Wind Shear – Fact or Fiction?

Boat performance and Wind Shear – what every helmsman needs to know!

Here’s a great article that the people at Nexus wrote some time ago. It was published on their web for a while, but since this is essential knowledge, here it is again…

Brought to you by the great guys at Nexus!

What Relevance Does Wind Sheer Have To Sailboat Performance?

What is wind sheer, and why is it important?

At any given time the wind speed and direction at the top of the mast of any sailing boat will be different from that at the base. This change in the characteristics of moving air as it approaches sea / ground level is know by meteorologists as wind shear, and has practical applications for the racing sailor. With the right software it is possible to analyse this effect and calculate the all-important Central Driving Point of the sail separately for port and starboard. With this information processed and fed to the helmsman the result is more effective use of the apparent wind, and greater boat speed!

So, do I need to consider Wind Shear? …. YES!

Does Nexus NX2 take account of wind shear?

Nexus’ NX2 instrumentation system uniquely incorporates a mathematical process that identifies the amount of wind shear at any given time, using the T.R.U.E* process. NX2 Race then displays the adjusted wind speed and direction data. If a boat’s polars are also loaded into the navigational system then NX2 can combine both data sets and display the most efficient course for the helmsman to steer.

* T.R.U.E. – Temporary Reference Update Estimation. When Activated, this function updates the wind calibration after every tack.

So, does Nexus NX2 take account of Wind Shear? … YES

What is the significance of this capability?

Accurate tactical decisions can only be based on accurate wind speed and direction data, and with access to such data the performance of a sailing boat can be substantially improved. With NX2 ‘steering by the needle’ becomes a reality, with or without the use of polar data, enabling the helmsman to get the very best out of his boat. With NX2 on your side, it could make all the difference between winning and coming second.

So, is this significant to the racing helmsman? … YES

The Invisible Sailing Wind

How many of us have noticed that our boats sometimes just don’t perform upwind the way we expect, even though we think that we’re doing everything right? More often that not, the problem is put down to the set-up of the rig, or blamed on instrument error, but keen sail racers everywhere understand that this problem cannot simply be blamed on some unknown factor. To understand what lies behind this meteorological anomaly, and figure out how we can counteract its effects, we need to look at the basic principles that underlie the movement of air across the surface of the Earth, starting with the global perspective.

Rotation of the globe

The Earth rotates to the east at a speed of 465 meters/sec at the equator. As with the surface of any rotating sphere this rotation creates the ‘Coriolis effect’ – first identified by Frenchman Gaspard-Gustave Coriolis in 1835 – which observes that an object moving straight ahead within a rotating frame of reference will appear to an observer to be deflected to one side (watch someone trying to throw a ball in a straight line while on a childs roundabout, and you’ll see that he had a point).

Due to the rotation of the earth, therefore, any air moving away from the equator in the northern hemisphere appears to drift westwards. Over time this deflection develops into a circular movement and the air currents begin to rotate (anticlockwise in the northern hemisphere, clockwise in

the south) as they slowly move towards the centre of a low-pressure area. (To understand why this is always the case, please see the more detailed explanation at the end of this document.)

The Weather ‘Battle’

The result of the Coriolis Effect on the atmosphere is a perpetual conflict between two immense forces. On the one hand, large masses of air are in constant motion as they move from areas of high pressure to those of low pressure; the result of differences in global temperatures. At the same time the Coriolis Effect is working to push the air back to where it came from through large circles. This conflict between the two forces creates a number of atmospheric phenomena that form the basis of the weather that we experience, and contribute to the inherent instability of weather systems.

Sailing Wind

So what does this mean for us sailors? Well, it means that there is more to the winds that drive our boats than just speed and direction!

Take the true wind angle and speed indicators. These are among the most useful measurements displayed by any boat’s instrument system. By measuring the speed of the boat, the apparent wind speed and the apparent wind angle, this basic information then provides the core data for the second stage functions, where the compass course and true wind angle are added for true wind direction. To then calculate the geographic wind direction, leeway and tide / current must be subtracted since a current will create a ‘tide wind’ component that will impact on the direction of the true wind.

Let’s Add Some Friction!

But when the wind blows close to the water surface, air speed is reduced because of friction. We also know that when air speed changes, the direction changes. This is because the balance of forces between the Coriolis effect and pressure gradient is altered, and the result is that a slower moving wind will turn more towards the low-pressure centre.

The impact of friction varies at any particular place and time, but it can be calculated by measuring air temperature, air speed, humidity and the friction constant (this constant is particularly difficult to know). However, the key result of friction is that the air speed and direction change with altitude, creating wind shear.

Wind Shear Creates A Problem

Wind shear presents a significant problem for racing sailors. Not only has it in the past been impossible to measure, but even now standard instrumentation systems measure the wind vector at the top of the mast, but have no way of observing its angle closer to the deck when in reality wind speed and angle change on their way down the mast. We cannot see this ‘sheared’ wind, but we feel its effects when our boats do not perform in the way that we expect.

This single wind measurement point at the top of the mast plays a critical role in how sailors measure the efficiency of their sails relative to the apparent wind angle and true wind speed. Yet when using polar tables, there is no avoiding the fact that the wind data fed into the system comes from the top of the mast and not from where the wind is most efficient for their sail trim. If the measurement of the efficient wind angle is not correct, then the resulting defective input into the polar table calculations results in an incorrect calculation of the target speed.

This image illustrates the distorting effect that occurs when the wind angle is measured at the top of the mast and then used to calculate true wind. The geographical wind direction will be offset by 3 degrees relative to the average sailing wind angle!

What Do We Do Then?

With the basic measurement point incorrectly positioned, sailors naturally often assume that it is the instruments that are misreading. The result is generally a good deal of dissatisfaction onboard accompanied by unnecessary sail trim to compensate for ‘low wind angle’ or ‘low boat speed’. We know that this only further reduces efficiency, so you may recognise some of these:

The instruments do not show the same
wind angles on port and starboard.

We are sailing far too low.
Trim the sail. What is wrong?

Wow, now we are sailing really high,
this boat is really fast now!

There is no power in the boat,
what is wrong?

The instruments are wrong, geographical wind
varies widely between port and starboard tack.

We’ve noticed that on the open sea,
the boat hits the waves harder on the starboard side.

Addressing The Problem

The first step to ensuring that your instruments are gathering accurate information is making sure that they are all properly calibrated. The boat speed function should always be checked first, followed by careful auto-deviation of the compass.

With that completed, it is important that the navigator recognise that the different tack angles and boat speeds between port and starboard are caused by several factors;

The wind transducer reports what is going on at the top of the mast but not at the efficient sail pressure centre or indeed the true direction of the sailing wind. Wind shear creates a difference in sail efficiency since the sail pressure centre is not the same on port and starboard tack. On the starboard tack the wind will hit the top of the sail at a wider angle and make the boat heel more in stronger winds. In lighter winds, this top wind may instead give extra forward power instead of heel force. The helmsman can compensate for this by sailing slightly higher, but such a strategy will increase leeway and the real VMG (Velocity Made Good) will drop.

Data that is fed into the computer’s polar table is offset by the wind shear. The computer therefore reports a target boat speed (TBS) calculated from top of the mast and not from where the most effective sail pressure centre is located. The TBS will therefore not be accurate and, by trusting the polar data without taking wind shear into consideration, valuable boat speed will be lost.

Wind Angle Errors From Up-wash And Heel

As air moves it anticipates upcoming obstacles through the change in the local air pressure ahead of that obstacle, and on a boat this has the effect of forcing the wind to bend before it passes over the sail. This phenomenon is known as up-wash and it has a noticeable impact on apparent wind angle and wind speed.

This is a change that can be difficult to measure. Many instrumentation systems simply ignore it and instead use a compensation table to adjust for the true wind, but the NX2 system can measure this important factor and apply the necessary compensation to the apparent wind angle. The resulting reported apparent wind-angle data is therefore adjusted for up-wash, heel, and for where mast twist has been added.

It is important to compensate for this wind angle error when calculating the true wind angle, and by allowing some twist at the top of the sail this force can be reduced and the boat sailed at the same angle to the wind but with a lesser degree of heel.

On the port tack, the sheared wind is ‘negative’, and it requires a flatter sail trim. The instruments will indicate that boat is sailing high and fast on this tack, but this is an illusion; the opposite to what is observed on the starboard tack. With less wind force in the top of the sail, the efficient wind pressure centre is moved downwards. The result is that, for a given heel angle, the boat moves at a wider wind angle. It is not sailing as high as the instruments indicate.

On the open sea, where the wind direction is more stable, the waves follow the surface wind direction. Since this wind speed is reduced by friction, it turns slightly more towards the low pressure. The result is that the hull hits the waves somewhat harder on starboard than on port (in the northern hemisphere). Trimmers are advised to add more twist to get increased power from the wind on the starboard tack.

By using the knowledge of wind shear and trimming their sails according to which tack they are on, sailors can derive a real advantage. This offset for ground wind can also be calculated with or without wind shear and for the top of the mast only. This will result in different lay line angles on the port and starboard tacks.

Using Target Boat Speeds From Polars

As we have described above, wind angle and speed are a function of the measurements made at the top of the mast and the balance of the sails (the sail trim). It is common among instrument manufacturers when adjusting for the normal wind angle simply to average the error to get equal readings on port and starboard. This can be an acceptable solution for sail racing where the navigator prefers to see a similar reading on the instruments on both tacks, but in reality there is a difference caused by wind shear, and this will have an effect on performance.

The result is that polars need to be asymmetric in order to compensate for this effect, but it is simply not practical to do so. There is however another way to compensate for the sheared wind and that is to let the computer “rotate” the polar table with the same angle as the offset from the sheared wind. This function is also supported in the NX2 Race software that comes with the NX2 system. NX2 Race comes preloaded with a range of polar tables for different boats, and navigators can create their own and add them to the list if required.

Sailing downwind using polars

Nexus’ NX2 system has been designed to include a very powerful function that takes full advantage of the data contained within a boat’s polar tables. By comparing the compass course with the optimum wind angle derived from the true wind speed plotted on the polar data, the needle on the NX2 Steer Pilot instrument shows the helmsman the most efficient VMG course to steer!

This data is re-calculated five times per second to give the helmsman continuously updated steering information, presented by a smooth, moving needle. We call this, ‘steering by the needle’. NX2 Race also accepts ‘polar table offset’, allowing compensation for a sheared wind angle.

When sailing downwind at low wind speeds the importance of maximising VMG downwind increases. One problem at low wind speeds is the accurate measurement of that wind speed as to do so requires a very sensitive, low friction transducer. Nexus’ Twin Fin transducer is designed with a ‘toe-in’ of 3 degrees in order to gain angle stabilisation. The three delta-shaped propeller blades use 100% of the available air power instead of a pulsation peak in a 120 degrees phase that is an unavoidable feature of the traditional transducers that use three, evenly spaced cups.

More About Pressure Gradient And The Coriolis Effects – The Technical Background

Wind is caused chiefly by differences in air pressure. Regional differences in air pressure are equalized from the lowest height, moving upwards, and this means that high-pressure air sinks to replace rising low-pressure air from low to high. The air temperature also has an impact on air pressure; rising (warm) air rotates anticlockwise, while sinking (cold) air rotates clockwise. This process is described by the following equations, where;

P = CρT (pressure exerted by the gas)
C = Constant
ρ = density of the gas = mass/volume
T = Temperature of the gas
PGF = Pressure Gradient Force

Forces that act on the air masses are as follows:

1. Pressure Gradient Force
PGF = ΔP / (ρ * d)

ρ – density of the air
ΔP – is the change in pressure
d – is the distance

2. Coriolis Force

CF = 2 * Ω * Vg * sin Φ

Ω – is the rotation rate of the earth
Vg – is the geostrophic* velocity
Φ – is the latitude that you are at

3. Centripetal Force (Of The Rotating Wind, Not The Earth)

CPF = m * v² / r

m – mass
v – velocity
r – radius

4. Friction

*) Geostrophic flows are the winds created by the Pressure Gradient Force on straight, parallel isobars, and acted upon by the Coriolis Force (ie. PGF is balanced by CF)

Gradient flows are the winds created by PGF on curved isobars, acted upon by CF and CPF on the rotating airflows:

At ground level (up to 1 – 1.5km), friction slows the winds down so that the CF action is reduced (the airflow crosses the isobars to the low pressure point)

Finally , as we can see, the CF (Coriolis force) is perpendicular to the wind direction and not to the PGF (Pressure Gradient Force). The friction factor is important in determining how much the wind is slowed down and its angle deviated towards the low-pressure centre. At sea the friction is low and fairly constant. On a larger perspective, a mast of 20-metres is still considered to be very close to the surface. It is also important to refer to the efficient pressure centre of the sail, which will be far lower then 20-metres. This is why wind shear will have a very important impact on the efficiency of the sail shape on both port and starboard.

(c) Copyright 2009 Nexus Marine.