# Apparent Wind on Americas Cup AC72’s

See here for the TV Schedule for all countries. Who’s taking it home?

AmericasCup.com

So how do they do it? I was watching it on Sunday – the wind was at about 15 knots, yet the boats achieved 43 knots angling downwind and 25 knots angling upwind. Wow – this is the most impressive thing in yachting. The engineering design effort and atmosphere over the past three years must have been intense, ground breaking and so innovative. This is the stuff that engineers and yachties live for. Even if it is to sit back and just watch (eyes wide open).

Back to the question – how do they do it? Now that we’ve been seen to remove the drag factor almost entirely by introducing the foils and getting the hull out of the water, we’re seeing that the limitation on boat speed is not hull design or even wind speed as one might have thought. It comes down to the angle that a boat can go into the wind.

First consider this – As the boat goes faster and faster, the wind that the boat feels “shifts forward”. Every one says that but how? what does that mean?  Here take a look at this:

http://www.nauticed.org/freesailingcourse-m1-2

It is the wind shifting forward as a car accelerates. The wind that the car feels is the shifted wind – called the apparent wind. “Apparently” this is what the car feels. So, same on a boat. The boat feels the new wind generated by it’s own speed. Unless it is stopped dead in the water, a boat will never feel the actual true wind. But the limiting factor is how much forward force can the winged sail garner out of the wings from the angle between the boat heading and the direction of the apparent wind. That angle can never be zero else the boat would be moving straight into wind and that’s not possible under these universal laws. The lift or forward component of the force to drive a boat forward relies on this angle. In traditional sailboats, this angel is about 30 degrees off the apparent wind angle.

As an extreme example, ice sailing sleds with vertical wings and only thin blades touching the ice can achieve an 11 degree angle off the apparent wind. Airplanes with an asymmetric wing shape can gain lift with around 5 degrees off the airflow. Now with hydrofoils mounted on the Americas Cup AC72 sailing catamarans, we’re seeing sub 20 degrees off the apparent wind.

With  few rudimentary calculations then  and with out calling the design teams at Team New Zealand or Oracle I calculated that the apparent wind must be about 19 degrees on a downwind angle and about 16 degrees on upwind. This was done by observing their tacking and gybing angles and plugging it all into the sine and cosine formulas. These formulas solve for the following  obtuse triangle. We observed the true wind speed, the boat speed and the angle off the wind and thus we solved for the remaining.

Here it is for the AC72 heading downwind

AC72 Americas Cup True vs Apparent Wind – Dowwind

And here is is for the AC72 heading upwind

Americas Cup AC72 True Vs Apparent Wind – Upwind

Given these formulas, there was no other solution than to come up with about 19 degrees of apparent wind heading downwind and 16 degrees apparent wind heading upwind. You might have also observed how tight in the sails were trimmed in the “downwind” heading. They were tight – even the non-winged headsail. This means the AC72’s were on a close haul heading downwind. Messes with your mind doesn’t it.

Now there are a lot of other factors that play into all this like sideslip, tide, etc and I’ve not done those calculations so please consider this as completely rudimentary. I only have a Masters in Engineering – and you can bet a whole pile of dimes that a PhD guy will come back to me on a full explanation. With all the equations and diagrams – please do – I’ll publish it here – so long as it’s not too complicated. What I’ve attempted to do is explain the question presented above. How does an America’s Cup AC72 go faster than the speed of the wind?

Here is an animation of the AC-72 performing tacking and gybing maneuvers. Watch the wind vectors through the maneuvers and also watch the boat speed increase as it bears away.

Interactive Animation

BUT WAIT – Look at the Apparent Wind!

What I’m seeing here is that the apparent wind vector is shorter than the boat speed. How can that be – the apparent wind speed less than the boat speed? Huh? Well actually who cares? Wind is just a force – it does not matter its speed. You now have to release your self from the boundaries of old monohull sailing with big hull drag. The only factors we are now dealing with is drag from the hydrofoils and the force that the wings can eek out of the wind it sees. As the apparent wind gets closer and closer to the front of the boat the accelerating force reduces. As the boat speed increases, the drag force increases. The boat will stop accelerating only when the drag force = the accelerating force.

The following animation shows the drag force increasing with speed and the accelerating force from the wings reducing. Note that there is a step jump down in drag force as the boat begins to hydrofoil. Had the drag force equalled the accelerating force before the boat hydrofoiled then the boat would not continue to accelerate.

Interactive Animation

So there you have it. With limitations on our minds now removed  – you can see that boat speed can just keep on increasing and increasing until the forward component of the force from the wings equals the drag force on the boat. As the boat gets closer and closer to the apparent wind angle the forward force component reduces. True wind speed matters only in that it helps to get the boat up on the foils but at high boat speeds, by looking at one of the vector diagrams above, an increase in true wind speed will only  marginally help to increase the apparent angle which marginally increases the forward forces. The biggest revolution has come from the introduction of the wing being able to gain lift from closer angles to the apparent wind AND in a massive reduction in drag. Think about drag next time you’re towing a dinghy behind your cruising boat on a close haul. This requires an increase in force and the only way then to maintain speed is to bear way – whoops.

With all this, I couldn’t help it – I’ve thrown in a basic (very basic) interactive animation of AC72  going around the course showing the above diagrams at strategic points around the course.

Interactive Animation

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If this really got your juices going then I highly suggest our Electronic Navigation Course.

The Electronic Navigation Course is Laid out in Eight Modules

Module 1 emphasizes that electronic navigation is an aid to the good sailor’s senses but can not replace them.

• Module 2 introduces and brings back some of the basics that you should already be familiar with in regards to navigation.
• Module 3 is an in-depth discussion of wind. In particular it delves into the calculation of true wind and shows how important true wind direction is when navigating.
• Module 4 is all about boat speed. How to navigate using optimum speeds and how to find your best course to achieve your destination in the fastest time. We define velocity made good on course and velocity made good upwind.
• Module 5 prepares you for the shotgun of jargon that will be delivered in module 7.
• Module 6 introduces technologies such as AIS, RADAR and Weather GRIBS and electronic chart overlays.
• Module 7 is a step by step walk through of a real GPS chart plotter unit. You’ll gain the confidence, working knowledge and user experience, through various animations, to fully work a chart plotter device and apply this to your sailing navigation.
• Module 8 will round everything out so that you’re confident in your ability to navigate using electronic instruments.

# The Wind Triangle: Apparent Wind vs True Wind

Posted by Director of Education on October 2, 2012 under Skipper | Comments are off for this article

Apparent wind is a vector summation of the boat velocity and the true wind velocity.This animation shows how the apparent wind changes with boat angle to the wind given no change in boat speed and true wind velocity.

Sailing concepts like this are discussed in depth in our NauticEd Skipper Course.

Click on the various points of sail buttons and the Animate button.

# Stopping Rounding Up Dead in it’s Tracks

Rounding up is caused by many factors. One is too much wind and force aloft which tends to heel the boat over. This reduces the amount of rudder in the water and thus the rudder’s effectiveness. Another factor in rounding up is the center of pressure of wind on the sails is too far aft which then pushes the aft of the boat downwind and thus the front of the boat up wind.

The NauticEd SailTrim clinic discusses this topic and so what we wanted to do was test it out for sure. So last weekend we took out a friend’s Beneteau 373 to test out an anti-round up theory. Read on to find out the results of our experiment.

First though, we must first understand wind shear. The phenomenon of wind shear is pretty easy. Wind moves faster at the top of the mast than is does at the water level because the stationary water slows the down the wind in close proximity.

Secondly,  consider the concept of true wind vs apparent wind. Which is best understood by imagining driving your car in a cross wind with your hand out the window of the car.   At stand still you would feel the wind coming from the side of the car. The faster you go, the more you feel the wind coming from the front of the car. But when a gust of wind comes (which is just an increase in true wind speed) then you would feel the wind shift back more to the side. When relating this to a sailboat, if your boat was standing still, the wind at the top of the mast would be the same apparent direction as at the cockpit level albeit, faster (from the wind shear phenomenon). However as your boat picks up in speed the apparent wind moves forward BUT because of wind shear it shifts forward less at the top of the mast. IE at the top of the mast the wind tends more to the direction of true wind direction because the true wind speed is higher.  Thus at the top of the mast the true wind is more aft than apparent wind. Aft means it is coming from a direction further towards the back of the boat. Get it?

So – whether you get it or not. The fact is: at the top of the mast the wind is higher in speed and more aft than at the cockpit level.

Figure A and B show the boat speed, true wind and apparent wind vectors for cockpit level and top of the mast. Obviously in both cases, the boat speed vector must be the same. The true wind vector is obviously the same direction but due to wind shear it is longer (faster) at the top of the mast. This results then in the apparent wind direction being more aft. IE in this case from 135 deg to 125 deg.

Wind shear and apparent wind phenomenom

Thirdly, you should understand that if a sail is sheeted in to tight it creates more heel. This then is exactly what is happening at the top of the mast.  Even though at the bottom of the sail you may have perfectly trimmed the sail, the top of the sail is sheeted in too tight against higher wind speed. No wonder you’re getting excessive heeling. And excessive heeling creates round ups.

This is now quite a revelation! It means that the top of the main needs to be “out” further than the bottom of the sail for it to operate efficiently. This  is usually indicated by the top telltale. Often the leeward telltale will be stalling at the top of the sail. Especially in high wind because of the phenomena above.

The top of the mainsail needs to go further out so that the starboard telltale can fly smoothly

Thus the top of the mainsail needs to be let out further so that the leeward telltale can fly smoothly. This is commonly referred to as twisting the sail out at the top. Except people believe you are just spilling out (wasting) the wind at the top. Not quite so now, as you’ve just learned. Twisting out the top of the sail is letting the top of the sail fly according to the direction of wind it is feeling.

In the illustration, you can see the top telltale on the downwind side is fluttering. If you let out the main at the top, the wind can reattach to the sail on the leeward side and the telltale will fly smoothly reducing the force aloft.

Understanding all the above. How do we stop rounding up?

Option one: Obviously the first and safe option in higher winds is to reef the sail.

Option two: Let out the traveler which is what most people do when hit by a gust. Just so long as you realize what you’ve done is not twisted the top of the sail out – all you’ve done is let out the mainsail from top to bottom and thus depower the mainsail. This reduces the force aloft and thus the heel. It also moves the center of effort of wind on the sails forward which reduces tendency to round up. The trouble is that you spend all day fighting gusts with still quite a few involuntary round-ups.

Option three: Let out on the mainsheet. Here again you’ve depowered the entire mainsail to handle the gust. Still, it works.

Option four: Permanently reduce the force aloft by letting out further on the mainsail and tightening up on the traveler. The trick here is to bring the mainsail bottom back in again using the traveler. Yes, bring the traveler to windward up past the center point. Most sailors are reluctant to do this because they’ve been taught that it detaches the wind on the leeward side. But not when you’ve let out the mainsheet. In effect, by letting out on the mainsheet, you’ve allowed the boom to rise up and the leech of the sail to slacken. This creates the desired twist at the top and allows the top of the sail to fly according to its apparent direction. At the same time, the bottom of the sail can fly according to its apparent direction.

By trimming the traveler and mainsheet together  you can manage the twist at the top of the sail as desired yet still keep power on the bottom of the mainsail. Keeping power on the bottom of the mainsail keeps your speed up which also increases the effectiveness of the rudder. Increasing the effectiveness of the rudder means it can hold more against any turning effect created by the shifting of center of pressure backwards. Wow – see how it is all connected.

What happened on our 15 knot gusty sailing day? Well, not one round up.

So to summarize, the sailing lesson here is when in higher winds bring the traveler up and sheet out the main. You’ll also need to release the boom vang a little. Letting the boom vang out allows the boom to rise which loosens the leech (trailing edge) of the sail and allows the top part to “twist out”.

This and many other finer sail trim concepts are discussed in NauticEd’s Sail Trim Clinic.

# True Wind versus Apparent Wind

Posted by Grant Headifen on April 3, 2010 under Crew, Skipper | Read the First Comment

We took this from a section in the Crew and Skipper Courses to easily explain the difference between true wind and apparent wind.

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Section 7.2 – True Wind versus Apparent Wind – Explained

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.

wind vectors, true versus apparent

# Further elaboration of true vs apparent wind

Posted by Grant Headifen on February 7, 2009 under Crew, Skipper | Be the First to Comment

There has been a request for further elaboration of the wind vectors as to why the true wind is always behind the apparent wind. Previously we used this diagram and the confusion is that the true wind vector is in front of the apparent wind vector.

The following diagram explains the vectors better perhaps. The answer is to look at the direction the arrows not their relative position on the diagram.

wind vectors, true versus apparent

Fig A and B are the same. But now in Fig B it is easier to imagine standing on the boat and holding your arm out pointing to the direction of the wind that you feel (apparent wind). Then taking your arm and pointing further aft. This is the direction of the true wind. The rule is consistent. No matter which sail point you are on, the true wind is always behind (coming from further aft) than the apparent wind. Again, think of the car example in the previous blog on this topic.

This morming as I am writing this blog an intersting second example came up. A friend just called to ask if I’d like to play tennis. However, it is quite windy outside today (good day for sailing). In the mucho talk that proceeded, he said that he hits the ball so hard that the ball would not deflect very much in the wind. If his flabby muscles really could hit the ball that hard, then he is correct  due to this true vs apparent wind discussion here. If the (true) wind is blowing from across the court, a faster moving ball feels less side wind and more wind from the front. A slower ball would feel more of the side wind and be affected more. As the ball travels, the true wind is coming  from the cross court direction where as the apparent wind (what the ball feels) is from the direction of my flabby friend.

# 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.

The following diagram illustrates this.