Technical Corner: Sailing Angles Downwind [Part 1]

When watching any top level conventional single-handed sailor sail downwind, there is one thing that you are bound to see: the angles. The sailors “transition” from broad reach to sailing by the lee and don’t often spend much time sailing dead down wind.

But why is sailing extra distance worth it? And how do they know when to transition? And why doesn’t this constant change in direction decrease their boat speed?

Hopefully, the following two-part article will shed some light on this challenging downwind technique. In the first part we are going to focus on why we sail at these angles, and the second section to the post will focus on when and how we change angle.

Why is sailing angles downwind more effective?

There are three main reasons why sailing using angles downwind is more effective than sailing straight downwind. These are:

  1. To maintain laminar flow going over the sail
  2. Using apparent wind to help give us clean air
  3. To link up small gain features

Maintaining laminar flow

A sail can produce a force in two ways, these are often categorised into “Laminar flow” and “Turbulent flow”. To understand why we want to maintain laminar flow we must understand the differences between the two.

A sail has turbulent flow when sailing directly down wind. In this situation your sail is at 90˚to the wind which means that the wind hits the windward side of the sail and pushes you downwind.

To do this the air flow bounces off the sail in all directions, causing eddies which we call turbulence. In addition to this, a small amount of air travels around the edges of your sail onto the leeward side, where they collide with each other and become turbulent.

Turbulent Flow acting on a sail

This turbulence and the induced wind (The wind caused by the boats movement), creates an air bubble on the leeward side of the sail. The boat then must push this air bubble downwind, decreasing the overall boats speed.

Alternatively, a sail has laminar flow when its overall angle allows for the air to split evenly around the edge of the sail and continue along the length of the material. This is how a sail predominantly works, creating lift due to a pressure gradient caused by this flow of air.

Laminar Flow acting on a sail

This laminar flow can be in both directions across the sail. When sailing on a broad reach this is from the luff to the leach, but when sailing by the lee (sailing past downwind, but not gybing) this is from the leach to the luff. The second of these two situations is sometimes called “Reverse flow”.

But most importantly, when our sail is working with laminar flow there is no turbulent air and the induced wind is not perpendicular to the sail. This means there is no air bubble on the leeward side and no decrease in boat speed associated with it.

We can now see that by maintaining laminar flow across our sail, by sailing on a broad reach, or creating reverse flow from sailing by the lee, increases our overall boat speed compared with sailing directly downwind.

Using Apparent wind to keep clean air

The second reason why sailing these large angles downwind is effective is the effect it has on your apparent wind. The apparent wind is the result of the true wind direction and induced wind caused by the boats movement forward.

For a more detailed explanation of this relationship, have a look at the “What forces act on my boat” section of my previous post: Pressure Shifts

When sailing directly downwind, the boats induced wind is in an opposite direction to the true wind. This means that the apparent wind which is acting on your boat is still coming from the same direction as the true wind. In a racing situation, if you are not at the back of the fleet, the chances are that there will be someone sat directly to windward of you. The wind shadow created by their rig will be decreasing your overall boat speed, we call this “Dirty Air”.

Apparent wind angle and dirty air when sailing directly downwind

However, when sailing at an angle down wind, the induced wind and the true wind are not in opposing directions. This change in direction means that your apparent wind is now coming from a new angle. There are two factors effecting this new angle compared to the true wind, this is which gybe you’re on, and whether your sailing by the lee or on a broad reach.

Apparent wind angle and clean air when sailing angles downwind

This ability to alter the direction of your apparent wind has massive advantages. It means that even if there are several boats sat directly to windward of you, you can always stay out of their wind shadow and keep “Clean Air”. This is another reason why sailing at an angle downwind is very effective.

Linking up small gain features

The final reason that sailing angles downwind is effective is due to the small gain features that are around you. A small gain feature is a term to describe a little feature that will benefit you in some way, for example: a gust or a large wave.

When sailing directly down wind, you’re only sailing in an extremely narrow section of water and have no control over whether one of these gain features is going to come your way. This isn’t a proactive or effective way to sail down wind.

Narrow channel of water used when sailing directly downwind

However, by sailing angles down wind we drastically widen the section of water in which we are sailing in. Increasing the amount of gain features that may come our way, and actively sailing the boat towards them.

Wider Channel of water used when sailing angles downwind

This allows us to link up lots of little increases in pressure or surf from one large wave set across to another. This proactive way of sailing downwind is far more effective and will drastically increase your overall position compared to the fleet.

Hopefully, this post has helped you to better understand the advantages gained from sailing angles rather than just sailing on a run directly downwind. In the next section of this article we are going to look at when and how we transition from a broad reach to sailing by the lee.

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