Americas Cup Apparent Wind

Posted by Director of Education on September 10, 2013 under About NauticEd, Crew, Skipper | Comments are off for this article

Apparent Wind on Americas Cup AC72’s


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

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:

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

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

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

  • Electronic Navigation Course

    Electronic Navigation Course

    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.

Weather Helm Vs Lee Helm – What is it? How to use it?

Posted by Director of Education on June 16, 2010 under Crew, Skipper, weather | Read the First Comment

During the Americas’ Cup campaign in New Zealand in 2003, I saw one of the best explanations of this on a TV interview with the Greg Butterworth, the Tactician for the Alingi Team.

Most of us sort of understand the concept and we’ve been left with the answer of “Well – weather helm is better because it’s safer.” But few explanations go into how it gives your boat a sailing advantage.

The definition of weather helm and lee helm is simple and it is easy to remember which is which. If you have a tiller, weather helm is when you have to pull the tiller to weather (toward the wind) in order to keep the boat going in a straight line. Lee helm is when you push the tiller to lee (downwind) in order to keep the boat going in a straight line. We’ve probably all felt this slight pressure required on the tiller when underway.

Your boat can be tuned to give weather helm or lee helm. Rake the mast forward  and you move the center of effort of the wind forward which causes your boat to want to turn downwind. Rake the mast back and you move the center of effort of the wind back causing your boat to want to go upwind to weather.

When your boat gets rounded up – you just experienced massive weather helm. No matter how much you pull the tiller to weather, you can’t stop the boat going to weather. Dumping the main sail moves the center of effort forward thus reducing the weather helm.

The basic perception of weather helm being safer comes from this effect: if you let go of the tiller, it will automatically go to center because of the water flowing over the rudder and because the rudder is pivoted at its leading edge. Now there is no rudder force to counter the desire of the boat to turn up wind to weather so the boat does exactly that. It turns to weather and rounds up slowing the boat down and reducing forces on the rig. Conversely, lee helm  means that if you let the tiller go the boat will turn away from the wind, heel over more increase forces on the rig.

So from a safety point, weather helm is good. BUT there is another advantage that we’re not generally taught. Holding the tiller to weather means that there is a slight pressure on the rudder to windward. This actually MOVES THE BOAT TO WINDWARD as it slices through the water. And we all know what that means, race advantage!

The Weather Helm Advantage

The Weather Helm Advantage

The illustration shows how the water pressure from weather helm creates a sideways force on the rudder tending to push the boat to weather.

Now Greg Butterworth went on to explain that there are other cool things you can do. One is to put a little trailing edge swinging control surface on the keel.

The illustration below shows this effect too. For us pilots, this is much like a trim tab on a wing of a small airplane. The trim tab creates the ability to adjust the lift at that point on the aircraft and thus create a balance of forces. The issue to remember here is that you’d need to trim the tab the other way when you tack over.

A control surface on the keel

A control surface on the keel

So there you have it. While we’ve all been understanding the lifting effects of the wind over the sail, the other fluid that we’ve ignored is the water under the boat and how we can gain lift from it too.

Next time you’re out sailing on a nice steady 10 knot breeze, come up on a close haul, trim the sails perfectly so that all your tell tails are flying smoothly. Then notice what pressure you’ve got on the helm. Note that if you’ve got a wheel, weather helm will be a tendency to apply downwind turning pressure on the wheel (which is the same as pulling a tiller upwind right?). Ideally you should have slight weather helm. If not, you should probably not jump right in and start raking your mast back. Talk to a mast tuning specialist in your area first.