Sometimes you'll find that the boat seems to be sailing better on one tack than the other. There are various things that could be going on; the boat trim could be effected by lots of gear in one locker, the helms person may be steering differently on opposite tacks, sail trim could be wrong on one tack, possibly the boat may not be symmetrical (although we hope that's not the case). However there's also a good chance that you're seeing differences in wind twist due to wind shear so that the boat is actually sailing in different apparent wind on opposite tacks. This article looks at how wind shear can effect twist, and how that effects the boat.
- Sail Twist and Wind Shear Wind Shear
- Sheared Wind Direction
- Wind Shear and Apparent Wind
- How Much Twist Can We Expect?
- Practical Sailing
Sail Twist and Wind Shear Wind Shear
The wind speed isn’t constant at all heights. Friction slows the wind down at the sea surface, in the next layer up friction with the slowly moving surface layer slows the wind down, and so on as we move away from the surface. This means that wind speed tends to increase with height. This is called wind shear.
The amount of wind shear (the amount by which the wind speed decreases as we get close to the surface) depends on the prevailing conditions. If the air mass is stable then it will tend to form well defined layers, this means that the layer near the surface stays near the surface and gets significantly slowed down. If the air mass is unstable there will be lots of vertical movement to mix up the layers, so any given parcel of air will spend some time at the surface and some time higher up, overall it will be slowed less so there will be less wind shear. In a stable air mass we can expect strong wind shear, in an unstable air mass week wind shear, and various amounts of shear in between.
The friction that the surface level air experiences depends on the nature of the surface. Over land there is more friction than over the sea, so there will be stronger wind sheer. If the wind is blowing off the land then close inshore there will be stronger shear than out to sea.
Wind shear is a well known effect. The “surface level” wind speed given in shipping forecasts is actually the predicted wind at 10m above the surface, where wind shear shouldn’t be having a significant effect. 10m is roughly the height of the masthead on a typical cruising yacht, so the met office are forecasting the wind that you’ll see on your wind instrument (assuming that the anemometer is in the usual place at the masthead).
Sheared Wind Direction
As soon as we start to talk about wind directions we need to think about the Coriolis force, at which point we need to know whether we’re in the Northern or Southern hemispheres. We sail in the Northern hemisphere so that’s the case we’ll talk about, just to avoid having to preface everything with “in the Northern hemisphere”.
Away from the surface the Coriolis force makes the wind blow more or less parallel to the isobars. The Coriolis force depends on the wind speed, if the wind slows down the force reduces and the wind flows more towards the area of lowest pressure. Buy Ballot’s law states that “in the Northern hemisphere if you stand with your back to the wind low pressure is to your left”, so as the wind slows down it turns to the left (which is anti-clockwise, so it backs). The slower the wind the more it backs.
Putting this together with the effect of wind shear on wind speed we can see that near the surface, where wind shear has significantly slowed the wind down, the wind will be quite strongly backed. As we move away from the surface the wind will be faster so won’t be backed so far.
- As we move down towards the surface the true wind slows and backs
- As we move up away from the surface the true wind speeds up and veers
We’ll call this change in direction the “true wind twist”.
The amount of true wind twist depends on how strong the wind shear is. If wind shear is strong then the change in wind direction will be large, if it is small then the change in wind direction will also be small.
Of course the weather forecasters know all about this. The forecast wind direction in the shipping forecast is the direction expected at 10m above sea level.
Wind Shear and Apparent Wind
A sailing boat sails in the apparent wind, and we know that this is a combination of the true wind and the induced wind. Because of wind shear the true wind will be different at different heights up the mast (the induced wind will be the same, because we hope that all of the mast is travelling along with the boat!), so the apparent wind will vary as we go up the mast which is why we need some twist in our sails.
Lets start by just looking at the effect of the changing wind speed (this is only part of the equation, though it’s often the only part that gets explained).
As we move up the mast, from the gooseneck to the masthead, the induced wind stays the same (we’re ignoring any pitching and rolling) while the apparent wind speed increases (because of wind shear). We know that for a fixed induced wind, increasing the true wind strength moves the apparent wind backwards, so as we move up the mast the apparent wind comes from further backwards. This gives a “twist” to the apparent wind, with the sails properly trimmed we aim for a matching sail twist.
On starboard tack the wind is coming from the starboard side. As we go up the mast the apparent wind moves backwards, more to starboard, which is a clockwise shift so the apparent wind veers as we go up the mast. On port tack things are the other way around. So:
- On starboard tack apparent wind veers as we move up the mast
- On port tack apparent wind backs as we move up the mast
Now let’s add in the true wind twist. We know that as we move up away from the surface the true wind increases in strength and veers. If we’re on starboard tack the change in true wind strength makes the apparent wind veer as we move up the mast, with the true wind veering as well the two effects add up giving increased twist on starboard tack. On port the two effects are shifting the apparent wind in opposite directions so they tend to cancel each other out. The twist due to the change in true wind strength is the bigger effect so there is always some twist in the apparent wind, but we can say that:
- On starboard tack there is more twist in the apparent wind
- On port tack there is less twist in the apparent wind
Of course in the Southern hemisphere things are the other way around.
This is one of the reasons why boats sometimes sail better on one tack than the other.
How Much Twist Can We Expect?
Exactly how much the wind twists depends on how strong the wind shear is and how the true wind strength compares to the induced wind (the boat speed).
The twist in the true wind will be greater on a day with strong wind shear because the air at sea level is slowed down more and so turns more towards the low pressure.
The strength of the Coriolis force varies with latitude, near the equator it is very weak so we don’t expect much twist in the true wind at low latitudes. Moving nearer to the poles the Coriolis force is stronger so we should see more twist in the true wind at higher latitudes.
Apparent wind depends on the relative strengths and directions of the true and induced wind. If the true wind is significantly stronger than the induced wind then it will dominate and the apparent wind will be close to the true wind. If the true wind is week compared to the induced wind then the apparent wind will be pulled further towards the induced wind. As far as apparent wind twist is concerned this means that when the true wind is strong compared to our boat speed apparent wind will be closer to the true wind all the way up the mast so there will be less twist. Conversely when the true wind is relatively week (or the boat speed high) the induced wind will have a stronger effect so there will be more twist in the apparent wind.
If the wind is blowing off the land then there will be stronger wind shear close inshore, so both the true wind twist and the apparent wind shift will be stronger close in shore.
Putting all of this together here’s what we can expect:
|Increasing boat speed||Increasing twist|
|Increasing wind speed||Decreasing twist|
|Increasing wind shear||Increasing twist|
|Offshore wind, sailing closer in shore||Increased twist|
|Increasing boat speed||Decreasing difference between tacks|
|Increasing wind speed||Decreasing difference between tacks|
|Increasing wind shear||Increasing difference between tacks|
|Increasing latitude||Increasing difference between tacks|
|Offshore wind, sailing closer in shore||Increasing difference between tacks|
It’s not possible to work out the precise amount of wind twist that we’ll actually get for any given conditions, but we can use our knowledge about how this changes to make decisions about how we sail the boat and trim the sails.
Sailing close hauled the wind twist moves the deck level apparent wind nearer to the induced wind, which is effectively a header (i.e. we need to turn further away from the wind to keep the sails trimmed). In the northern hemisphere we can expect to see more wind twist on starboard tack than on port tack, so we will need to sail further off the wind on starboard to keep the sails trimmed. In conditions that give strong wind shear we will be able to sail closer to the wind on port tack than on starboard.
Strong wind sheer occurs in stable air masses, which are often associated with high pressure and light winds. So on settled light wind day we’re likely to find the conditions for strong apparent wind twist and a significant difference in twist between tacks. If you’re making poor progress on starboard tack on this sort of day then it’s worth trying port tack to see if things improve (assuming, of course, that this doesn’t send you off in completely the wrong direction).
As the boat speed or wind strength change the amount of apparent wind twist will also change. Keep an eye on your sails and be prepared to re-trim to match the twist that you’re currently experiencing.
If it looks like conditions will give a significant difference between the twist that you see on each tack be prepared to have different sail trim on each tack. This could mean having the port genoa car further back than the starboard one to give more headsail twist on starboard tack, you may also find that you need to ease the main sheet and move the main traveller up a little on starboard tack compared to port to give the main sail some more twist.