Clear Air — The Most Valuable Thing on the Start Line
PMR Sailing | Learning to Race — From First Start to Better Results
Sometimes being two metres behind is faster than being one metre ahead.
Picture the scene. You are approaching the starting line with ten seconds remaining. You have judged your distance reasonably well, your sails are set, and you are moving towards the line with what appears to be a very good chance of starting ahead of several other boats.
The starting signal sounds.
You cross the line almost perfectly, perhaps even a metre ahead of your nearest competitor.
Excellent! Or is it?
Within thirty seconds, something strange happens.
The boat immediately to windward of you begins moving ahead. Your sails are full, but the boat no longer seems to accelerate properly. You pull the sails in slightly, hoping to point higher, but that only makes matters worse.
Your competitor, who crossed the line two metres behind you, is now sailing faster, pointing higher and moving steadily ahead.
What went wrong?
You had a good start in terms of timing and position, but a poor start in terms of airflow.
You were sailing in disturbed air, often called dirty air, while your competitor had managed to find clear wind.
It is one of the most important lessons in sailboat racing, and one that can make an enormous difference on the relatively narrow waters of the River Thames.
1. A Sailing Boat Needs More Than Just Wind
When we learn to sail, we quickly discover that the sails must be adjusted according to the direction of the wind.
Sailing close-hauled, we trim the sails in. Sailing on a reach, we ease them out. On a run, the sails are eased much further.
As sailors progress, they begin to appreciate that it is not simply the strength of the wind that matters.
The quality of the airflow is equally important.
A sailing boat performs best when its sails are receiving an appropriate, reasonably steady airflow. That allows the sail to develop the aerodynamic forces needed to drive the boat forwards.
When another boat interferes with that airflow, three things can happen:
The wind reaching your sails may be weaker.
The direction of the wind reaching your sails may change.
The airflow may become more turbulent and less consistent.
Any combination can reduce your boat speed.
You may still feel wind on your face. Your sails may still appear reasonably full. Yet your boat is no longer performing as well as it should.
This is particularly noticeable when sailing upwind, where even a small loss of boat speed or pointing ability can have a significant effect on your progress towards the windward mark.
2. What Exactly Is Dirty Air?
Imagine two boats sailing close-hauled towards the windward mark.
The first boat is sailing through relatively undisturbed wind. Its sails alter the airflow around them, producing the aerodynamic forces that move the boat.
However, the air leaving those sails is not exactly the same as the air approaching them.
The airflow has changed direction, and the boat has created a disturbed region around and downwind of its rig.
The second boat may find itself sailing through some of this disturbed air.
Its sails are now receiving air that has already been affected by the first boat.
The result is what sailors call dirty air.
A wind shadow is not quite the same as a shadow on the ground
When a tree casts a shadow, we can see where sunlight has been blocked.
Wind is different.
A sail does not simply stop the wind. It changes the speed and direction of the surrounding airflow.
Therefore, the wind shadow of a sailing boat is not a perfectly defined area directly behind the sail.
Its location and strength depend on:
Wind direction and strength.
The angle at which the boats are sailing.
The size and shape of the sails.
The distance between the boats.
The relative positions of the boats.
The surrounding wind conditions.
The important lesson is that the disturbed air may affect another boat even when that boat is not directly behind the first one.
On an upwind leg, a boat can suffer from disturbed airflow while sailing slightly to windward and astern of another boat.
This is one reason why the starting line can become so tactically complicated.
3. Why Dirty Air Can Ruin an Apparently Perfect Start
Let us consider two boats, A and B, sailing upwind on the same tack.
Boat A crosses the starting line slightly ahead.
Boat B starts a couple of metres behind but has a clear lane, allowing its sails to work efficiently.
Initially, A appears to have the advantage.
However, suppose Boat A is close to another boat whose sails are disturbing the wind reaching A.
Boat A loses a little speed. The helm tries to maintain the same pointing angle, but the sails are no longer producing the necessary driving force.
The crew may then have to bear away slightly to rebuild speed.
Boat B, meanwhile, continues sailing in clear air.
It accelerates, maintains a better speed and may be able to sail a more effective course towards the windward mark.
Within a surprisingly short distance, Boat B can overtake Boat A.
That is the meaning behind our opening statement:
Sometimes being two metres behind is faster than being one metre ahead.
Of course, a boat two metres behind is not automatically in clear air. Everything depends on its position relative to the wind and the surrounding boats.
The important question is not simply who crossed the line first.
It is who can sail fastest, on the most useful course, during the first minute after the start.
4. Why the First Minute Is So Important
The moments immediately after the starting signal are often among the most important in an upwind race.
Boats are close together. Everyone is accelerating. Most competitors are attempting to establish a position from which they can sail towards the windward mark.
At this point, three advantages become particularly valuable:
Boat speed: You want to accelerate quickly and then maintain your normal racing speed.
Pointing ability: You want to sail effectively towards the windward mark without being forced to sail lower than necessary.
Freedom of movement: You want enough space to respond to wind shifts and make tactical decisions.
Dirty air can damage all three.
A boat that loses speed may find itself slipping back into even more disturbed air from the boats ahead.
The sailor then faces a difficult choice.
Remain on the current course and hope conditions improve, or tack away and try to find clear wind?
The longer the decision is delayed, the more difficult the situation can become.
This is sometimes described as being trapped in a bad lane.
A good lane is not simply an empty piece of water.
It is a position where you can sail at full speed for a useful distance without being forced to alter course unnecessarily.
5. The Invisible Battle Between Windward and Leeward Boats
There is another fascinating feature of dirty air.
It does not always affect the windward and leeward boats in the same way.
When two boats are sailing close-hauled alongside each other, each can influence the airflow reaching the other.
A boat sailing to leeward may disturb the airflow around the windward boat, producing an unfavourable change in the apparent wind that makes it difficult for the windward boat to maintain its course and speed.
Sailors often call this the lee-bow effect.
Conversely, a boat to windward and slightly ahead may place the leeward boat in its wind shadow.
Which boat gains the advantage depends on their relative positions, sail configurations, wind strength and how effectively each boat is being sailed.
In light winds, the aerodynamic interference may extend further and have a particularly significant effect on performance.
A practical example
Suppose you are sailing close-hauled on starboard tack, with another boat slightly ahead and to leeward.
You may be tempted to point higher to get past.
However, if the other boat is disturbing the airflow reaching your sails, attempting to point even higher could make matters worse.
Your boat loses speed and the other boat moves further ahead.
In this situation, the correct response may be to bear away a little to rebuild speed, provided doing so gives you room and useful airflow.
Alternatively, a tack into clear air may offer a better opportunity.
The important skill is recognising when your boat is genuinely slow because of its trim, and when it is slow because another boat is interfering with the wind.
That distinction takes practice.
6. Why Clear Air Matters Even More on the River Thames
On open water, a sailor who finds themselves in dirty air may have plenty of room to tack away, sail into clear wind and establish a new position.
River sailing can be rather different.
At Upper Thames Sailing Club, we race on a river bordered by trees, buildings and varying bankside vegetation.
The wind is rarely as straightforward as the forecast might suggest.
Even before another sailing boat interferes with it, the airflow may have been disturbed by the landscape.
Trees can create sheltered areas and turbulence. Gaps between trees may allow stronger gusts to reach the water. The direction of the wind may vary considerably across the width of the river.
Now introduce a fleet of racing dinghies.
Each boat creates its own area of disturbed airflow.
Suddenly, finding clear air becomes a rather more complicated exercise.
The riverbank problem
Imagine that the best wind is currently towards one bank.
Naturally, several sailors decide to sail towards that side of the river.
The boats become concentrated in a relatively small area.
The leading boat may be enjoying reasonably clear wind, but the boats immediately behind begin interfering with each other's airflow.
A sailor who remains a few metres further out towards the centre of the river may have slightly less wind pressure but considerably cleaner airflow.
That sailor might consequently make better progress.
However, there is another complication.
The River Thames has a current.
An attractive piece of water may offer good wind but an unfavourable current. Another part of the river may offer slightly worse wind but more favourable water movement.
Therefore, the best racing decision involves balancing three factors:
Wind strength + Wind quality + River current
And, of course, the distance you need to travel.
The fastest route is not necessarily the shortest route, nor is it automatically the route with the strongest wind.
It is the route that gets you to the next mark in the least time.
A personal reflection
One of the things I find particularly interesting about sailing on the Thames is that you can experience completely different wind conditions within a surprisingly small area.
It encourages constant observation.
I might look at the wind indicator, the movement of the water and the sails of nearby boats, all while trying to work out whether there is a better route towards the next mark.
It also makes me think more carefully about other boats.
Rather than simply asking, "Can I get ahead of that boat?", I find the more useful question is often:
"Where will I have the clearest wind in thirty seconds?"
That is a small change in thinking, but an important one in the transition from simply sailing a race to racing intelligently.
7. A-Raters and the Enormous Influence of Sail Area
This subject becomes particularly interesting when we consider an A-Rater such as our own Champagne.
An A-Rater is a very different proposition from an ordinary training dinghy.
Its long, narrow hull, lightweight racing construction and particularly tall rig are designed to produce excellent sailing performance.
The large sails create the aerodynamic forces needed to drive the boat at impressive speeds.
But those sails also influence the airflow around the boat.
Why a large sail creates a significant wind shadow
A large, efficient rig interacts with a substantial volume of moving air.
As the sails generate aerodynamic forces, they alter the flow around and downstream of the rig.
This can produce an extensive area of disturbed air that affects smaller boats sailing nearby.
It is tempting to imagine that a sail twice the size automatically creates a wind shadow twice as long.
That is not how it works.
The affected area depends on sail geometry, wind conditions, relative positions and other aerodynamic effects.
Nevertheless, a large racing rig can create a substantial wind shadow, particularly in lighter conditions.
For a smaller dinghy sailing close behind an A-Rater, this can be extremely frustrating.
The smaller boat may be sailing perfectly well, with sails correctly trimmed and crew weight sensibly positioned.
Yet it can lose speed simply because it is sailing through air disturbed by the larger boat.
Champagne approaching a crowded starting line
Imagine we are approaching the starting line in Champagne.
Ahead of us are several smaller racing dinghies.
Our immediate objective is to make a good start, accelerate cleanly and establish a useful position.
However, we must consider more than the position of our bow.
We need to think about the effect of our rig on neighbouring boats, and the effect of their sails on our own airflow.
Although our rig is particularly tall, it is not immune to disturbed wind.
Another A-Rater positioned poorly relative to us could interfere with the wind reaching our sails.
And in the light, variable conditions frequently encountered on the river, even a relatively small reduction in effective wind can make a noticeable difference.
A boat that depends on maintaining momentum does not benefit from being repeatedly slowed by disturbed air.
The sailing team must therefore consider both the immediate start and the conditions the boat will encounter during the first part of the race.
Teamwork aboard Champagne
This is where sailing an A-Rater becomes particularly rewarding.
With three people aboard, we have an opportunity to divide our attention between different aspects of the race.
One person may be concentrating primarily on steering and maintaining speed.
Another can concentrate on sail trim and the information provided by the tell-tales.
The third can help observe surrounding boats, wind shifts and potential areas of clear air.
The precise division of responsibilities will vary with conditions and experience, but the principle remains valuable.
Good racing is not simply about steering quickly. It is about ensuring that the whole crew understands what is happening around the boat.
A useful communication might be:
"Boat ahead is disturbing our wind. Clear air to starboard if we can find room to tack."
Or:
"Keep this lane. We have good pressure and the boats behind are dropping away."
These observations help the helm make decisions without having to look everywhere at once.
8. How to Find Clear Air Before the Starting Signal
The best way to escape dirty air is often to avoid entering it in the first place.
That begins before the starting signal.
Look beyond the starting line
Many inexperienced racing sailors concentrate almost entirely on the line itself.
They are thinking about whether they will cross too early or too late.
More experienced sailors also consider what happens immediately afterwards.
Before committing to a starting position, ask yourself:
Which boats are likely to be directly ahead of me after the start?
Will I have room to accelerate?
Am I likely to be trapped between two boats?
Can I sail towards the favoured side of the course?
If I lose my wind, is there an escape route?
The last question is particularly important.
A start may appear excellent until the boats on either side prevent you from doing anything useful.
Avoid the temptation to join the crowd
Suppose the committee-boat end of the starting line is favoured by a small amount.
Naturally, many boats are likely to gather there.
It might seem sensible to join them.
But if the result is a tightly packed group in which you cannot accelerate properly, the theoretical advantage of starting at the favoured end may disappear.
Starting slightly further down the line, in a position where you have room and clear air, could be considerably better.
This does not mean you should ignore line bias.
Rather, you should understand that line bias is only one part of the starting decision.
You need to weigh it against your ability to sail at full speed after the starting signal.
That balance is an established principle of racing-start strategy.
9. A Simple Example: The Crowded Start and the Clear Start
Let us imagine a race with twelve boats.
The starting line is relatively short, and the wind is light.
There are two possible approaches.
Start A: Crowded
Boats tightly packed, with little room to accelerate or escapeStart B: Clear lane
More space to accelerate and maintain boat speedIn Start A, a boat crosses the line almost exactly on time but is surrounded by other boats.
It cannot accelerate freely. The sails are affected by disturbed air. It has limited room to change course.
In Start B, a boat crosses the line two seconds later but has a clear lane.
Suppose, purely for illustration, that:
The crowded boat averages 2.5 knots during the first minute.
The clear-air boat averages 3.5 knots during the same period.
Converting knots into metres per second:
Speed in metres per second = Speed in knots x 0.5144
Distance travelled = Speed x Time
Over 60 seconds:
Crowded boat = 2.5 x 0.5144 x 60 = 77.2 metres
Clear-air boat = 3.5 x 0.5144 x 60 = 108.0 metres
Illustrative distance travelled in 60 seconds
Assumes constant illustrative speeds; actual results vary.
The clear-air boat travels approximately 31 metres further through the water, before allowing for its later starting time, headings or current.
A two-second starting disadvantage at 3.5 knots represents only about 3.6 metres of travel.
If the faster boat can maintain its speed advantage, that initial disadvantage can disappear very quickly.
Naturally, real racing is more complicated. Both boats may be travelling in slightly different directions, and changing current or wind conditions can alter the result.
Nevertheless, the principle is clear.
A few metres gained at the start are of little value if you immediately lose the ability to sail quickly.
10. What Should You Do When You Find Yourself in Dirty Air?
Even experienced racers sometimes end up in poor air.
Perhaps another boat tacks ahead of them.
Perhaps they underestimate the wind shadow of a larger boat.
Or perhaps the wind changes direction, leaving them in a position that was perfectly acceptable only a few seconds earlier.
The important thing is to recognise the problem and respond intelligently.
Option 1: Sail into clear air
Sometimes only a modest change in position is needed.
If there is sufficient room and the course allows it, you may be able to bear away slightly, accelerate and sail clear of the disturbed airflow.
However, this is not always possible.
On a narrow river, sailing lower may put you close to the bank or another competitor.
Option 2: Tack away
If you cannot escape while remaining on the same tack, changing tack may be the best solution.
A tack costs some speed, particularly in light wind.
But continuing for several minutes in disturbed air may cost considerably more.
The decision involves comparing the short-term loss of speed during the tack with the potential longer-term benefit of sailing in clear wind.
You must also consider the racing rules, other boats and whether your new course will take you towards a useful part of the river.
Option 3: Wait for a change
Sometimes there is no sensible immediate escape route.
You may be close to a mark, constrained by other boats or expecting a useful wind shift.
In these circumstances, remaining where you are briefly may be preferable to making a hurried manoeuvre.
However, waiting should be a deliberate tactical decision rather than simply hoping that the problem will disappear.
Option 4: Protect your lane
Perhaps the most effective tactic is not allowing another boat to place you in bad air.
Look ahead.
Anticipate where nearby boats are likely to sail.
Where possible, preserve enough space to keep moving at full speed.
That is often easier than recovering after you have already lost momentum.
11. Three Common Mistakes Beginners Make
Mistake 1: Trying to point higher when the boat is already slow
A sailor notices that another boat is getting ahead and responds by pulling the sails tighter and turning closer to the wind.
Unfortunately, if the sails are already struggling in disturbed airflow, this can make the problem worse.
The boat slows further and may begin to make considerable leeway.
Better approach: Check your tell-tales, assess your speed and consider whether you need to bear away slightly to rebuild momentum.
Mistake 2: Staying directly behind a larger boat
A smaller dinghy may follow a larger boat upwind, hoping to benefit from its chosen route.
But the smaller boat may find itself continually sailing in disturbed air.
Better approach: Look for an opportunity to establish a separate lane with clear wind rather than simply following another boat.
Mistake 3: Winning the starting line but losing the first beat
An inexperienced racer may be delighted to cross the line ahead of the fleet.
But the real objective is not to be ahead for one second.
It is to establish the speed, position and freedom to make good progress towards the first mark.
Better approach: Judge your start by where you are thirty seconds or a minute later, not simply by your position when the starting signal sounds.
12. A Practical Exercise to Try at Your Sailing Club
Understanding dirty air becomes much easier when you experience it deliberately.
This exercise is suitable for two comparable boats and can be used during a coaching session in an appropriate area of open water, away from other traffic.
Exercise 1: Sail side by side
Start with the boats sailing upwind on the same tack, separated by a generous safe distance.
Gradually vary their relative positions, while maintaining safe separation, and observe changes in boat speed, heading and tell-tale behaviour.
Try repeating the exercise in different wind strengths.
Exercise 2: Follow another boat
Have one boat follow another at a safe distance, initially in a position where the following boat experiences disturbed air.
Observe the change in speed.
Then allow the following boat to move into a clear lane and compare its performance.
Exercise 3: Two practice starts
Set up a short practice starting line.
On the first attempt, concentrate on crossing as close as possible to the starting signal.
On the second attempt, concentrate on establishing clear air and maintaining speed.
Compare the relative positions after one minute rather than only at the start.
These exercises should be undertaken with suitable supervision, clear separation and proper regard for other river users. There is no need to position boats dangerously close together to demonstrate the effects.
What should you observe?
Ask the sailors to notice:
Whether their sails require different trimming in disturbed air.
How quickly the boat accelerates when it finds clear wind.
Whether the boat can maintain its pointing angle.
How strongly the effect varies with wind speed.
How long it takes to recover speed after escaping the wind shadow.
If you have access to GPS tracking or suitable boat-speed instruments, it may be possible to compare the results more quantitatively.
Even without instruments, the exercise can be remarkably revealing.
A sailor who has experienced the frustration of being trapped in dirty air is much more likely to recognise the same situation during a race.
13. What the Best Racers Are Really Watching
When watching an experienced sailor preparing for a start, it is easy to assume that they are concentrating entirely on the countdown.
In reality, they may be watching several things simultaneously.
They are assessing line bias, looking for wind shifts, judging boat speed and deciding how much room they need to accelerate.
But they are also watching the surrounding boats.
They want to know who is likely to accelerate first, who might tack, which gaps are closing and where they will find clear air after the start.
This is a particularly useful lesson for those progressing from recreational sailing into racing.
At first, we learn how to control our own boat.
Then we learn how to observe the wind and water.
Eventually, we begin to appreciate that every other boat is influencing the conditions around us.
That is when sailing becomes a much more interesting tactical activity.
14. Five Things to Remember About Clear Air
1. A perfect start is not necessarily the earliest start. Crossing the line on time is valuable only if you can maintain speed afterwards.
2. Dirty air can affect boats alongside as well as behind. It is not simply a neat shadow extending directly downwind.
3. Light winds make clear air especially valuable. Losing pressure in already marginal conditions can have a substantial effect on performance.
4. A crowded favoured end may be worse than a clear lane elsewhere. Always balance starting-line advantage against the ability to accelerate and sail freely.
5. Think ahead rather than react too late. It is much easier to avoid bad air than to recover after several boats have passed you.
Conclusion: The Race Begins with the Wind You Can Actually Use
There is something rather satisfying about crossing the starting line ahead of the rest of the fleet.
For a brief moment, you may feel that you have everything exactly right.
But racing success is rarely decided by a single moment.
A sailing race is a continuous sequence of decisions, and the most valuable starting position is often the one that gives you the best opportunity to make your next decision.
A good start provides more than a favourable position.
It provides clear wind, boat speed and freedom of movement.
On the River Thames, where the wind is constantly being influenced by the trees, banks and surrounding boats, these advantages can be particularly valuable.
And when sailing a powerful, large-rigged A-Rater such as Champagne, understanding how the sails influence the surrounding airflow becomes another fascinating part of racing.
It is easy to become preoccupied with gaining that final metre on the starting line.
But perhaps we should ask a different question.
Would I rather be one metre ahead, struggling in disturbed air, or two metres behind with room to accelerate into clear wind?
I know which I would prefer.
The best start is not always the one that puts you ahead at the gun. It is the one that gives you the best chance of being ahead at the first mark.
Follow the Learning to Race series at PMR Sailing, where we explore practical racing skills, sailing on the River Thames and our experiences aboard the A-Rater Champagne.
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