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Everything you wanted to know about Dinghies and Sailing

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Basic principles of sailing

The evolution of sailing

Basic sailing physics

Why doesn't the boat go sideways?

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The basic principles of sailing

This page introduces the basic physics of sailing and will give you a simple grounding on how the sails work.

The evolution of sailing

Sailing and sail design have evolved over hundreds, even thousands, of years. Trial and error seemed to have led the way up to until fairly recent times, when sail design began to be more fully understood. The Bermudan Rig (common triangular jib and main) is a natural evolution from the Gaff Rig (the sail is raised to a gaff. Once people realised that the boat went faster with the more sail area they put on the gaff, the next logical step was to do away with the gaff and fill the space between the boom and the mast with sheet cloth.

The shape of the sail was another process of trial and error. The bird's wing was studies by many early pioneers of flight, and indeed the principles of flight form the same principles as those of sailing. The most efficient shape of sail to present to the wind is the natural wing - the flow of air across the front and back sail is least disrupted when presented to the wind.

The Basic Sailing Physics

The sail is presented to the wind and catches some of that wind. So the wind that is caught has to go somewhere. The wind travels along the sail and flows out the back of the sail (called the leech) but because the wind has been caught in the sail it actually creates a high pressure. On the other side of the sail there is no wind, and so this creates a low pressure.

Therefore, the side of the sail capturing the wind is a high pressure area and the side of the sail that is sheltered from the wind is a low pressure area. The wind in the high pressure area pushes the boat sideways, whilst the low pressure area has the effect of sucking the boat sideways.

These forces push the boat sideways and an element of forward thrust is added by the wind flowing across the sail and out of the leech. Immediately we can begin to see the phenomenon of sailing beginning to work. The sail is being presented to the wind at 90 degrees and the boat is not being pushed in the same direction as the wind, instead the forces on the sail are causing a sideways movement and a degree of forward propulsion.

Why doesn't the boat go sideways?

What stops the boat from just going sideways? The answer is also down to some simple physics. The water is actually pushing against the side of the boat (the hull) and resisting the sideways movement of the boat.

To help this resistance, the centreboard can be lowered into the water to increase the resistance of sideways movement. So the boat moves forward, partly because there is some forward movement from the action of the wind in the sail, and partly because the sideways forces are being resisted and some of that energy is transferred to the forward movement.

You will never entirely remove the sideways movement, but when sailing close to the wind your centreboard will help to prevent you from making leeway.

The Workings of the Jib

Let's not forget the jib. What effect does the jib have? The jib is in fact at its most effecient when you are sailing close to the wind. The jib needs to be tensioned to compliment the mainsail because the effect of the windflow over the jib has a direct effect on the efficiency of the airflow over the mainsail and vice versa.

We won't go into the technicalities too deeply, just remember not to tug the jib has hard as you can. Think about the two sails as one engine that must work in harmony. The jib delivers airflow from its leech whicjh makes airflow over the lee side more efficient. The turbulance at the front of the mainsail is also exploited by the jib and this helps the efficiency of the sails.

In very simple terms, imagine the leech of the jib spraying water over the lee side of the main. You don't want the water to splash off the main but to run smoothly over the lee side, this should help you to get the relationship between the jib and the mainsail right when you are sailing.