Technical Corner: Pressure Shifts

I am starting off the technical corner section of the blog by looking at Pressure Shifts. How changes in the wind strength or pressure effects the angle of the wind we are sailing in. Hopefully we will be able to answer the following questions:

  • What forces act on my boat?
  • What is a pressure shift?
  • How can I benefit from them?

Have you ever been sailing upwind, sailed into a lull and thought you have been headed away from the wind, tacked over but still been on a header on the opposite tack?

For many lake sailors and keen racers, the answer is almost certainly a yes. But what’s going on here? Is it bad luck, or a coincidence or is there something more to it?

What forces act of my boat?

To understand what is really going on here we must understand a few basic principles about the Aerodynamic forces acting on a boat sailing upwind. The wind that you feel when sat stationary in your boat is called the “True Wind”, this is the naturally occurring flow of air that we often refer to as the wind.

However, when a boat is sailing the movement forward creates a flow of wind directly opposite to the direction of travel, this is called the “Induced Wind”. The wind that you feel whilst driving down the motorway at mph isn’t a sudden hurricane, it is the induced wind due to your rapid movement against a mass of air.

True Wind and Induced Wind

Now these two forces are not acting on your boat in the same direction, one is coming directly in line with your sailing angle and one is coming from “Up wind” … Wherever that is!?

So, these two forces now combine to create a new force from a single direction, this result is called the “Apparent wind”. This resultant force is the wind that you feel when your sailing upwind and that flows over your sail to give you movement. This is best displayed in a vector diagram as seen below and to the right, as this allows you to clearly see how these forces change each other and the angle between them.

Apparent wind and Aerodynamic vector diagram

What is a pressure shift?

As I have mentioned, the apparent wind is a result in the true wind and the induced wind. Therefore, if there is a change in direction and strength of the true wind then the apparent wind will also change. So, let’s look at a few examples of how a change in wind strength may alter the apparent wind direction.

To do this we are going to have to make some assumptions and estimates on boats speeds and sailing angles, as these change depending on what boat you sail but the physics is still the same. These calculations use a derivative from the law of Cosines, for all you math geeks out there, but can also be done using online calculators!

We are sailing upwind in 10 knots of wind, sailing at an impressive 5 knots boat speed and at 30 degrees off the true wind angle. This means that the resultant apparent wind speed is 14.5 knots and its angle is 10 degrees right of the true wind. This is our base condition which we are going to use to see what impact changing the wind strength has.

Base Condition Vectors

Firstly, we are going to sail into a 15 knot gust, a wind increase of 5 knots, with no change in true wind angle, sailing angle or boat speed. This would over time create an increase in boat speed, but for these examples we are going to focus on the few seconds that the boat hasn’t caught up with the change in pressure yet.

This now means that the apparent wind speed is now 19.5 knots and its angle is 7 degrees left of the true wind.  Therefore, not only do we have an increase in pressure, but we have a 3 degree lift, even though there has been no change to the true wind direction at all! The boat will then speed up and apparent wind angle will move back to its original position.

15 Knot Gust Vector diagram

Now let’s look at what happens when we sail into a 5 knot lull, a wind decrease of 5 knots. Again, with no change in true wind angle, sailing angle or boat speed. Here we would see an apparent wind speed decrease to 9.7 knots and larger angle change to 15 degrees right of the true wind. So not only have we got a decrease in pressure, but there has been a 5 degree header but all with no actual change in the true wind direction. The boat will then slow down, and apparent wind angle will move back to its original position.

5 Knot Lull Vector Diagram

Therefore, we can see that a gust feels like a lift and a lull feels like a header, even thought there has been no change in true wind direction. But its key to remember that these “pressure shifts” that we feel only lasts a few seconds, until the boat has either sped up or slowed down.

How can I benefit from them?

So how does this affect us when we are sailing in gusty conditions, and how can we benefit from them compared to other sailors? If we can identify that we are sailing in conditions prone to give these pressure shifts, we can respond to them in a beneficial way.

If we sail into a lull at the same time as feeling headed then don’t tack straight away, hold your angle and see if your gradual decrease in boat speed will bring the breeze back to its original position. This will save a costly tack for a non-existing shift.

Secondly, if we are sailing into a gust, well-practised and effective use of main-sheet trim can also help you out. Most sailors will feel a pressure created lift and want to roll up into the breeze, gain some height and drop back down. However, this a misleading gain and often ends in the boat feeling stalled and slow.

The correct action is to ease the sail slightly, maintain a flat boat and then squeeze the sail back in. This ensures that we keep the “flow” attached to the sail, that there is no detrimental leeward heel, or un-required steering …resulting in a rapid increase in boat speed. This can also often bring tactical advantage, bringing your bow forward and popping you out of the group of boats around you.

So next time your sailing in gusty conditions, think twice as to if the shifts are due to an actual change in wind direction or if they are just pressure driven shifts!

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