Jamie Forslund & Dan Vanderbilt

The laws of physics can be found in many aspects of sailing. The sails propel the vessel in much the same manner as an airplane wing creates lift. The shape of the sail favors a difference in air pressure, so in one way the energy that propels the sail boat comes from "bending" the wind. Sailors also use laws of physics to their advantage when they manipulate the rigging of the ship. For example, block and tackle are used as pulleys to allow raising and adjustment of heavy sails. In this way the required force can be reduced, but the rope will have to be pulled a longer distance.

The power of a sailboat comes from the way in which the sail catches the wind. A sail is in fact a vertical wing. It operates in the same way as a wing on a plane. A sailboat uses this wing (the sail), and the centerboard (which projects downward into the water) to propel it forward. Figure 9 shows the forces that are acting on the centerboard. The flow of water around the centerboard creates a pressure difference.

This pressure difference is the same as the pressure difference around the sail-high pressure on the side towards the wind, and low pressure on the side away from the wind. In nature, high pressure moves towards low pressure; as a result, there is a perpendicular force exerted on the sail. This is shown in figure IO. The forces on the centerboard act in combination with those on the sail to power the sailboat forward.

The combination of the centerboard and sail do result in a forward motion, but there is some slipping. The term for this slipping is "drift". As shown in figure 10, the boat will not travel in the exact direction in which it is pointing. Drift will also be influenced by other elements such as waves and current. The stronger the waves and the stronger the current, the more drift the boat will experience. This is an important concept to be aware of, especially if there are hazards nearby.

The last aspect of the physics of sailing that the manual will discuss is the concept of true and apparent wind. These terms refer to the wind and its changes due to the wind direction and the boat speed. True wind is defined as the direction of the wind to a stationary observer. Induced wind is the wind experienced due to the movement of the boat. A good analogy is riding a bike. When riding a bike on a day with no wind, the rider still feels a wind. This wind is induced by riding the bike through the air. A boat experiences wind in the same manner. The combination of these two winds is the actual wind experienced by the boat. This wind is called apparent wind. Figure 12 outlines the three winds and how they relate to one another.

A pulley is basically a type of lever that is used to change the direction of force or multiply forces when used correctly. They are made with a rope in our case, wrapped around a grooved wheel. The wheel works as the source of the change of direction or the increase in force.

With only one pulley, force required to move the mass is unchanged by the pulley. All the pulley does is change the direction of the force, you are pulling down rather than lifting (better for your back, easier with sails).

Because the mass is being supported by 2 ropes, the force you have to exert is halved, but the amount of rope you have to pull is doubled. Each newton of input will support 2 newtons (newtons are a measure of force, the force it takes to move one kilogram of mass one meter per second per second) of load, in other words, if you pull just as hard with a single pulley, you'll only get half as far.

The more pulleys you add to the mix, the less force you have to exert to move the object, but that depends on how you use them. If the pulley does not support the weight of the object, than it will merely change the direction from which you are pulling the rope from.

Here is an example of a four pulley system in which there are two pulleys on the same shaft connected to the weight. Notice that the labels on the ropes represent an equally distributed weight. An important thing to note is that though the person pulling the rope and lifting the object is exerting less force, the same amount of force as simply lifting the weight is still being exerted. In the case below, the leftover force is provided by the ceiling. Also, since every pulley supports some weight of the object, the force is multiplied by 4, thus the amount of rope you are going to pull is multiplied by four as well. But the force you have to put out is divided by four. Do you get the pattern?