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THEORY
 
Understanding apparent wind

exerpt from Bluefinz article
by Denis Wee

Apparent Wind is a term that most sailors have heard about but very few really understand.

The behavior of apparent wind under different conditions has a profound effect on the performance of all sailing craft. This is especially true in the case of windsurfing where the speed of the board very often exceeds that of the true wind. A sound understanding of the principle of apparent wind will help in improving a sailor's technique on all points of sailing, whether it is in sailing upwind, reaching or broad reaching. An awareness of the effects of apparent wind is also important when performing actions like pumping and gybing.

In this article I have adopted a rather theoretical approach to the treatment of the subject. The use of vector diagrams is necessary in order to show how the speed and direction of the apparent wind are affected by changes to the speed and direction of the board. I hope that the reader will be able to understand enough of this article to help him improve his overall windsurfing technique.

This is the beginning of 7 brief articles, explaining apparent wind and the forces acting on board and rig. Articles begin on:
http://www.bluefinz.com/jm/index.php?option=com_content&task=view&id=16&Itemid=10001

Centreboard Secrets

by Duke Duyck
TWC Instructor Emeritus

We hardly ever notice the Centreboard (CB). It sits somewhere underneath and is easily forgotten. What is it? What does it do? What is it for? Do we really need it?

The CB is a "long narrow plank" that sticks out under the (windsurfing) board, somewhere in the middle and behind the mast. That is if you push the handle backward - so it can descend. If you do not, it remains hidden inside the board.

Beginners will find that it helps them get more stability, especially on a narrow board.

The main reason for its existence is to prevent the board from side slipping as the wind pressures the sail. This is called Lateral Resistance, which is provided by both the Centreboard and the Skeg (= Fin). Without lateral resistance it would be impossible to sail up-wind. To test this you can try to sail up-wind end then raise your CB and try it again. Some boards will not move up one inch. There are others, however, which are hardly affected unless the wind is very light. Why is that?

Long boards with sharp rails will go up-wind without a Centreboard, especially if you push down the down-wind rail slightly. That down-wind rail will provide enough lateral resistance to keep the board on-track. This may not work too well when the wind is light.

Why do we not have a Centreboard (CB) on a shortboard?
The main reason is the wind strength. Shortboards are designed for high winds. In high winds most of the board is out of the water and only the last few feet are in. That means that the Skeg will provide all the necessary lateral resistance.

Another good reason is that as the wind gets stronger, we need to lift the Centerboard up, because it will cause driving resistance, which would slow down the board but also, more importantly, it can cause the board to flip. i.e. the CB gets pushed up by the driving force of the water, which makes it act like a wing. When we use a short board we don’t want any of that!!!!

We do not need the CB either when we sail on a run, except maybe for stability. While on a broad reach, we may have it halfway up.

This brings us to gybing.

What do you do with your CB when you gybe?

In light winds, you will probably have the CB down all the time. So when you gybe, you can keep it down. But gybing a long board with a long CB is not easy. You need to step back on your board as much as you can, to sink the tail, while leaning your rig backward. Then while you are leaning your mast over to windward to initiate the turn, you can help the board turn by pushing down the windward rail. What does that do to the CB? It puts it on an angle and allows the board to pivot around it, while the CB is on the leeward side.

In heavier winds, you may prefer to have your CB up. Do this also, if you have trouble turning your long board. You still need to go to the back end of the board and sink the tail. But this time, you don't need to sink the windward rail. You can actually sink the leeward rail instead, as you need to do on a short board. That will give your body a bit more stability as it rides through the turn.

Finally, don't forget to pull up your CB occasionally, to free the weeds it may catch.

Source URL: http://torontowindsurf.com/learnto/pdf/Centreboard%20Secrets.pdf