Lesson 1

How does it sail, anyway?

The wind blows on the sails, and makes the boat move. This is pretty basic stuff, and it’s very clear when you think of the wind blowing from behind (i.e. when you sail "before the wind").

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Early vessels used only this method, using the sails more for steering than for anything else. As the time progressed, people realized that it is possible to arrange the rigging in a way that will enable the boat to sail at different angles to the wind – thus making longer trips possible without having to rely on the direction of the wind. Now it is possible to sail at an angle of up to 38 degrees. In order to understand how it is done, we have to discuss the notion of apparent wind.

When a boat is stationary (when it is anchored, for example) then we only feel the "true wind". It is coming from its true direction. When a motor boat is moving fast, the wind felt while on it is "the wind of its own speed". A sailboat uses a combination of both, and this is called the "apparent wind". The telltales will only show the direction of the apparent wind – this is a very important point. It’s important, because the direction of the apparent wind will change if the strength of the true wind changes.

Have a look at the following vectors (remember that the "movement" is in fact the wind created by the boat's movement):

While the true wind stays the same, the movement remains stable, too. But once the strength of the true wind increases (let’s say because of a gust), the wind of the boat’s own speed remains the same, and the apparent wind will change direction. It will blow more astern (from behind). In this situation the boat will increase its speed, and the apparent wind will change direction again. When the true wind’s strength will go back to its previous state, the apparent wind will change again (will blow more from the front, because once again the wind of the boat’s own speed remains the same). If you don’t realize these mechanics, it’s easy to become annoyed at the wind changing the direction constantly. Once you remember that the telltales show only the apparent wind, and that this wind’s direction may change with the strength of the true wind changing, you’ll then be able to steer your boat in the most advantageous way. In other words, you’ll use your wind and your sails more efficiently.

Why do you have to know this to understand why we now can sail against the wind?

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When the wind blows astern, the boat is driven forward by the force of resistance to the wind. Thus, the bigger the sail area, the bigger the resistance, the faster the boat will move. Think about spinnakers – they are very large sails with a parachute-like shape – we use them (most often!) when sailing "before the wind".

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However, when the wind blows from the front, the sails shouldn’t offer resistance anymore. The should deflect the (apparent) wind, and this is achieved by making the sail flatter. In this way, the force driving the boat forward is the pressure built up on the windward side (the side of the sail on which the wind blows), and the low pressure formed on the other side of the sail (the leeward side). All this results in suction. (Think about airplanes – it really is the same idea!) These two pressures form a force that is called "sailing thrust", or a "lift" (it is often said that you’re "sailing in the range of lift"). In order that this force can have a full effect the sails must be set in the most advantageous angle to the wind (around 10 to 25 degrees to the wind).

At this point two forces act on the boat: the force of propulsion (parallel to the axis of the boat’s length), and the transverse force (vertical to the axis of the boat’s length). The former is advantageous – it propels the boat, the latter is disadvantageous – it causes the boat to make leeway and heeling, thus it pushes the boat on its side. Modern boats, however, can make up for this force greatly, and they often can reach very high speeds without making leeway (the keel/centerboard help), and with very little heeling (especially when the crew is helping out by sitting on the windward side - the reason I'm on the leeward side on this picture, is that I need to see both the jib and the main, which is possible only from this spot, the rest of the crew IS sitting on the windward side).

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Photo by: Zbyszek Turkiewicz

Lesson # 2 "The Beaufort Wind Scale"