Saturday, April 30, 2016

Apparent Wind Fun

Windsurfers know about apparent wind. I mean the physical concept, not the "apparently, there is no wind when I want to sail". Apparent wind is the combination of "true wind" (the wind we feel when standing still) and "induced wind" (the wind we feel when we stick the hand out of the car window). We can all add the direction vectors in our head, and end up with the strength and direction of the apparent wind, right?

Or so I thought. I learned about apparent wind a while back, when the original Windsurfer was a new board. Back then, we were typically going much slower than the wind. Let's look how the apparent wind changes for a windsurfer going at half of the true wind speed:
When our slow windsurfer is going upwind at an angle of 60 degrees to the wind, the apparent wind has an angle of about 45 degrees. When turning downwind to a 120 degree course, the apparent wind changes angles to about 90 degrees. That's a 45 degree change in apparent wind angle for a 60 degree change in course. Our sailor will have to open up his sail as he turns downwind almost to the same degree as he changes his course.

The curve above applied more than 30 years ago when I learned windsurfing, and it stuck with me. I never really considered how faster gear would change things until recently, when the apparent wind issue came up in our "International House of Speedsurfing" on Cape Hatteras. So I took a closer look, starting with the GPS tracks from Roo, the fastest sailor in the house. The program GPS Action Replay Pro lets you look at "Polar plots", which show the maximum speed at different sailing angles:
There's lots of interesting information in this plot: The maximum upwind angle was close to 60 degrees
  • The top speed was reached at an angle just below 120 degrees (it was quite choppy)
  • Maximum upwind speed was around 20 mph, maximum speed near 120 degrees was 34 mph.
The wind speeds that day were in the high 20s, with gusts just above 30, so Roo's top speed was just a tad faster than the wind speed. Let's look at the speed vs. board angle on a graph:

Board speed and apparent wind speed are show relative to to maximum wind speed. When going upwind at a 60º angle to the wind, the apparent wind was 40% higher than the true wind; when on a downwind speed run at a 120º angle, the apparent wind dropped to be roughly the same as the true wind. 

What I found somewhat surprising was the lowest line in the plot, which shows the apparent wind angle. It changed from just below 40º when going upwind to just below 60º when on a speed run. That's a change of just 20 degrees for a direction change of 60 degrees! Compared to the 45 degree change that we had seen for our slow windsurfer in the first diagram, that's a lot less. Our speedsurfer barely has to open up his sail by only 20 degrees when going from pinching hard upwind to full speed downwind!

Another thing I found surprising was that even when pinching upwind, the apparent wind angle was still about 40º. That's more than I would have thought. Over the years, I have had several experienced speedsurfer point out that windsurfers often tend to oversheet (once they got used to going fast). Roo also caught me oversheeting when he followed me to check my technique. He blamed the sail I was using, since it has a short boom and was developed for "light weights" (which I am not). But a wrong idea about what the apparent wind direction was probably also caused me to oversheet! 

I got a few of things to look out for when speedsurfing in the future: 
  • don't sheet in so much when going upwind or on a beam reach
  • don't open the sail too much in a sling shot
  • double-check your angles by comparing them to faster sailors, and/or by using GPSLogit and an Android phone to announce the actual speeds when sailing, and/or move your hands and harness lines closer together when speedsurfing to develop more sensitivity for the sail.
I suggest that my speedsurfing friends stop reading here. But anyone interested in loops should keep reading, because apparent wind strength and angles also play a role on speed loops. 

I admit that my interest in learning to loop had waned a little bit in the last couple of year, but the recent reminders that life can be short and come to a sudden, unexpected end have re-kindled my interest. So I started to think about loops again; to watch loop videos over and over; and to replay lectures about looping in my head.

One of the confusing things about looping is that spin loops can be initiated in many different ways. Some of the suggestions I have heard are:
  1. Jump high and sheet in hard!
  2. Whatever you do, try the first loops when going deep downwind!
  3. Stomp on the tail to practically stop the board, while moving the depowered sail to windward-forward!
  4. Take off at a slight upwind angle to get height, let the wind turn the nose of the board downwind a bit, then just hold on for the ride!
Some of these suggestions directly contradict others. But there is solid support for each of these suggestions. The last suggestion may be the one that is heard the least often, but it comes from a PWA pro who routinely throws some of the most beautiful loops ever seen, often on perfectly flat water. All the suggestions have helped many windsurfers to learn the loop. Nevertheless, some experienced teachers who have taught dozens of student to loop are convinced that other approaches to teaching are dangerous/no good/wrong. How confusing!

Well, apparent wind can also be very confusing to students. So there probably is a role for apparent wind in the loop! Let's see what we can find.

We will start with a simplified description of the loop: The loop is a catapult where the board is in the air and the feet are in the straps. 

We all catapulted when we learned to use the harness. Most of us did not like that, and quickly learned not to catapult. But we probably all remember that a sudden gust of wind can cause catapults. Sheeting in too quickly can have the same effect. Right away, this explains approach #1 to the loop: "Jump high and sheet in hard".  You'll get a lot more power in the sail and will get catapulted. Just make sure you are high enough in the air! A 10 ft wave on Maui to jump of works really well here. But for flat water loops, this approach can be downright dangerous.

Approach #2 ("do it downwind") is advocated by many windsurfers who have seen others get hurt by approach #1, or got hurt themselves. By going onto a deep downwind course before jumping, the chances of hitting your gear and getting hurt are drastically reduced. However, the downwind approach has one big problem: it is hard to jump when going deep downwind! Also, your apparent wind will be much less than the true wind, so there is just not enough wind to cause a catapult!

I know several very good windsurfers who tried to learn the loop with the downwind approach, but got nowhere. However, I also know several high-level freestylers who learned the loop with a variation of the downwind approach: they were working on Grubbies, and ended up looping instead. I have also seen "first Grubby" videos where the ending looked more like a loop than a proper Grubby, so the number of "Grubby loopers" is probably much higher.

One thing that you must do when learning the Grubby is to stick the nose of the board into the water to "create a rotation point", as the Tricktionary calls it. In a good Grubby, the board rotates 180 degrees, then slides backwards fully planing with you leaning forward on the board and backwinding the sail clew first to initiate another 180 degree rotation. But when learning the Grubby, there's a good chance that you kill most of the speed when sticking the nose into the water, with your board turned just a bit. As a result, your apparent wind increases to the true wind speed, and you get catapulted. Congratulation, you just landed your first loop!

The 3rd approach, "Stomp on the tail", is usually just a part of the entire instruction. I have seen some beautiful loops that clearly take this approach; but I have also seen beautiful loops that do not include tail stomps, even on flat water. This has confused me. What had me even more confused was a description "you stomp on the tail and stop the board, but your body keeps moving forward". That simply did not work for me. I can stomp on the tail to get a nice board wheelie, but all my weight must be on the back foot to do so, which means my body stops just as much as the board.

So let us look at what happens to the apparent wind if we go deep downwind, say to 150º off the wind, and stomp on the tail to kill speed. That's easy enough to do with plenty of apparent wind calculators on the web; I used this one. We'll assume that the true wind is 25 mph, and that our initial speed is 20 mph (typical for choppy conditions on freeride gear).
  • Initial apparent wind:
    12.6 mph, angle 97.5º
  • Apparent wind after slowing to 10 mph:
    17.1 mph, angle 133º
So the apparent wind has increased by 4.5 mph, and moved around by 35.5 degrees. If we had the sail luffed at the start, if will now be fully powered: we get catapulted! If we happen to the the board out of the water and pull up with the back foot, we'll get our first loop.

Now finally to the 4th approach: jump slightly upwind, move the rig towards the wind, let the nose turn downwind, and hold on! This loop advice is somewhat unusual, but it comes from Tonky Frans, not someone to take lightly when it comes to loops! Let's see what happens to the apparent wind here, again assuming 25 mph wind and 20 mph board speed:
  • Initial apparent wind:  29.6 mph, angle 45.3º
  • Apparent wind after the nose turns 20º downwind: 26.9 mph, angle 57.5º
This time, our apparent wind direction changed by only 12.2 degrees. But keep in mind that the nose of the board turned 20º downwind, so that the total angle change is actually 32.2 degrees! If the sail was luffed at the start, it is now nicely powered - without any active sheet-in! We're just holding on to the sail so that it does not get ripped out of our hands. Note that the apparent wind is much stronger than in our previous 150º downwind scenario (26.9 mph vs. 17.1 mph), meaning that the catapult will be more violent. That's good, because we have to turn the nose of the board an extra 50 degrees!

In all 4 approaches to the loop, we must get the sail suddenly loaded up, so that we get catapulted around. In the first approach, the load-up is initiated by actively sheeting in; in all the other approaches, the load-up is "automatic", caused by changes in the apparent wind. In reality, most loops will probably use a bit of a mix of the "active" and the "automatic" sheet-in, but at least in theory, either approach alone will work. Regardless how the sail is loaded up, the mast must pushed to foward-windward with an extended front arm: more to windward for a horizontal rotation, more to forward for a vertical rotation (I hope you jump high enough if you do it this way!). So, pick your approach and go looping!




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