True & Apparent Wind

If an instrument for measuring the speed and direction of the wind is mounted on shore then the readings obtained are those of the speed and direction of the true wind. If we take this same instrument, and mount it on a boat that is moving through the water, then the readings will be quite different from those taken on shore. These readings show the speed and direction of the apparent wind relative to the boat. I have used the term "relative to" because the apparent wind is applicable only to the particular boat we are looking at and is not the same for other boats in the same area.

Let's look at it in greater detail. If the true wind is blowing at 20 knots from the south and the boat is travelling at 15 knots in a southerly direction, then the wind speed measured on the instrument will be the sum of the two speeds, i.e. 35 knots and the direction of the wind will be from the south. Similarly, if the boat is travelling at 15 knots in a northerly direction, then the wind speed measured will be the difference of the two speeds, i.e. 5 knots coming from the south. Most people will have no difficulty understanding this. The situation gets a bit more complicated if the boat travels at 15 knots in an easterly direction. What wind speed and direction will the instrument show? To arrive at this answer, we will have to resort to using something called a vector. A vector is a line with an arrowhead. It can be used to represent the speed and direction of anything you like, whether it is the wind, a boat or a board. The length of the line represents the speed and the direction of the arrowhead shows the direction in which the wind or boat is moving.

In the diagram above, the vector TW represents the speed and direction of the true wind, BV represents the speed and direction of the boat and AW represents the speed and direction of the apparent wind. TW has a length representing 20 knots coming from the south. BV has a length representing 15 knots going to the east. AW is the result of something called vector addition and represents the apparent wind relative to the boat. Its length represents 25 knots. The angle between AW and BV, shown as ^aw, is 53 degrees. The apparent wind therefore has a speed of 25 knots coming from a direction 53 degrees south of east.

It is the apparent wind that acts on the , not the true wind, and that is why it is so important to understand how it behaves. In the rest of this article we will look at how the apparent wind changes in speed and direction as the speed and direction of the board changes relative to the true wind. Forces Acting on the Sail

Most readers would probably find this section of the article much too technical as it requires some prior basic knowledge of the physics of forces and their resolution into right-angled components. It is however a subject that is unavoidable if anyone wishes to have a more in-depth understanding of how a sail actually drives the board forward.

I do not propose to go into the theory of how lift is created in a sail as there are several ways of looking at it and I do not fully understand them myself. I shall confine the discussion only to two areas - a) the effect of the wind striking the sail at various angles and b) the interaction of the forces of lift and drag.

The diagram above shows the sectional profile of a sail with the mast on the right and the leech on the left. The Chord Line is an imaginary line drawn through the mast and leech of the sail profile. The arrow AW represents the apparent wind. The angle ^aa between AW and the chord line is referred to as the Angle of Attack, a term borrowed from aeronautical engineering. Actually, the way a sail works is very similar to the way an aircraft wing works. The action of the wind on the sail generates lift in the same way as lift is generated when air flows over an aircraft's wing.

If the angle ^aa is zero, very little lift is generated and there is no power in the sail. As the angle ^aa increases, the lift in the sail increases and we feel an increased pull in the sail. The increase in lift is in direct proportion to the increase in the angle ^aa, however if this angle is increased beyond a certain point (about 15 degrees), the sail rapidly loses power and stalls. This is commonly referred to as over- sheeting. Dinghy sailors rely on watching the luff of their or on tell tales to warn them when they are in danger of over-sheeting. Windsurfers are more fortunate. They can feel the loss of pull on the sail and therefore know when the sail has stalled. The sail should be quickly sheeted out to regain power. Experienced windsurfers are constantly trimming their sails to get maximum power from them. This is particularly important in marginal planning conditions and is probably the biggest single reason why some windsurfers can get planning while others cannot.

There are actually two forces acting on the sail. One which we have already talked about is the lift which acts in a perpendicular direction to the Chord Line. The other is the drag on the sail caused by the wind flowing over it. This force acts in the same direction as the apparent wind. Both lift and drag are proportional to the square of the wind speed, so a wind speed of 20 knots will generate four times the lift and drag as a wind speed of 10 knots.

In the above diagram, a-b is a vector which represents the magnitude and direction of the lift generated in the sail. b-c is a vector which represents the magnitude and direction of the drag on the sail. These two forces added up vectorially will result in a combined force represented by vector a-c. There is an angle between a-b and a-c, shown as ^da which I will call the drag angle. The higher the drag, the larger will be this angle.

The force a-c can be resolved into two forces, one in the direction of travel of the board - the forward component represented by a-e, and the other at right angles to it - the lateral component represented by a-d. It is the forward component that is of interest to us because that is what drives the board forward. The other tries to push the board sideways. The angle at which the sail is sheeted in, i.e. the angle between the Chord Line and the board line of travel, is shown as ^sa in the diagram. What is important to note is this that as the sail is sheeted out, the forward component becomes larger and the lateral component becomes smaller. Conversely, as the sail is sheeted in, the forward component becomes smaller and the lateral component becomes larger. When angle ^sa is equal to angle ^da, the forward component is zero and there is no force on the sail to drive the board forward. When the angle ^sa is smaller than angle ^da, the forward component is negative and the board will move backwards.

It follows from this discussion that, for the same forward component of force, a sail with less drag will have a smaller drag angle and can therefore be sheeted closer in as compared with a sail with higher drag. Such a sail can therefore be sailed closer to the apparent wind at a higher speed.

The interaction of lift and drag also explains a commonly known fact, that in strong a small sail will go faster than a large one. What happens is that a sailor using a large sail becomes overpowered and he cannot fully sheet the sail in. The angle of attack is, say, only two thirds of optimum. The drag, on the other hand, remains practically unchanged. The ratio of lift to drag is drastically reduced and the sailor experiences a loss of speed. By switching to a smaller sail which the sailor can fully sheet in, the angle of attack will be at an optimum, thus producing a lift which will be about the same as with the larger sail, but the drag on the smaller sail will be a lot less. There is therefore a greater forward component of driving force on the sail and the board travels faster. Of course, a heavier sailor who is able to fully sheet in the larger sail will be able to take advantage of the increase in lift in the larger sail.

Sailing on a Beam Reach

To understand the effect of apparent wind on a windsurfing board let us look at the case of a board that is sailing on a beam reach as shown in the diagrams below. They show in sequence how the apparent wind changes in speed and direction with different board speeds while the true wind TW remains constant at 20 knots at an angle of 90 degrees to the board.

In the first diagram the board is travelling at a speed of 15 knots. The interaction of board velocity BV and true wind produces an apparent wind, represented in speed and direction by the vector AW. Its speed is 25 knots and it is at an angle ^aw = 53 degrees.

As the board speeds up, the apparent wind increases in speed and moves further forward. This is shown in the next diagram. Here the board has reached a speed of 20 knots. The apparent wind has now increased to 28 knots and is now at an angle ^aw = 45 deg.

In the third diagram we assume that the speed of the board has gone up even more to reach a speed of 25 knots. The apparent wind speed has now reached 32 knots and its angle ^aw has gone down to 39 deg.

What this shows is that as the board speed increases, the apparent wind speed also increases. With a higher apparent wind speed, more power is generated in the sail and this drives the board to a higher speed which in turn creates a higher apparent wind speed. Does this therefore mean that the board speed and apparent wind can go on increasing indefinitely? The answer is no. There are three factors which limit the speed of the board. The faster the board travels, the higher will be the drag of the board in the water and the higher the drag caused by the sailor's own wind resistance. More importantly, as the board speed increases, the angle of the apparent wind ^aw decreases. The sail has to be sheeted further and further in so that it maintains a correct trim in relation to the angle of the apparent wind. This results in a smaller and smaller forward component of driving force on the sail. The speed of the board will reach a limit when the driving force on the sail will just be sufficient to overcome the sailor's wind resistance and the drag of the board in the water. In order to go any faster, we will have to increase the angle ^aw by going into a broad reach. In the next section we will see what happens when we do that.

Sailing on a Broad Reach

If we start bearing away from the beam reach to go onto a broad reach what happens to the apparent wind is shown in the next three diagrams. The true wind is still at 20 knots. In the first diagram, the board has turned away from the wind so that the angle ^tw is now 110 degrees. The apparent wind has dropped from 32 knots to 26 knots but its angle ^aw has increased to 46 deg. In spite of the drop in wind speed the increased angle of the apparent wind allows the sail to be sheeted out slightly, thereby increasing the forward component of driving force on the sail which results in a higher board speed.

We shall assume the board speed has now gone up to 30 knots. The apparent wind speed will also be 30 knots and its angle ^aw has decreased to 39 degrees as shown in the second diagram. Once again, because of the smaller apparent wind angle, the sail has to be sheeted in, reducing the driving force on the sail and the board cannot go any faster.

Figure 8 shows what happens if we keep bearing away until the true wind angle ^tw reaches 140 degrees. The apparent wind speed will drop to 19 knots and its angle ^aw will increase to 41 degrees. This much-reduced apparent wind speed leads to a loss in sail power and the board will start to slow down. This causes a further reduction in the apparent wind speed and a further increase in the angle ^aw. At a board speed of 20 knots, the apparent wind speed will be 14 knots and the angle ^aw will increase to 70 degrees. If we continue to bear away from the wind, the apparent wind speed will drop very rapidly and the board will eventually stop .

At this point in time, I should point out to readers that the board speeds I have assumed are only for the purpose of illustrating what happens when the board is sailed at various angles to the true wind. In practice, such board speeds may or may not be actually attained.

To summarise what I have said earlier we see that, as we bear away from a beam reach, the speed of the apparent wind will start to decrease, however its angle ^aw will increase thus allowing the sail to deliver more power to drive the board forward which in turn increases the speed of the board. As we continue bearing away downwind, there comes a point when there is no further advantage to be gained by a larger apparent wind angle and the lower apparent wind speed causes the board to slow down. When sailing on a very broad reach, the board will slow down so much that it cannot continue planing. To keep it on the plane the board will have to be turned back upwind. Exactly the same thing happens if the speed of the true wind decreases.

In practical situations the wind is seldom constant. There will be gusts and lulls. To keep a board on a plane and at the same time to sail as broad a course as possible, the sailor will have to luff up on the lulls and bear down on the gusts. Competitive sailors will pump vigorously going downwind even in strong winds, not so much to gain speed, but to stay on the plane while sailing as broad a course as possible. This is where a big sail will have a clear advantage over a smaller one. A sailor carrying a larger sail will be able to keep planing on a broader course.

Apart from changes in the true wind speed, there could be other reasons for changes in the speed of the board. The most common cause is the need to negotiate the waves. When sailing downwind, the waves are usually travelling in the same direction as the board. The board will slow down when it is climbing the back of a wave and speed up when it is surfing down the face of a wave. The angle ^aw of the apparent wind will therefore be alternately increasing and decreasing. The sail will therefore have to be correspondingly sheeted out and sheeted in to maintain the optimum angle of trim.

Sailing Upwind

It is well known that in marginal planning conditions it is possible to plane on a beam reach but it will not be possible to plane going upwind. This fact is particularly relevant to sailors who are keen participants in Formula Windsurfing races where the minimum wind speed at the starting line before a race can be started is 7 knots. Most modern boards equipped with large sails can plane in about 6 to 7 knots of true wind. Let us look at what happens to the apparent wind under these conditions.

We will start by looking at a board on a beam reach planing at a speed of 11 knots in a true wind speed of 7 knots. The apparent wind will be 13 knots at an angle ^aw of 32 degrees. If the board is now turned upwind so that it is now sailing at an angle of 60 degrees to the true wind the apparent wind will immediately go up to 16 knots but the angle ^aw will be reduced to 23 degrees. In spite of the increase in the apparent wind speed, the smaller angle will not provide sufficient forward driving force on the sail to keep the board planing. It therefore stops planing and its speed will drop back to about 6 - 7 knots. To keep planing, the board will have to be turned downwind again towards the beam reach position. As the true wind speed increases, it will be possible to plane at angles progressively closer and closer to the true wind. The higher the true wind speed, the higher the board will point.

The implication of this in racing is that in marginal planing conditions (6 - 9 knots) a board that is planing upwind will have a velocity made good (VMG) to the windward mark that is less than the VMG of a similar board that is slogging upwind at a speed of 6 - 7 knots. In such conditions the sailor has to make a decision whether to continue planing or to start slogging. The situation becomes more deceptive if the wind at the start of the upwind leg is 10 knots or more but gradually drops back to 8 knots or less. The sailor may then not be aware that although he is still planing in the lower wind speed, he is not pointing as high and is losing ground in the windward direction.

A Word About Pumping The action of pumping the sail introduces an additional element into the wind vector diagram. Its effect is to move the apparent wind backwards and increase the angle of attack. This is beneficial when sailing upwind because it enables you to point higher. If you are trying to fetch a windward mark that seems to be just out of reach, you will notice that pumping the sail all the way to the mark will enable you to round it quite easily.

When going downwind on a very broad reach, the sailor has to ensure that pumping is done carefully and correctly if it is not to be counterproductive. The apparent wind is already aft of the beam position and the pumping action will move it even further back. It is therefore very easy to exceed the optimum angle of attack and get into the oversheeted position. This is particularly noticeable when sailing downwind on a longboard in very light winds. In such conditions the sailor should try to avoid a fanning action which results in sheeting the sail in and instead pump the sail by pulling it without changing the angle of the plane of the sail while at the same time making sure that there is always maximum resistance on the sail. If the resistance decreases during the pumping stroke, it will indicate that the sail has become oversheeted. It helps if, prior to the pumping stroke, the sail is sheeted out beyond the zero angle of attack position and as it is pulled in it will increase the angle of attack and go through a little beyond the maximum. This will help to ensure that the pumping stroke is optimised.