yachting apparent wind

Yachting Monthly

• Digital edition

Apparent wind: How to predict it and use it to your advantage

• Katy Stickland
• July 1, 2022

Predicting changes in apparent wind will make you a safer, faster sailor, says Martin Watts

Knowing what to do when the apparent wind changes could be the difference between accelerating smoothly and broaching badly. Credit: Graham Snook/Yachting Monthly

Newcomers to sailing soon learn to appreciate the differences in apparent wind on different points of sailing, writes Martin Watts.

We’ve all slogged to windward in a Force 5, clad head to toe in waterproofs as spray flies over the deck, only to see a boat sailing in the opposite direction, downwind , with the crew lounging around in shorts and T-shirts enjoying what looks more like a Force 3.

However, the apparent wind can also change whilst sailing on a leg of a route, and understanding these changes and how to respond to them can improve not only speed, but also the efficiency of how a yacht is handled, comfort on passage as well as safety by avoiding losing control in broaches and other incidents.

Changes in true wind direction are obvious, but there are other reasons for apparent wind changes.

As wind increases in gusts, the apparent wind angle will move aft. Credit: Maxine Heath

I’ll start by looking at what causes these changes and then how we respond to them.

If the true wind speed suddenly increases in a gust, then not only is there an increase in apparent wind speed, but the apparent wind angle (AWA) changes in direction, moving further aft with respect to the yacht.

Conversely, if there is a decrease in the true wind speed, then the apparent wind speed decreases, but it now moves further forward in relation to your boat.

There are also changes in the apparent wind due to variations in the speed of the yacht.

If the yacht’s speed drops, then the apparent wind decreases and moves further aft.

If the boat slows down, the apparent wind will also move aft, while an increase in boat speed will bring it forward. Credit: Maxine Heath

If the yacht speed increases, then the apparent wind speed increases and the apparent wind angle decreases, moving further forward.

This effect can clearly be seen on the foiling catamarans who have their sails sheeted in all the time.

In practice, any change in apparent wind speed and angle are usually a combination of both of these mechanisms.

The common sight is a strong gust hitting a yacht.

Continues below…

Why twin headsails are best downwind

Crossing the Atlantic from Cape Verde to Barbados took us just 13 days in our 1998 Lagoon 410, Cushla Na…

Sail trim in light airs: 7 tips to keep you sailing

John Parker, owner of the OneSails GBR (East) loft shares his top tips on sail trim to make the most…

Sailing in waves: top tips to keep you safe at speed

Sailing in waves can make for a jarring, juddering experience and long, uncomfortable passages and at worst, a dangerous, boat-rolling…

How to cope with gusts and squalls

Spikes in wind strength can range from a blustery sail to survival conditions. Dag Pike explains how to predict which…

The extra power and the fact that apparent wind is now more beam-on to the yacht means the sails are over-sheeted and there are large lateral forces that make the yacht heel, which result in a less efficient hull shape requiring more rudder to be used to keep the yacht going straight.

This all slows the boat down, and so the apparent wind shifts even more beam-on. The rudder eventually loses grip, and we end up with a broach.

So how should you respond to these changes in the apparent wind speed and direction?

The actual response will vary on the yacht, the crew and the conditions, and on whether you lean more towards the ‘racer’ or ‘cruiser’ ends of the spectrum.

Apparent wind: dealing with gusts

Gusts are formed in a variety of ways, but in open water, a gust is likely to have been formed by a downdraught around a cumulus cloud after a cold front.

If this is the case, the wind direction of the gust may be the same as the higher-level wind direction.

In the northern hemisphere, this will typically be veered compared to the sea-level wind.

‘Cats paws’ on the water show how gusts fan out as they hit the surface. Credit: Martin Watts

The result is that if you are sailing on starboard tack, you may get a lift to windward, while on port tack you would be headed.

It’s slightly more complex than this, however, as downdraught gusts fan out as they hit the water and so it helps to identify where the yacht is in relation to the gust fan so that the change in wind direction can be predicted.

If you are sailing towards the centre of the gust, on either tack, you are likely to be headed, while sailing away from the centre, you are likely to get a lift.

If you are hit by a gust when sailing upwind, the increase in true wind speed will move the apparent wind aft, giving you a lift in which you can luff up slightly, or if you are on course and don’t need to make more progress to windward, you can ease the sails to gain more boat speed and avoid excess heel, or do a combination of both.

If you are less concerned about performance, it is easier to just luff up slightly, which makes use of the lift, and also spills some power from the sails.

When a gust hits, its leading edge will often bring the strongest wind, so in blustery conditions, it pays to be looking upwind spotting approaching gusts, even if you’re just cruising.

Upwind, it pays to be prepared and not caught off guard by a gust. Credit: Graham Snook/Yachting Monthly

This gives you a chance to luff up significantly before the gust hits, so that the sails are lifting slightly and the yacht is not overpowered as the gust hits.

You can then slowly bear away again, keeping the boat under control and maintaining good boat speed.

As boat speed increases again and the apparent wind moves forward, both groups may end up sailing at a slightly lower heading than before.

In contrast, if you aren’t ready for the gust, you will heel and increase rudder angle, slowing the boat down, and if you then luff up, you will do so into the lighter wind behind the gust, further losing speed.

When sailing on a reach, easing the sails and bearing off slightly, so that the direction of effort is more in line with your heading, will give the biggest increase in speed.

Racers steer to keep the boat ‘under the sails’, reducing heel to keep the rudder gripping particularly when fully powered up.

They may even bear off more to stay in the gust, particularly in light winds, knowing that they will be coming back up to the original course line with a better apparent wind direction.

When reaching steer to keep the boat under the sails, bearing away in gusts. Credit: Graham Snook/Yachting Monthly

For a cruising sailor, the response to a gust when reaching can be rather different to this and is more similar to how they responded when sailing upwind.

Luff up slightly, or a lot before the gust hits if the gust is large to spill power from the sails, and then bear away as the gust eases.

Then as boat speed increases and apparent wind comes forward again, racers will sheet in, while cruisers may be content to bear away slightly to keep the sails full.

Things change again when running deep downwind.

With the wind astern and the sails sheeted right out, they are being pushed along, but there is no aerodynamic flow along the sails and they are not generating any lift.

The force on the sails is in the same direction as the boat’s heading, so there is no heel.

Rounding up towards the wind, far from depowering the sails, will do several things at once: it allows an airflow to develop and with it a significant increase in power; the apparent wind will increase as you are no longer running before the wind; and a large heeling moment, which could easily lead to a broach.

On a run, don’t be tempted to luff up to depower the sails; it will have the opposite effect. Credit: Graham Snook/Yachting Monthly

This is all exacerbated by the fact that there is a temptation to set more sail when downwind when things feel calm, only to find that you are hugely over canvassed as soon as you turn towards the wind.

So generally, the response is to stay heading in the same direction, with a key aim now being to stop the yacht burying its nose in the back of wave.

If you need to reduce power, sheeting in is a more responsive technique, or alternatively you could try reefing down to match your sailplan to the gusts.

Slowing down

When the true wind speed drops, when going upwind, the apparent wind moves forward and so the initial impression is that you have been headed.

The automatic reaction is to bear off, but you must do this gradually so that the yacht does not lose momentum as a result of large rudder movements and disrupting the airflow over the sails.

When the true wind speed drops on a reach, the apparent wind also comes forward, so you’ll need to sheet in or bear away to keep the sails pulling.

Wake from other boats will cause you to slow down and the apparent wind to move aft. Credit: Graham Snook/Yachting Monthly

One of the common reasons boat speed suddenly drops is because of wake from powered vessels.

The first response is to try and minimise the effect of the wave by steering accordingly.

As the yacht’s speed drops and apparent wind moves aft, ease the sails or luff up so that the apparent wind angle is the same as before the wake hits, so that the sails remain set correctly.

Heavy displacement craft like tug boats create particularly short and sharp ‘boat stopper’ wakes. Credit: Alamy Stock Photo

Then as the yacht’s speed increases, bear away back on to the original course.

In light winds, it may pay to keep sailing a bit higher, with more apparent windflow over the sails, to build up boatspeed more quickly before bearing away.

Waves and tide

The effect of yacht speed on the apparent wind direction is clear when sailing upwind with the waves or swell coming downwind.

As the yacht climbs the front of the wave, it slows and so the apparent wind moves aft, so the helm can luff up slightly to keep the sail trim right and this can help the boat punch through the top of the wave.

As the yacht goes down the other side of the wave, it accelerates and so the apparent wind moves forward and so the helm must bear away to keep the sails drawing properly.

Apparent wind can also be dramatically affected by changes in the tidal stream. Sailing with the tide will increase speed over the ground and with it, apparent wind.

As wave size increase, actively helming over each wave will keep the boat moving much better. Credit: Richard Langdon/Ocean Images

In terms of direction, the wind angle will move forwards, so sheeting in or bearing away will be necessary.

The more marked impact is that sailing upwind with the tide, the wind speed can increase dramatically, while on a run the apparent wind may drop a Force or two by comparison.

Things get more interesting when sailing across the stream.

With the tide on your leeward side, you would be favourably pushed towards the wind, ‘lee-bow effect’, while with tide on your windward side both apparent wind speed and progress to windward would be reduced.

Traditionally, the mark of a good helmsperson was that they could sail a course with a steady compass bearing, so that the navigator could make good dead reckoning calculations.

Martin Watts and his wife, Irene, have chartered in the Med and Caribbean. They own a Hanse 341 and sail from Largs, Scotland

Now with GPS and chartplotters , the yacht’s position is known, and the helm and crew have a greater flexibility to respond to apparent wind changes to maximise boat speed.

So, the yacht can be pointed on its desired course, the sails set accordingly and then the helm left to look around to predict the wind and wave conditions and steer to what feels right for the yacht and the conditions.

The aim is to be able to predict and react to any apparent wind changes, rather than just respond after the event.

Then, after a period of time, compare the actual and desired position or look at the cross track error (XTE) and alter the course heading and adjust the sails to suit.

Enjoyed reading Apparent wind: How to predict it and use it to your advantage?

A subscription to Yachting Monthly magazine costs around 40% less than the cover price .

Print and digital editions are available through Magazines Direct – where you can also find the latest deals .

YM is packed with information to help you get the most from your time on the water.

• Take your seamanship to the next level with tips, advice and skills from our experts
• Impartial in-depth reviews of the latest yachts and equipment
• Cruising guides to help you reach those dream destinations

Follow us on Facebook , Twitter and Instagram.

Apparent wind

by Niclas | Oct 21, 2015 | sailing basics , tactics | 1 comment

Understanding how Apparent Wind works and its impact on how you should efficiently sail the boat is crucial, and even though it may seem theoretical at first it’s well worth mastering this subject. You may get some “aha moments” while reading the theory as you would have seen the effects on the water already.

What is Apparent Wind?

The wind you experience while moving with the boat is called Apparent Wind . It’s the combination of the True Wind and the  Boat speed Wind .

Apparent wind changes in wind gusts

So, what happens when your boat sailing close hauled suddenly is hit by a stronger breeze – a wind gust?

The first thing that happens is that the True Wind component increases, and hence the Apparent Wind  also increases and changes direction. This means that your leeward telltales will suddenly start spinning, the boat will be heeling over from the stronger wind and it feels like you are getting a lift, although the True Wind still comes from the same direction as before.

When the boat is hit be the more powerful breeze it will either accelerate and eventually the True Wind direction will become more or less the same as it was before the gust, or if the boat has already reached its maximum speed it won’t accelerate further. Hence the way to respond to these gusts will vary if you’re in e.g. a small planing skiff or in a large keel boat.

and in lulls…

The opposite obviously applies when sailing into a lull (less wind). Suddenly the Apparent Wind changes direction and your windward telltales will start spinning around. It’s tempting to bear away, although if it’s a very short lull it may be better to keep your course waiting for the wind to fill in again. There may also be a temptation to tack as it feels like you’ve been headed. The decision whether to tack or not should then be based on where the best wind strength is and other tactical aspects such as what other boats are doing. Just be aware it’s not a real header you sailed into.

The boat will also slow down in the lighter breeze and eventually the Apparent Wind direction will be similar to before. Again, changes in boat types will react differently, where a large heavy boat will keep its boat speed much longer than a dinghy would.

Why fast boats don’t sail dead downwind

Let’s now look what happens when we sail downwind. See how the Boat Speed Wind   and the True Wind are opposite resulting in a small force of Apparent Wind speed. At times at the end of a gust, or surfing down a wave, you may even sail faster than the wind and suddenly all power goes out of the sails.

For fast planing boats going dead downwind is rarely a good option as they will be so much faster reaching that the extra distance they will need to cover is more than well compensated for in better boat speed. By sailing at more of a reaching angle these boats will generate more apparent wind which in turn allows them to sail much faster.

Apparent Wind and current / tide

If you’re sailing in a current your boat speed component will also be affected by the moving water. We’ll look further into this in another article.

I’m sure there’s a lot more to be said about apparent wind, but this hopefully should serve as an introduction. As always we welcome questions and comments to the article.

Share this:

I have a 12 ft non planing dinghy. Does it make sense to reach on a downwind? Thanks!

Blogroll - Sailing blogs

• Americas Cup info site
• Sailing World Magazine

Rules and regulations

• RORC, including the IRC handicap rule
• The rules of racing

Sailing organisations

• Americas Cup
• Australian Olympic sailing team
• International Canoe Sailing Organisation
• International Sailing Federation
• Yachting Australia

tips and tricks

• Sailing world experts section
• boat handling
• sailing basics
• Sydney sailing

• Apparent Wind vs. True Wind

If we place the anemometer ashore, then the data on wind speed and direction will give the so-called true wind. However, if we use the same device on a moving ship, the data will be different. The device will then be measuring the apparent wind. And it is precisely this wind that is so crucial for the correct sail trim and why it is so important to us.

How does apparent wind come about? 

It is a combination of wind direction and speed, but also the direction and speed of a moving object. For a better idea, let's use the example with a steam locomotive. If a locomotive is stationary and no wind is blowing, the steam will rise straight up. However, if the wind is blowing, for example from the right, then the steam will go to the left. But what happens if the locomotive starts moving? In addition to the real wind, the wind created by the movement of the locomotive itself also begins to act on the steam. This is the wind from front to back. The effect of both winds, the real wind and the wind created by the movement of the train, will add up and the steam will be affected by what we call the apparent wind. The steam will not go directly to the left but obliquely. The faster the train travels, the more there will be an apparent wind from the front and the steam will eventually go straight back instead of to the left.  

What does the difference between apparent and true wind depend on? 

The above example shows that the difference between true wind and apparent wind will always depend on our course, and its strength will be different with a tailwind or crosswind. Plus, it changes depending on the speed at which we sail. During the voyage, we don’t change power depending on the direction of the real wind (according to the weather and other influences) but also the apparent wind, which is related to our course and speed, which is constantly changing.

Imagine this 

1) When the wind blows at 10 knots and you are piloting the boat at a speed of 5 knots directly against the wind, the wind will feel 15 knots (10 + 5), i.e. the apparent wind will be 15 knots. It will seem like the wind is blowing strongly.

2) But if the wind is blowing at the same 10 knots but you have a tailwind of 5 knots, the apparent wind only be 5 knots (10-5), because you are travelling with the wind. It will seem that the wind isn’t blowing so much. 

Trim trim trim

So when it comes to really using the boat effectively, we can't adjust the sails to the current conditions and just take a siesta. Basically, it is necessary to trim constantly, because the conditions that affect the trim are constantly changing. So if you didn’t know it before, you must always trim the sails according to the apparent wind, not the true wind.  

FAQ Apparent vs. True wind

Come and test the theory in practice and rent a boat soon.

Contact me and we will choose a boat with perfect sails for your windy adventure.

Denisa Nguyenová

Sales Consultant

[email protected]

• Course Login

RYA Online Courses

Apparent wind explained, apparent wind / true wind.

The issue of wind direction and strength becomes complicated as soon as a boat starts to move. Telltales and wind vanes on mastheads only indicate the true wind when a boat is stationary.

As soon as it starts to move they react to a combination of wind and the boat’s movement – apparent wind.

Fig 12a shows a vector diagram for a boat on a beam reach. The long side of the trapezoid represents 10 knots of true wind drawn to scale while the short side of the trapezoid shows the boat speed, 6 knots. The long diagonal represents the apparent wind. Note that the length of the line now shows 12 knots of wind speed and the angle of the wind crossing the boat has decreased to 60°. In order to maintain the same entry angle to the wind, the boat’s mainsail and jib had to be trimmed on -meaning, pulled in – progressively as the boat accelerated.

Fig 12b shows the apparent wind for the same conditions with the boat sailing close-hauled, at 45° to the true wind. You can see that the apparent wind builds significantly and the entry angle closes down as the boat accelerates forward to a full speed of 6 knots.

Fig 12c shows the same boat broad reaching at six knots in the same wind. This time the apparent wind decreases but the entry angle still closes down.

In all three of the illustrations the sails had to be considerably re-trimmed as the boat increased speed, and boat speed varies all of the time. Wind speed and direction are rarely constant, so it follows that, to keep a boat moving efficiently through the water, the sails need almost constant re-trimming to cope with the constantly changing conditions.

Remember, as soon as you start moving the windex or woolies will only show you the apparent wind. The same is true of the more basic electronic wind insruments. To display true wind information the wind instrument must interface with the log, or speed input.

The above is an extract from the RYA book “SAIL TRIM hanbook for Cruiers” which is avaialble from Sailtrain.

© Copyright Sailtrain Ltd 2024, All Rights Reserved | Sitemap | Privacy Policy

Understanding Apparent Wind

Also in Technique

• How to start sailing shorthanded
• Winch servicing
• Repairing delaminated core
• Splicing Dyneema
• DIY custom bug screens
• Anchoring and mooring a catamaran
• Co-owning a boat
• Installing a steering wheel
• Helming skills
• Captain your own charter

Also from Staff

• Tor Johnson
• Learn to Sail Better
• New boat: Aureus XV Absolute
• Remembering Hobie
• Catalina 275 Sport
• New boat: Saphire 27

• Cruising Compass Media Advertising & Rates
• Blue Water Sailing
• Multihulls Today
• Subscribe Today

Calculating the True Wind and Why it Matters

There are several ways to define the wind. For weather work at sea we care only about the true wind. This true wind is the speed and the direction of the wind relative to the fixed earth under the ocean. Tied up at the dock, we feel the true wind. Once we get underway, however, our own motion changes the wind we feel, and then it is called the apparent wind.

Many sailors claim that the apparent wind is all they care about, and that can be well argued when it comes to setting sails and judging performance. But to know about the weather patterns that are causing the wind and how the wind is changing, we need to know the direction of the true wind as accurately as possible. Shifts in the true wind are usually the first sign of changing patterns. If we do not figure this properly, we can miss an important shift. This is not a simple observation though, which is the point at hand. Slight changes in true wind speed affect boat speed, and in turn, the apparent wind speed and direction, which can easily mask a small but important shift in the true wind direction.

To help practically illustrate these points, let’s bring in some shorthand:

AWS = Apparent Wind Speed (relative to the boat) AWA = Apparent Wind Angle (relative to the bow, 0 to 180, starboard plus, port minus) AWD = Apparent Wind Direction (relative to true north)

S = Knotmeter speed (relative to the water) H = Heading (relative to true north)

DFT = Current Drift (speed, relative to fixed earth) SET = Current Set (direction it flows toward, relative to fixed earth)

SOG = Speed Over Ground (relative to the fixed earth) COG = Course Over Ground (relative to the fixed earth)

TWS = True Wind Speed (relative to the fixed earth) TWD = True Wind Direction (relative to true north)

Since wind directions are almost always discussed in terms of true directions, not magnetic, we’ll forget the compass for now and consider all directions as being true. Our actual use of compass directions in navigation does indeed complicate things a bit, but that can be sorted out later as it is not really related to the subject at hand.

To review the issues involved, we’ll start with a basic example. It can be dead calm at the dock, and I take off under power headed due north with my knotmeter reading 5.0 knots, and sure enough, I will have 5.0 knots of apparent wind right on the bow. S = 5.0, H = 000, AWS = 5.0; AWA = 0, implying AWD = 000.

If I then turn to H = 090, I will still have AWS = 5.0 and AWA = 0, but now the AWD = 090. So, if there is no wind at all, I am creating it all myself.

A bit closer to the point at hand, I could do this same thing, still with S = 5 knots and then I notice that I have AWS = 7.0 knots, still with AWA = 0. Something changed. I check the GPS and see that my SOG = 7.0 knots, and that accounts for the extra wind. I am in a current that is moving the boat at 2.0 knots. Now I need to look at the COG. If the COG is exactly equal to my Heading, then this current is directly on my stern, pushing me forward at DFT = 2.0, SET = 090.

Now, if I shut off the engine and slow to S = 0, with H = 090, I should see AWS = 2.0, still with AWA = 0, providing the COG = 090, and SOG = 2.0 (still H = 090), and I can conclude that I have measured two things: The true wind is calm, and the current is setting toward 090 at 2.0 knots. If this were not the case, one of these numbers had to be different.

When TWS is not zero, this analysis gets more complex and a vector triangle must be solved, but the key point is always the difference between COG and H. If COG = H, meaning you are moving the direction you are headed, then all of the standard vector triangle solutions for finding true wind will work fine. You just substitute SOG for the S that is in the equations or plotting routines.

Generally, these formulas and plotting routines solve for true wind angle (TWA) based on AWA, AWS and S. Then you apply the TWA to H to get the TWD. That all works fine in those cases, and there are numerous resources online, such as the one included in what we call the NIMA Nav Calculators, which is a free download at  www.starpath.com/navpubs .

Also, it seems to me that the typical equations we see in books (including my own) that use some form of the Law of Cosines might not be able to handle all the various combinations of directions in this case. It seems safer to get the answer from x-y coordinates, and so we present these formulas, written in a way that can go directly into a spreadsheet or calculator.

AWA = + for Starboard, – for Port AWD = H + AWA ( 0 < AWD < 360 )

u = SOG * Sin (COG) – AWS * Sin (AWD) v = SOG * Cos (COG) – AWS * Cos (AWD)

TWS = SQRT ( u*u + v*v )

TWD = ATAN ( u / v )

Remember, in a spreadsheet all the angles have to go in as radians, i.e. COG = COG(º)*(Pi/180). In a spreadsheet you can write AWD = MOD(H+AWA;360).

However, it is likely simpler to plot it with actual bearings, rather than as a relative plot using COG as 000. Below is a sample. It is the green triangle that you plot, i.e. plot SOG/COG and plot AWS/AWD and connect the end points to get TWS/TWD.

The spreadsheet format that computes everything will make it easier to experiment with various interactions of sailing and current to see how this affects the final outcome. You can download a copy of this spreadsheet with the equations in it at the tech support page for Modern Marine Weather ( www.starpath.com/weatherbook) .

Keep in mind that these measurements assume the instruments are calibrated and the wind sensors are located away from disturbing wind from the sails or other rigging on a power driven vessel. We have seen cases where the masthead instruments are affected by updrafts from the sails, which is why some race boats use a rather large arm holding the instruments a good distance off of the masthead.

One easy test is to measure true wind on one tack compared to the other tack as you tack back and forth in smooth water. This exercise might expose other important issues too, namely that your speed varies noticeably on each tack, implying that the speed sensing is not purely symmetric––assuming the sails are. In big waves, you often expect the speed to be different on opposite tacks, but if the speed sensors and sail trim are working properly, the true wind should be the same on each tack or gybe.

Ben Ellison has pointed out ( www.panbo.com ) that many performance sailors prefer to know the wind vector relative to the water as opposed to the fixed ground below it, which is what we would get by simply using S for SOG and H for COG. This is the way many instruments compute this data, though many models now offer options. He proposes to call this wind the Water Wind, as opposed to True Wind (which as shown above is not correct), and then do away with the term True Wind and call that Ground Wind.  There is clearly logic in this terminology, and it drives home the point that we should think on these differences. We will have to see how the sport and industry responds.

I am old fashioned in these matters, and will likely stick with true wind, and just check that it is computed properly when it matters. But then, I still use GMT when I should say UTC1.

David Burch is the director of Starpath School of Navigation, which offers online courses in marine navigation and weather at  www.starpath.com . He has written eight books on navigation and received the Institute of Navigation’s Superior Achievement Award for outstanding performance as a practicing navigator.

Administrator

You might also like.

• Fountain Pajot-Dufour Yachts Group Makes Commitment to Carbon Neutrality by 2030

Orca 2 Nav System Expands Capabilities

Replacing a Sailboat’s Propeller

Read the Summer-Fall Edition of Blue Water Sailing

Read the fall 2023 edition of blue water sailing, recent posts.

• What Capacity Pumps Are Large Enough?

• Race to Alaska Changes to Biennial Format in 2026

Please Visit Our Sponsor’s Webpages

• Media Advertising & Rates

Published by Blue Water Sailing Media, a division of Day Communications, Inc., Middletown, RI

Publisher & Editor: George Day

Blue Water Sailing Media publishes Blue Water Sailing magazine, Multihulls Today and other titles.

Cruising Compass Advertising Sales:

George Day, Newport, RI [email protected] 401-847-7612

• Arcona 460 Sinking Update. Builder Launches Inquiry
• New Channel Opened Through Baltimore Bridge
• Lagoon Launches New 60-Foot Cruising Cat
• Survey of the Week

© 2014 Blue Water Media. All rights reserved. | Admin

• Learn to Sail Memberships
• Private Lessons
• Online Learning
• Group Sails, Racing & Events
• Memberships
• Reciprocal Privileges
• Members Only
• Tahiti May 2024
• Croatia September 2024
• British Virgin Islands December 2024
• Skippered Sails
• Corporate Events

Apparent Wind

By andrew lesslie.

A question came up on the Spinnaker Forum about why the wind always feels less when it’s behind the beam.   This is an opportunity for us to dive into the concept of  apparent wind .

But first a quick definition.    True wind  is the wind we would feel if our boat were stationary, say if we were at anchor.  But if we’re out racing, we have our sails trimmed and we’re using that wind to create motion.     The wind we feel on our faces when we’re moving through the water is a combination of the air flow we feel from our own motion, plus the air flow from the true wind.  We call this combined wind the  apparent win d .  Apparent wind is the wind the boat feels and the wind that we trim our sails to.  We sail in apparent wind, so it’s kinda important that we understand it.

We can begin to appreciate the way that true wind speed and direction, combine with boat speed and heading to give us apparent wind and direction by considering the diagrams that follow.    In these illustrations we keep the boat orientation constant and rotate the wind around the boat.   Thus we preserve the sailors point of view as looking up the page in each example.

Credit : The illustrations I’ve used in this article were taught to me long, long ago by my Junior Race Program coach, Mr, Peter Conway.  I’ve not seen this style of illustration used in any books on sailing or racing, but think it offers a simple, accessible way to understand the relationships between True wind, Boat motion and Apparent wind.  While I’ve created these illustrations using computer tools, Peter would draw these on the concrete boat ramp using a yardstick, a piece of string and colored chalks.  As I was drawing each of these diagrams on my Mac, in my mind I could see the yardstick, string, chalk, and a half dozen wet, cold and shivering kids!

Calculating (or plotting) Apparent Wind Angle and Speed from True Wind Angle and Speed and Boat Speed and Direction

Note!  For ease, this explanation uses a simplified case of a constant boat speed and a constant wind speed.  Of course boat speed is not the same on all points of sail and we’ll expand the discussion to cover that later in the discussion. In this first case, let’s consider a vector diagram representing a boat at various angles to the true wind on Starboard tack.   For those who have forgotten high school trig, a vector diagram is an illustration where the length of a line represents velocity and the direction of the line represents an angle.  By considering the way the vectors combine, we can predict what we as sailors will experience on the boat.  Understanding how a boats motion combines with the wind is important to developing our understanding of how sailboats perform and especially to developing our racing strategy.

We can resolve vector combinations either by scale drawing or using mathematical formulae.    I’ve used scale drawings here so that we can lead the discussion into using Polar Charts later in the discussion.  I’ve included the mathematical formulae later so we can plug them into our computer or programmable calculator. (Or ignore them completely!).

First we draw a line representing our boat speed.   This is the blue line and it’s ‘five knots long’ and pointing directly up the page, representing our direction of travel.   We then draw an arc with its center at the tip of our boat speed vector, with a radius of 10 knots.   This arc represents the True Wind Speed and the range of possible wind angles we can sail, from close hauled to dead downwind.

We have selected four points on this arc and each represents a True Wind Angle.  We’ve plotted 45° (sailing to windward), 90° (sailing across the wind), 135° (a broad reach) and a mystery angle that we’ll get to in a little while.

This diagram helps us to readily determine the Apparent Wind Angle and Apparent Wind Speed simply by drawing a line from the base of the boat speed vector (the blue line) out to any point on the True Wind Arc.   The length of this line indicates the Apparent Wind Speed and the angle (in red) is the Apparent Wind Angle.  

That’s it.  It’s that simple.  Once chart and we can measure the AWA and AWS for any TWA at this Wind Speed and Boat Speed.

From this chart, we can see that when sailing at 45° to a 10 knot true wind we experience an apparent wind speed of 13.7 knots at an apparent wind angle of 30°.  These numbers are important to racers because we trim to wind speed and angle.  

Let’s consider other angles.    

If we sail at a right angle to the true wind, our AWS and AWA vector is the broken line connecting the base of the boatspeed vector to the 90° TWA point on the True Wind arc.  In this case we measure our AWS as 11.1 kts and our AWA as 65°

If we sail at 135° to the true wind, our AWS is 7.5 kts and our AWA is 108°

At our mystery angle where the apparent wind is abeam, we reverse the calculation.   By plotting, we connect the point on the TWS arc to the point of our boat speed vector and measure.   To sail with the apparent wind dead on the beam, we would sail to a TWA of 118°.   For example, if the wind is coming from 030°, we would sail a course of 30°+118°=148° to have the wind on the beam.

If we’re sailing one-design, these calculations help us understand if we’ll be able to carry a spinnaker on a given leg.   If we’re offshore racing on a boat with a full sail inventory, an idea in advance of the TWS and TWA to expect for each leg of the course gives us an idea which of our upwind or downwind sails we’ll want to have ready and which we put in the sail stack.

Let’s look at a downwind example, then use it to figure out whether we can carry our spinnaker in a Merit One-Design race.

Determining whether we can carry a spinnaker on a leg of a course

If we know from our experience that we can reach with our spinnaker to an apparent wind angle of 120° we can reverse the plot and work out what true wind angle we can carry it.   We can then compare the wind direction with the bearing of a leg on our race course and work out of the leg will be deep enough to set the spinnaker.   Let’s do that.

Now let’s put that into practice.    We’re racing out on the South Bay in a 10 knot wind that’s blowing from the NW.  The next leg of the course is from X to 2.  Will we be able to set the spinnaker?

We can use Navionics, OpenCPN, our paper chart or whatever to determine that the course from X to 2 is at a bearing of 175°(T).  This image is using the ‘measure’ feature in the free version of Navionics on the web ( https://webapp.navionics.com ). The data is the same as the mobile version and some kind user has put the SYC racing marks into the app.  

Other apps offer similar functionality, but you might have to add your own marks.

We also know that NW, when expressed in degrees is 315°(T) and the course from X to 2 is 175°T.  Yes, the math is simple in this case, but plotting it on a compass rose makes it crystal clear.

From our earlier calculation, we know the limit for our spinnaker is 145° and our calculation above says the TWA on the leg from X to 2 will be 140°.

This creates a tactical dilemma.   Do we try to fly or not?  Marginal spinnaker legs are challenging and we might opt for caution.  Or, we might hoist, sail as deep as we need to, knowing we’ll be sailing 5° or low of the mark and will need to drop the spinnaker and head up under jib later in the leg (consider this a strong tactical hint!).

Creating an AWA / BoatSpeed cheat card from a Polar Diagram

Post race discussions around the sailing club fireplace often turn to polar charts and how they can be used to improve racing performance.   I’ve never seen anyone consult a polar chart while out racing, but it’s certainly possible to extract the data and use it to create ‘reference cards’ for a given boat.

To keep this as relevant as possible to One-Design racing at Spinnaker, I’ve used the Polar chart for the J/24, as the performance is close enough in most conditions (The J/24 is stiffer and quicker upwind, but off-wind, they’re equal).

Below is a picture illustrating the different components of a Polar Chart.

The background of the chart shows a range of true wind angles and check marks radiating out from the origin that in this case represent boat speed.  The plot lines represent the speed that the boat can achieve at a given true wind speed and angle (boat speed curves).  A plot is drawn for each wind speed, as indicated on the lower axis.  This chart has plots for windspeeds of 8, 10, 12, 16 and 20 knots.

To read the chart pick a windspeed and a true wind angle.  Let’s use 10 knots and a TWA of 75°.  We look for the point where the 10 knot plot line crosses the 75° TWA line and we read off the value from the boat speed scale (distance from the origin).   We see that the prediction is that we should be able to sail at 6.5 knots. Easy, huh?

Where the date from polars is really useful is in downwind spinnaker legs, where we have an option to sail at a variety of angles to the wind.  It should be a given for races in a spinnaker fleet to know that the fastest way to get from the windward to the leeward mark is not a dead down-wind course.   Note that when sailing with jib and main only, the direct line usually is the fastest course.

What’s challenging about polar charts is that by convention, they’re plotted against True Wind, but we sail in apparent wind.  I have no idea why the decision was taken to plot against true wind, it wasn’t me who made that decision!  Plotting against apparent wind would have been much more useful.

So.    How can we convert from TWA to AWA? If you buy a boat with a high-end chart plotter, these calculations are built into the computer that lives inside it.  You merely have to load the polar data for the boat into the computer and it’ll tell you exactly what angles to sail and whether you’re sailing too high or low.   But where’s the fun in that?  And even if you have one, it’s good to understand the calculations it’s doing on your behalf, right?

Let’s take our J/24 polar diagram and overlay our true to apparent vector triangle.  We read the boatspeed directly off the speed plot and see it’s 5.1 knots.  So we draw the blue boatspeed line at 5.1kts and then connect the base to the arc representing 10 kts of true wind speed.   Then we plot the apparent wind line to the other end of the boatspeed vector and we have our apparent wind speed and direction.

So now we know TWA, TWS, Boatspeed, AWA and AWS for the optimal downwind angle for 10 knots of wind.   Why would we care about knowing the AWA? Because on a boat that has no instruments, but has a masthead fly (like, say a Merit 25) we now have a way to judge whether we’re sailing the best angle downwind.

In 10 knots of wind, we should strive for a boat speed of just over 5 knots at an apparent wind angle of 142°.

Building out a complete AWA, AWS table

We can repeat this plot (or calculation) for each windspeed that’s listed in the polar table and the outcome is shown below.   Note that this is a standard table built from the rating certificate for the J/24.  Also below is an illustration of TWA and Gybing angle.

How would we use this? When racing a Merit 25 with a spinnaker, there is much to be gained (or lost) on the downwind legs and judging gybing angles is less familiar to most skippers than judging tacking angles. If you like keeping things simple, here’s what you’ll want to know:

In light winds of 6-8 knots, sail downwind with the windex showing apparent wind 10°-15° behind the beam and  no faster than 5 knots .   If you’re going faster, head lower. In moderate winds from 10-12 knots, sail downwind with the windex showing apparent wind about half way between abeam and dead astern and no faster than 5.5 knots.   If you’re going faster, head lower. If sailing under jib and main ,  sail almost directly downwind, heading up only enough to keep the jib from collapsing ,  AWA 165°-175°

Doing the math For those who prefer calcs to measuring lines on charts, here are the formulae to resolve AWS (A) and AWA (?).

Learn About Upcoming Events

• 451 Seaport Court
• Redwood City, CA 94063
• 650-363-1390
• Rich Ferrari, The Chief
• Bob Diamond, School and Vacations Director
• Dana Ochstein
• Instructors
• Redwood Landing Marina

Copyright © 2021  Spinnaker Sailing Inc. All Rights Reserved.

Downwind Strategy: Use Apparent Wind Clues

Dave Dellenbaugh’s Speed and Smarts , Issue 130 (May/June 2014) has a very informative diagram showing how to use apparent wind clues to sail faster and smarter downwind. We present Dave’s diagram as a quiz, with his answers reprinted in the links below (used with permission).

Apparent Wind Clues – Quiz

Boats X and O are sailing downwind at their optimal downwind sailing angles. Note the direction of the masthead fly/telltales on boat O.  Answer the questions using the clues in the diagram below. 

1. What is the approximate direction of the true wind?  ( click here for answer )

2. Is Boat O in Boat X’s bad air? ( click here for answer )

3. Will Boat O intercept Puff A? (Assume Puff A is moving down the course in the direction of the true wind.) ( click here for answer ) 

4. If Boat O gybes and sails her optimal downwind angle on port, draw a line indicating her approximate heading. ( click here for answer )

5. If boat O gybes and sails her optimal angle on port, will she intercept Puff B? (Assume Puff B is moving down the course in the direction of the true wind.) ( click here for answer )

6. Draw a line indicating O’s current “ladder rung.”  ( click here for answer )

7. How can O use her apparent wind to determine when she reaches the layline for the leeward mark? (The layline is the line that allows her to sail directly to the mark at her optimal sailing angle.) ( click here for answer ) 

Key Principles for Using Apparent Wind

Apparent wind direction is indicated by your telltales/masthead fly.

The telltales and masthead fly respond to apparent wind, which is the combination of the true wind and the wind created by the boat’s motion. For more on this topic, see our post Understanding Apparent Wind: Visual Resources. 

Use the apparent wind direction to see what’s coming

The apparent wind is the “real” wind you are sailing in. When you look upwind to see what’s coming, look in the direction of the apparent wind.

• If an upwind boat’s sail plan is in the direction of your apparent wind, you are in that boat’s bad air, unless you are more than 5-10 boat lengths ahead.
• If a puff or lull is moving toward you from the apparent wind direction, you will likely intercept it.

Project your apparent wind direction to look ahead

When sailing downwind, projecting the direction of your apparent wind  forward gives you strategy clues.

• When your apparent wind points to boats ahead, you are giving them bad air and slowing them down. Use this to attack boats ahead.
• Your projected apparent wind shows your new heading if you gybe and sail the same relative angle to the true wind. Use this to determine when to gybe to sail toward a given point, such as the mark.  

Look at your wake to find puffs to intercept when gybing

Strangely enough, your wake points in the direction of the apparent wind you would see if you gybe and sail at the same relative angle to the true wind. Study Dellenbaugh’s diagram, item 5 , to see this. Use this to see puffs you might intercept by gybing.

The true wind determines who’s ahead

Just like in upwind sailing, you can tell who’s ahead on a downwind leg using the concept of ladder rungs. Ladder rungs are imaginary lines perpendicular to the true wind direction. On the downwind leg, boats on lower ladder rungs (further downwind to the true wind) are ahead.  

Use a line parallel to the ripples on the water determine the ladder rung you’re on compared to boats around you.  According to Dellenbaugh, you may also be able to use a line parallel to your boom, since many sailors trim their mainsail perpendicular to the true wind.

For more, see our post  Ladder Rungs: Understand the Race Course.with a Visual Approach

Related Posts

Play the Shifts Downwind – Check Your Understanding

Downwind Strategy Mistakes: Four Ways to Get Passed

Choosing the Leeward Gate: Race Course Notes

Leave a comment cancel reply.

You must be logged in to post a comment.

This site uses Akismet to reduce spam. Learn how your comment data is processed .

Insert/edit link

Enter the destination URL

Or link to existing content

No products in the cart.

Sailing Ellidah is supported by our readers. Buying through our links may earn us an affiliate commission at no extra cost to you.

True And Apparent Wind Speed Explained

True wind speed is the actual wind velocity measured by a stationary object. Apparent wind speed is the wind velocity perceived by an object moving through the air, such as a boat or yourself. In other words, apparent wind speed combines the actual wind and the effective wind created by your motion.

So why is this so important in sailing?

Let me explain.

Understanding the difference between true and apparent wind

What might feel like a chill and relaxed Sunday sail off the wind can turn into an intimidating and challenging experience if you have to turn around and sail upwind. When you understand how the wind works on a moving object like a sailboat, you can consider it when planning your trip or deciding whether to go or wait for better conditions. 

The angles we sail relative to the wind and the wind created by our speed over the ground equals the apparent wind we experience onboard. And on a sailboat, we sail and trim the sails according to the apparent wind.

Our apparent wind speed changes when we adjust the course and angle towards or away from the wind direction. Our boat speed also affects the apparent wind angle. This can be confusing in the beginning. To fully understand how the wind speed and direction affect us on a sailboat, we first have to familiarize ourselves with the compass rose and learn some terms.

The compass rose explained

We often talk about wind direction in terms of the cardinal points in the compass rose.

The most normal to use are the following:

Knowing your compass rose is essential when sailing and navigating at sea!

These are commonly known and used by most. However, specifying the angle in degrees is much more accurate. It is also what you will read from your sailing instruments and the weather forecasts.

True wind explained

So, precisely what is true wind?

True wind is the wind that affects you when you are stationary . When we talk about wind in general, we usually refer to  true  wind speed, direction, and angle. Using nautical terms makes it easier to identify the specific element we are talking about.

These are the terms used about true wind:

• TWS  – True Wind Speed
• TWD  – True Wind Direction
• TWA  – True Wind Angle

The wind speed is usually measured by a wind anemometer in knots, meters per second, kilometers per hour, or miles per hour. For us sailors, knots are the most commonly used measurement as it makes calculations regarding speed in knots easy. Here you have the conversions:

Wind speed conversion:

10 knots = 5.14 m/s = 18.52 km/h = 11.50 mph

If the wind is blowing 10 knots north-easterly, the wind comes  from  the northeast, which is a 45-degree direction (TWD), blowing 10 knots (TWS). If you sail on a south-easterly heading, the vessel’s angle to the wind (TWA) is 90 degrees.

However, since we move in direction and speed when we sail, the apparent wind always behaves differently than the true wind.

Apparent wind explained

So, precisely what is apparent wind?

Apparent wind is the wind that affects you when you are moving . It combines the true wind and the effective wind created by your motion. The apparent wind will vary from the true wind depending on the angle and speed of your movement.

Here are the terms used about apparent wind:

• AWS  – Apparent Wind Speed
• AWD – Apparent Wind Direction
• AWA  – Apparent Wind Angle

Think about this:

You are standing still, and the wind blows 10 knots. If you start to run against the wind, it feels stronger. If you run away from the wind, it feels weaker. That is because even though the actual breeze is consistent, the apparent wind changes.

Let’s give some examples.

Apparent wind vs. true wind

Imagine the following scenario:

The  TWS  is  15  knots, and the  TWA  is 0 degrees, or straight from the north. You are sailing at 5 knots straight south, away from the wind. Your  AWS  will, in this case, be 10 knots.

If you turn around 180 degrees and motor straight north into the wind, your  AWS  will increase by the 5 knots of speed you are making and rise to 20 knots.

So, what about the angles?

Let your brain process this for a bit.

The elements work against us when we sail to some degree. This is where most fresh sailors’ plans fail. I have been there myself. However, don’t be disappointed in your boat’s ability to sail close angles. You can read more about how high a sailboat can point here.

Just take the facts for what they are and work with them to plan your routes.

The correlation between actual and apparent wind speed

Finding the correlation between actual wind speed and apparent wind speed in an angle requires us to calculate some vectors, which is unnecessary.

Luckily, Most boats today have wind instruments that do this job for us and show us all the information we need.

However, if you want to squeeze your brain around vector calculations, this is a good source from Spinnaker Sailing.

The most important thing is to know how to use this information. When you sail downwind, it might seem like it is just blowing a breeze. But that is only until you turn around and realize your apparent wind has increased significantly. 

You need to know and prepare for this. Maybe you should reef your sails before turning around?

When you sail, you sail to your apparent wind, as that is the wind that affects your boat in motion. But always keep an eye on the actual wind speed, especially when sailing downwind!

The video below shows an example of smooth 20 kt wind on a 90-degree beam reach while enjoying the Caribbean waters sailing above 8 knots!

Final words

Most people are familiar with true wind or actual wind. Understanding apparent wind is a bit more complicated. I hope you now understand the difference between the two and why it matters. Now, it is time to continue and learn how to sail at different angles to the wind. That brings us to the five points of sail, which you can read about in this guide .

Sharing is caring!

Skipper, Electrician and ROV Pilot

Robin is the founder and owner of Sailing Ellidah and has been living on his sailboat since 2019. He is currently on a journey to sail around the world and is passionate about writing his story and helpful content to inspire others who share his interest in sailing.

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Apparent Wind

The very first sailing physics topic that I need to convey to you is the idea of apparent wind velocity. This notion is absolutely fundamental for an understanding of sailing vessel movement. Fortunately, it is a simple idea and one that already will be very familiar to those of you who sail. I will state the case, nevertheless, because we will require the concept very soon when seeking to understand how wind provides drive for square-riggers. Even if you are thoroughly at home with apparent wind, there may be something for you to learn in this section, and it will serve to tell you about my notation.

Albert Einstein was a keen recreational sailor (fig. 2.1). This fact is pertinent to my discussion of apparent wind because apparent wind is all about relativity, a subject that the good professor (who appeared a little windblown most of the time) knew something about. Interestingly, the relativity theory pertinent to apparent wind is not that of Einstein but of a much earlier scientist whom Einstein greatly admired: Galileo. Galilean relativity is basic common sense: if you are driving along at 30 mph and another car is coming toward you at 40 mph, then your relative speed is 70 mph. Prof. Einstein would disagree,1 but that's life; we will go with Galileo. Now, to extend this idea to a two-dimensional sea surface rather than a one-dimensional road, we need to describe velocities by vectors. The notion of apparent wind is explained simply in vector notation in figure 2.2a.

I need to be careful about nomenclature: to a physicist speed refers to how fast an object is moving, whereas velocity refers to both how fast

and in what direction it is moving. So, in figure 2.2a the arrows represent wind velocity, whereas the arrow lengths represent wind speed. The angle between boat velocity, v, and true wind velocity, w, is avw. (Throughout the book I will denote vector quantities like velocity with underlining.) The wind velocity felt by the moving boat is not w but rather w% = W—v; the boat moves through the wind and so feels a wind velocity that depends on its own speed and direction, as well as that of the wind. Consider what happens if aw = 0°, in other words, if the boat is

Figure 2.2. (a) Our boat moves with velocity v, in a wind with true velocity w. She feels an apparent wind w'. (b) The boat has now changed direction and is heading upwind with the same speed: note how the apparent wind speed increases. (The dashed line shows how vector addition works.) In both cases the boat heading is closer to the apparent wind direction of origin than to the true wind direction of origin.

running before the wind. In this case she feels a following wind of reduced speed, as you might expect. If, as illustrated in figure 2.2a, the point of sail is a broad reach, then apparent wind velocity, W, has a different direction, as well as a different speed, from true wind velocity, w. If, as in figure 2.2b, the boat is close hauling (i.e., sailing into the wind), the apparent wind speed is greater than true wind speed. In the two cases shown in figure 2.2 the boat speed is the same; only the direction is different. Yet this difference produces very different apparent wind ve locities. The effect of boat velocity is always to make the apparent wind seem more head-on than the true wind, which is fairly obvious if you think about it.

Continue reading here: Wind Force Boat Force and Isaac Newton

Was this article helpful?

Related Posts

• Analysis Square Rigged Ship Motion
• Apparent Wind vs Course - Catamarans Guide
• Understanding Apparent Wind
• The Age of Exploration Carrack and Caravel
• The Slot Effect - Science of Sailing

• Create an account

Remember Me

• Forgot your password ?
• Forgot your username ?

Fields marked with an asterisk (*) are required.

Today's Open/Close Times based on tide predictions

red means the Club will be closed. Note that current low tides are around 0.4 feet higher than predictions.

Day Leader's WhiteBoard

Demystifying Apparent Wind - Part 1

A very common problem my students have voiced is that they have trouble figuring out where the wind's coming from. On the one hand, it shouldn't be so hard - turn your head and feel the wind on your ears. When it's the same on both ears, you're either looking straight into the wind or straight away from it. Or (less accurate) look at the yarn on the side stay.

Yet it's not so simple. You think of the wind as coming from the same direction, but when you sail, it doesn't seem to do that. It seems to shift a lot. And "you told me we couldn't sail higher than about 45 degrees into the wind, but that yarn on the stay has a much smaller angle than that". And (yet more observent) "We're sideways to the waves, so we should be on a beam reach, but the yarn says we're still somewhat upwind".

Welcome to the world of apparent wind. This is a simple concept to explain and  understand at a high level, but very hard to get at a detailed level. Everyone gets the simple explanation - you're peddling a bike at 10 knots on a calm day, what do you feel? A 10 knot headwind. The speed you're generating adds to the wind speed to create the wind you feel, the apparent wind. The apparent wind is what you and the bike feel. Peddaling 10 knots in a 10 knot headwind, and you're pumping against a 20 knot apparent wind. Doing the same in a 10 knot tail wind, and Bob's very much your uncle.

Those with a math background easily grasp that this is a vector algebra problem - the boat wind speed adds up with the true wind speed as vectors, where both the speeds and the directions interact. But even if you get that, it's truly hard to see how it all plays out on the water. And I'm speaking as someone with a graduate education in mathematics. In editing this blog, I realized that I had messed up a calculation in my first draft. If you sit down to do the calculations, you have to determine what your boat speed will be at a given true wind speed and point of sail (angle of the boat to the wind). Polar performance diagrams will show this, but good luck finding these for any of our dinghy's.

So my point is that it's quite difficult to build a mental model of this. Instead, you can get a feel of how it happens on the water without trying to understand why, exactly. I did the math from a guess of a polar diagram for a boat similar to a Laser Bahia at 5 knots wind. If you're not racing, it doesn't matter how accurate this is. But it should be pretty typical.

Remember that the wind the boat feels (and you feel) is not the true wind, it's the apparent wind. So you're trimming the sails for the apparent wind, and the true wind is in practical terms irrelevant.

I'll work through some examples at various points of sail. Here's what it looks like close hauled:

You're sailing 45 degrees to the true wind, but you're sailing about 30 degrees to the apparent wind. Look at the yarn on the side stay, and you'll see that angle. Remember it, as it's one guide to show you that you're going as high as you can into the wind (a better guide is the jib tell-tales when the jib is sheeted in hard, but unless you're sitting forward in the boat, you may not be able to see them - in that case, a good crew can really help).  It feels really fast, as your boat speed is pretty much addingdirectly  to the true wind speed to produce apparent wind speed. If you're trimmed correctly, your main sail is sheeted in hard, so the lift force from the sail (perpendicular to the sail) is more or less sideways to the boat, causing a fair amount of heeling force, which you have to balance with your weight.

Now let's go to beam reach:

Beam reach is when the apparent wind is 90-degrees (sideways) to the boat. Because of the way boat speed and true wind interact, you've got to go downwind 130 degrees to the true wind (40 degrees past perpendicular) to get to a beam reach. Beam reach is the fastest (non-planing) point of sail. The sail is still working like a wing (more power than the downwind parachute mode), but the properly trimmed sails are out farther, so the lift force (always perpendicular to the sail) is more aligned with the direction the boat is going. More propulsion, less heel. It doesn't feel as fast as going close-hauled, as the apparent wind speed is less. But the boat speed is faster.

Now let's go downwind to a dead run:

As we turn through a broad reach and get the wind behind us for the first time, everything seems to slow down. That's because when the wind is behind us, our boat speed cancels some of the true wind, so the apparent wind speed slows down. Note that here it's less than half of the apparent wind close-hauled with the same true wind. If you're racing, you'd better be prepared for a big increase in apparent wind speed as you round the leeward mark.

Here's an animation that puts all of this together (click on it to see it):

How much does any of this matter? If you're an experienced sailor, you just know it. You're tuned to the apparent wind, and you're setting your boat for that. So when you fly the kite, the apparent wind moves way forward, and you deal with that. But if you're just learning sailing, it does matter, as it explains the confusing things you're seeing on the water. It gives you a visual model of sail trim.

When you subscribe to the blog, we will send you an e-mail when there are new updates on the site so you wouldn't miss them.

About the author

John Bongiovanni

Related posts, separating the gybe from the turn, slow sailing, treasure island cruise, sailing small circles, how to make the most of your weight in high winds, comments 14.

The yarn on the shrouds will also tell you the precise apparent wind direction, in anything over a few knots.

It would be a great senior project for someone to make some polars for our dinghies! There are enough people in the club with GPS trackers and portable anemometers that this should be feasible.

What is a "polar"?

Great idea, Nathan.It's not hard to do (conceptually). I'd certainly help out anyone who wants to try it.

That didn't work, try here.... https://books.google.com/books?id=iQQoAAAAQBAJ&pg=PA377&lpg=PA377&dq=29er+sail+polar+performance+diagram+bethwaite&source=bl&ots=5SRWSwo4mB&sig=MLWFyGWK9jA2KmZu7HzpKjNmDKE&hl=en&sa=X&ved=0ahUKEwii44yI6b3JAhVD12MKHZVnC1kQ6AEIJjAB

This is what I used for the hypothetical dinghy in the blog: [url]https://jjbong.smugmug.com/Other/CSC/Boat-Speed/n-DtfQhK/i-H4gWz7w/A[/url]

Hi John. Great writeup, thanks for sharing. One question, you wrote: "Beam reach is when the apparent wind is 90-degrees (sideways) to the boat. Because of the way boat speed and true wind interact, you've got to go downwind 130 degrees to the true wind (40 degrees past perpendicular) to get to a beam reach." All the diagrams I've seen define beam reach as 90 degrees to the true wind, not the apparent wind. I thought that if you're at 90 degrees to the true wind, no matter how fast you go, you're still on a beam reach... you don't have to keep going down to stay on a beam. Isn't that correct?

Yes, I use a diagram in "ground school" to teach points of sail, with a wind circle (true wind) and a model sailboat. So I guess I teach that a beam reach is 90 degrees to the true wind, but I do that because to do otherwise would be too confusing. And students don't know enough to make the true/apparent distinction. Let me put it a different way. If you're sailing, how do you know you're on a beam reach? You can look at the woolen tell-tale on the side-stay and see it at 90-degrees to the boat, or you can feel the wind equally on both ears as you look perpendicular to the boat, or whatever. What you're looking at is the apparent wind. Next time you're out in any visible waves, note the angle of the boat to the waves when you're on a beam reach (the waves are approximately coming from the true wind direction), and you'll see that on a beam reach you're more than 90-degrees to the true wind. You and the boat feel apparent wind only. It's not easy to tell where the true wind is coming from. Let's assume it's constant direction and velocity (never!). In the moment, there's no way you can determine it (other than looking at a windsock or a smokestack - there's nothing on the boat that indicates it). At best, you can tack close-hauled to close-hauled, noting the course on each tack and splitting the difference. I hope this makes sense.

To learn true wind, head into the wind until the boat stops and look at the jib.

Indeed, more efficient than tack and split the difference. But it still shows that there's nothing you can look at in the moment to see the true wind (the wave direction is an approximate average historical, but not instantaneous).

Great article. It can also be trippy if you're sailing a catamaran or skiff and your apparent wind lets you sail faster than the actual wind speed!

If the wind is 20 knots and the boat speed is 10 knots and we're on an apparent beam reach, how far off the true wind are we?

It's a (relatively) simple trigonometry problem. The answer is 120 degrees. Here's the math: https://jjbong.smugmug.com/Other/CSC/Boat-Speed/n-DtfQhK/i-3n8zzSs/A

Understanding true vs apparent wind

Here is an easy description of true wind and apparent wind. Put you hand outside the window of your car traveling at 60 miles per hour on a still day and your hand will feel a 60 mile per hour wind. That’s apparent wind yet the true wind is zero. What if the car was driving into a 20 mile per hour head wind? Your hand would feel 80 mph. Or if the wind was blowing from behind at 20 mph, your hand would feel 40mph.

Now what about a cross wind of 20 miles per hour? Well we need to do a little Pythagorean theorem work on this. What is the square root of the sum of 60 squared plus 20 squared? Your hand would feel 63.24 mph and mostly from a direction in front of the car. If the car accelerated to 100 mph your hand would feel 102 mph again mostly from the front. If the car decelerates to 10 mph your hand would feel 22 mph mostly from the side of the car and if he car stopped you’d feel the full true wind of 20 miles per hour from the side of the car. What ever your hand feels is the apparent wind. The apparent wind equals the true wind when your car is not moving.

When determining direction of the wind, the faster the car goes the more the apparent wind direction comes from the direction of travel of the car. Again imagine the cross wind. At 1 mph the apparent wind feels almost like the true wind from across the car. As the car accelerates the wind feels more and more like it is coming from the front.

This is similar to a boat. The faster the boat sails into the wind, the more the apparent wind speed increases and the more it feels like it is coming from the front of the boat. As a general rule of thumb then, when sailing the true wind is about 15 degrees more towards the back of the boat. IE point to where you feel the wind is coming from then point 15 degrees further back and that is about where the true wind is coming from.

• Recent Posts

• RYA Day Skipper with NauticEd - April 1, 2024
• NauticEd uses the SailTies GPS Tracking App - March 29, 2024
• Sea of Cortez Flotilla – February 2025 - March 8, 2024

You might also like

The sentence “As a general rule of thumb,….” describes the direction of true wind in exactly the opposite way of your illustration. The illustration shows that true winds is generally 15 degrees ahead of apparent wind. Which one is correct, the illustration or the text?

Good question, one that had me thinking for a second. Actually, the text and the illustration are both right. the text actually says the true wind is 15 degrees more towards the rear of the boat.

It’s a matter of the addition of the vectors. If you look at the actual angles, the true wind is coming from behind more than the apparent wind. I’ll answer it in the next blog rather than here so that I can add an illustration to explain better. But essentially if you are sailing along and point to the wind that you feel (apparent wind) then point about 15 degrees more towards the rear of the boat – you’ll get the approximate true wind. Again – this is a general rule and Pythagoras takes precedent here.

Comments are closed.

TWEET ABOUT

FIGHT CHILDHOOD CANCER

NauticEd is a fully recognized education and certification platform for sailing students combining online and on-the-water real instruction ( and now VR ). NauticEd offers +24 online courses , a free sailor's toolkit that includes 2 free courses, and six ranks of certification – all integrated into NauticEd’s proprietary platform. The USCG and NASBLA recognize NauticEd as having met the established American National Standards. Learn more at www.nauticed.org .

The NauticEd Vacations team are Expert Global Yacht Charter Agents – when you book a sailing vacation or bareboat charter through NauticEd, we don’t charge you a fee – we often save you money since we can compare prices from all yacht charter companies. PLUS, we can give you advice on which destination or charter company will suit your needs best. Inquire about a Sailing Vacation or Charter .

Online Sailing Courses Sailing Vacations | Charters Practical Sailing Courses Sailing Certification | License

Sign up for 2 FREE Sailing Courses Try sailing in Virtual Reality! Gift a Friend a Sailing Course Sailing Events | Opportunities

About NauticEd Contact Us NauticEd Support Privacy Policy