The Baltic Sea’s Unusual Waves

The waves in the Baltic are unusual because the Baltic is a shallow sea, as shown in the following diagram. 

First of all it’s important to realise that waves are not just something which take place on the surface of the sea, but are also eddies that extend perhaps up to four or five times the height of the wave downward under the surface.

The above chart shows the wave height in November. Waves over 3 feet can be seen in the middle and at the end of the month. Each day the waves are a different height. Some of the waves are very tall, because the Baltic is a shallow sea.

Let’s have a look to see what happens to waves in shallow water.

 

If we look at the diagram which shows how wave motion is influenced by water depth, we can see in figure 1 how circular motion extends many meters under the surface, This circular effect decreases with depth, but even many wave heights under the surface is still quite prominent.

Figure 2 shows what happens to a wave approaching shallow water. When the circular motion makes contact with the seabed, the circles at the bottom get compressed, putting pressure on the circular motion above. This pushes up the waves making them taller.

 

Travelling Faster than the Wind

How is it possible that a Sailboard can travel much faster than the Wind? 

When windsurfing, have you ever felt that you are travelling much faster than the wind? Is it possible? Yes, it is! Let’s see how.

Firstly, it’s very important to have an understanding of what the fins fitted under your surfboard do? Without the fins, the surfboard would drift uncontrollably. Look at the diagram above. Imagine the wind direction coming from B. Without fins the surfboard would drift in the direction CE , but with the fins, the board will have to travel parallel to the fins, i.e. in the direction CF. This explains why the board doesn’t travel in the direction of the wind. But this doesn’t explain why the surfboard travels faster than the wind.

If the wind where coming from A at 10 knots the surf board would be pushed from C to D ( a distance of 10 n.m. in 1 hour ). However, the wind is coming from the direction of B but the surfboard is unable to travel the 10 n.m. a long CE but can still travel this vector distance by travelling CF, approximately 30 n.m. So we can see that with a 10 knot wind, it is theoretically possible for the surf board to travel at 30 knots.This is a mathematical way of looking at it.

Before we can look at this in a logical way, we need to understand the term ‘relative wind speed’.

‘Relative wind speed’ is the apparent speed and direction of the wind you would feel if you had your eyes closed, and you were on a moving platform.

Imagine that the wind is coming from behind us and we are on a stationary sailboard in the middle of the sea, then the relative wind speed which we would feel on the back of our head would be 10 knots.

So now let’s imagine the sailboard increases speed to 3 knots in the direction of the wind. What will the relative wind speed be now? Yes! You’ve guest it. 7 knots.

Let’s say the sailboard increases speed to 6 knots. Again, what will the relative wind speed be? 4 knots.

Now, this gets very interesting, what happens to the relative wind speed when the sailboard reaches 10 knots? Of course, the relative wind speed becomes zero. This means that there will be no more wind pushing into the sail.

Now let’s change the wind direction. Imagine that the wind is not coming from behind, but is coming from the left, and we are stationary in the middle of the sea, then the relative wind speed which we would feel on the left half of our head would be 10 knots.

 So now let’s imagine the sailboard increases speed to 5 knots with the wind on our left. What will the relative wind speed be now? 10 knots. Any increase in the speed of the sailboard perpendicular to the wind will not change the relative wind speed.

Let’s say the sailboard increases speed to 10 knots. What will the relative wind speed be? 10 knots :)

To conclude, no matter what the speed sailboard is provided that it is travelling perpendicular to the wind, the wind energy pushing the sailboard will remain constant.