Sailing Manual

by Paul Kylander

Part II : Basic sail theory and concepts

Topics covered in Part 2:

Physics of Sailing
Introduction to Steering and Sail Adjustment
Basic Sailing Terminology

In part II the manual will introduce some of the basic physics involved with sailing, steering and controlling the tiller and sheets and finally some basic sailing terminology. Please keep in mind that knowledge of the physics of sailing is not needed to sail properly.

Physics of Sailing

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 centreboard (which projects downward into the water) to propel it forward. Figure 9 shows the forces that are acting on the centreboard. The flow of water around the centreboard 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 10. 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 11, 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.

As mentioned earlier, the knowledge of these basic physical principles are not needed to be a proficient sailor. They do, however, help sailors understand how the water and the wind effect a sailboat.

Introduction to Steering and Sail Adjustment

There are two important parts of the sail boat that sailors must be aware of before starting to sail. First, the sheet. This is not the sail, but the rope that controls the position of the sail. By pulling in the sheet, the sail moves towards the center of the boat, and by letting out the sheet the sail moves away from the center of the boat. Figure 13 and figure 14 outline the movements of both the sheet and the sail.

The arrows indicate the movement of the sheet and the resulting movement of the sail. The second part of the boat to be aware of is the tiller extension. This is the handle that sailors use to steer the boat. This part of a sailboat exists so that the helmsman (the person who steers the boat), can hike out over the side of the boat to keep it flat while sailing. Steering a sailboat is done by either pushing or pulling the tiller. In Part III of this manual, there will be more detailed information on positioning in a sailboat, and on steering a sailboat.

Basic Sailing Terminology

Unlike the physics of sailing the terminology of sailing is something which sailors must be familiar with. Most sailing terminology refers to the position of boats in relation to the wind. Please use the glossary (Appendix II) located near the end of this manual to help clarify any terms that may be unclear within this section.

Figure 16 identifies four sailing terms that refer to locations on the boat. To identify the left side of the boat sailors use the term port. Conversely, to refer to the right side of the boat sailors use the term starboard. The front of the boat is called the bow, and the rear is called the stern. Figure 16 also indicates the terms leeward and windward. These two terms refer to the sides of the boat in relation to the wind. The side towards the wind is called the windward side, and the side which is away from the wind is called the leeward side.

When sailors use the term tack, they are referring to the side of the boat on which the sail is positioned. Figure 16a, shows the two different tacks; port tack and starboard tack.

A boat is on a port tack when the wind flows over the port side of the boat first, and then hits the sail. A boat is on a starboard tack when the opposite occurs.

The terms used to describe a boat's movement in relation to the wind are outlined in figure 17. It is a good idea to be familiar with all of these terms, as there is no standard term used.

At this point, most of the basic sailing terms have been covered. There are terms which have not been discussed yet. If there are, they may be found in the glossary located at the end of this manual.

Please see Appendix I for Part II test, drills and required skills.

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