Most boats, with a few exceptions, have one, two or three sails. Every sailboat has a main sail; this is the larger sail which is always directly attached behind the mast - it is controlled by the main sheet.

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The jib sail is the small sail in front of the mast; it can be adjusted with the jib Sheets. The third sail is called a spin sail (not shown on the picture below) and is used by advanced sailors to increase the boat's speed in specific instances.

Sailboats don't have a steering wheel; instead they have a rudder which is controlled by a tiller. When the tiller is pushed to the right, the rudder will move to the left causing the boat to turn left.

Large boats have a keel, while small boats have either a centerboard (cannot be physically removed from the boat) or a daggerboard (can be easily removed from the boat). Regardless of the type of boat, these sharp blades project from the bottom of the boat into the water and stop slide slipping - a phenomenon that will be explained later on. The front of the boat is called the bow while the back, or aft, is the called stern. The right side of the boat is referred to as the starboard side; the left is called the port side.

Points of Sail

After covering the essential terms of sailing, it's time to learn how a sailboat works. As mentioned earlier, a sailboat is powered by the wind. Before getting too technical, let us get an idea of how a boat functions. On a sailboat there is no motor or steering wheel. Therefore, the sails must be adjusted or trimmed according to the boats angle or position to the wind. There are of course many other adjustments that should be made, (centerboard, mast rake, weight...etc) but for now we will only discuss the appropriate sail shape relevant to the boats position to the wind.

The first thing every sailor must always know before heading out onto the water is where the wind is coming from. Without this crucial piece of knowledge, it would be impossible to start sailing. On the picture below, the wind is blowing straight down the lake. This concept may be difficult to grasp, but a clear way to think about it would be to imagine the wind as a gigantic fan blowing in a straight line.

There are five different points of sail. In other words, there are five different positions the sailboat can be in. The first is when the bow of the sailboat is facing directly into the wind. This is known as being stuck in Irons or the No Go Zone. Both sails, the main and jib sail, would simply luff (a term used in sailing to describe sails that are flapping). Since the boat does not get propelled forward in this case, it would most often drift down wind. Therefore, since a boat cannot travel upwind it has to "zig-zag" its way forward (see the figure on the right). This is slow and very challenging for a beginner. When the bow of the boat is at an angle of roughly 45� to the wind, then it is on a beat or a close hauled. Not every boat can point at exactly 45�: the angle tends to be smaller or greater depending of the model of boat. When sailing on a Beat, both sails - the main sail and the Jib Sail - should be pulled all the way in on the Leeward side. This means that the main sheet and jib sheet on the leeward side of the boat should be pulled tight. As you can see in the illusration on the left, leeward is a term used to decribe the side of the boat where the wind hits last, and describes the second point of sail. The windward is the side of the boat is where the wind hits first. Thus, third position of sailing is when the boat is perpendicualar to the wind. This point of sail is called a Beam Reach and both of the sails should be released half way out. The forth and fastest point of sail is when the bow of the boat is 135 � from the wind. This point of sail is called a Broad Reach and sails should be three quaters of the way out. Finally, when the bow of the boat is 180� from the wind, it is on a run or running before the wind. The sails should be let all the way out. In this situation, the sails could end up on the same side or on oposite sides; termed butterflying.

When a boat moves closer to the wind - an example being when it goes from a beam reach to a beat - it is called heading up. Consequentially, bearing off is when the boat moves further away from the wind. Hopefully, you now have a general understanding of how to sail a boat. It might not all be clear, however just like one can't learn how to swim on land, one can't learn to sail without being in a boat!

One Wind, Three Vectors

Wind is a force. In sailing, there are three different kinds of winds: true wind, boat wind (velocity) and apparent wind. As a matter of fact, all three of these winds are vectors. True wind is the wind that would be felt if you were to sit on an anchored sailboat. It is the wind that is blowing over the water - the wind produced from the "giant fan". Boat wind is the wind created when the boat is moving. It's similar, but not as strong as the wind felt if you were to put your hand out of a car window while the car is moving. In adding both of these vectors together, the resulting vector would equal the apparent wind. The apparent wind is what would be felt when you are sailing, and more importantly is the wind that must be considered when you are trimming (a term used in sailing that means adjusting) the sails on your boat.

Force One; the Sails

Many people don't realize that sailboats work much like airplanes. When a boat is sailing downwind on a run, the wind and the boat are travelling in the same direction. The wind is moving faster than the boat and the moment the air hits the sails, it is decelerated. Of course, the sails will push back against the wind, however not at the same force. Therefore the resultant vector of force will be forward. This explains the way a boat moves downwind, but what about upwind? This is where the complicated physics principals are introduced.

It was previously mentioned that there were three different types of wind: true wind, boat wind (velocity) and apparent wind. In order for the air to travel around the sails, it must deviate in direction. This will produce a change in wind velocity and consequently harness a lift force. Let us consider the true wind to be the initial velocity, vi, and the apparent wind to be the final velocity, vf. The difference between these two velocities is the boat wind or velocity, ?v. The acceleration of the air is equal to the change of this velocity over the time taken for this change to occur. Now that acceleration is known, we can use Newton's first and second law, F=ma to determine the force that the sail exerts on the air. This vector is equal in magnitude but opposite in direction to the force that the air produces on the sail.

Let us go back to the concept of lift force. The sail of a sailboat works in the same way as a wing on an airplane: they both follow Bernoulli's Principal. This law states that as the speed of a moving fluid or gas increases (or decreases), the pressure within the space of the fluid or gas will decrease (or increase). Thus the windward side of the sail will have a higher pressure then the leeward side of the sail. This will cause the air to travel slower on the windward side than on the leeward side. Also, air and fluid travels from areas of high pressure to areas of lower pressure. Since the air on the windward side has more pressure, it pushes against the sail as it attempts to reach the side of lower pressure. Moreover, because the wind on the leeward side is moving so much faster, it cannot push back with the same force on the windward side. This occurrence causes the sail to receive a force that is perpendicular to the direction of the wind. Without the resistance of a keel, centerboard or daggerboard, the force would not be sufficient to push the sailboat against the wind.

Force Two: The Centerboard

A centerboard counters the effect of the lateral force produced by the air interacting with the sail by producing hydrodynamic lift using the forward momentum of the boat. Wind is a very large force vector. When the wind pushes on the sails of the boat, the centerboard generates a force of its own in the opposite direction. From the side, the centerboard has a large, flat area. When the water hits the centerboard; it has to split and go around the object. This effect is known as pressure drag. Water will resist this action and apply a sideways force, Fk.. This will cancel out the sideways force produced by the sails, Fw.

The keel of a boat works a lot like the sail of a boat; they both create lift. The water on the windward side of the centerboard generates more pressure and moves a lot faster than the slower water on the leeward side. This lift, combined with the lift from the sails, allow the sailboat to move forward.

As the boat slices through the water is creates another kind of force; drag. Since water is a polar molecule, the water that touches the sides of the centerboard grabs more water with it. Consequently, this creates even more drag. This force will increase as the velocity of the boat increases. When the forward force produced by the moving boat becomes equal with the drag force, the boat has reached its maximum speed and cannot accelerate anymore.

Conversely, when a sailboat sails downwind, it doesn't follow these principals exactly the same way. When a boat is sailing downwind, the tendency for the boat to slide sideways on a run is greatly reduced since the wind and the boat are both traveling in the same direction. Therefore in an ideal situation, there is no keel resistance. The flow of the water is parallel to the centerboard and this causes an insignificant pressure drag. The area on the front edge of the centerboard is so small that the water has practically nothing to push against. Racers will often raise their centerboards completely when sailing downwind to eliminate drag force and increase speed.

When navigating upwind, the centerboard should be fully extended to prevent slide slipping or lee way (another sailing word that means moving away from the wind). On a beam reach, the centerboard should be raised half way up; on a broad reach, it should be raised as much as three quarters of the way up to maximize the velocity of the boat and reduce drag.

Combination of Forces

If you were to pop a water melon seed between your fingers, the seed would fly straight ahead. This happens because the combination of two large, opposing forces, propel the seed forward. The same kind of idea happens with a sailboat. The combination of the lateral or keel resistance and the sail force produce a forward force.

The horizontal components of the water and the wind vectors may cause the boat to tilt or heel to leeward. The sideways components of these two forces are of equal size, but in opposite directions and therefore cancel. However, in the process they create a phenomenon called torque in the clockwise direction. This is the tendency for a force to causes a rotation about an axis. Every sailboat is buoyant because it has the ability to float above water. The weight of a boat combined with its buoyancy creates another pair of forces that make torque in the counter clockwise direction. When a boat begins to heel, the weight at the bottom of the keel moves to port and counteracts the tipping motion.

See You On the Water!

There are many other factors that affect the way a boat moves. These factors can include mast rake, sail trim, rudder resistance, sail shape, CE (center of effort) and CLR (center of lateral resistance), weight, hull shape, etc... We do hope that you now understand the basic physics behind sailing a boat. More importantly, we hope we have sparked your curiosity about sailing. We look forward to seeing you on the water!