Coodanup Pictures

We had two days in a row of sailing at Fangy's Weed Farm in Coodanup, Western Australia. If you wonder why it's called "Weed Farm", check this picture:

Nina stood in the weeds today for a while to take some pictures:

Droned at the Weed Farm

The little drone I bought for our trip wanted action. The wind played along - at about 15 mph, the Spark would be able to fly against the wind. Mike was game - he wanted to get "droned". When we explained to him that "droned" sounds a lot like the German "(zuge)droent, he stated that he wanted to get droned in the weed farm. So to Coodanup we went.

Unfortunately, I had never flown the "flying camera" for any noticeable distance before, so I misjudged how far out I had to fly. I stayed way too close to shore - about 200 meters out, while Mike and Nina sailed about 800 meters out. Here's a picture that shows the problem:

The green tracks are from the drone, the red and blue tracks are from Mike and Nina. No suprise I could not really see them on the phone's screen! When I play the video on the computer, you have to look really hard to find them. See for yourself:

Australia!

We have been in Western Australia for a week now, and I finally got some time to blog. We've been busy getting a phone, getting it to work, shopping for a car, getting it fixed, windsurfing, and getting more windsurfing gear for Nina so she can freestyle and wave sail a bit.

There are lots of fantastic things that deserve their own blog post - the city of Perth; the weather; the highly civilized driving on perhaps the best road system I have ever seen; wonderful bakeries and other great food; Christmas decorations (yes, they definitely deserve their own post!); and, most of all, the wonderful people we have met. But, this being a windsurfing blog, I'll start with the windsurfing.

We had arrived Friday afternoon, and Sunday was Nina's birthday. So she got to pick what we'd do, which ended up being ... checking out a car! It was a station wagon that we ended up buying, but it fortunately was on the way to Mandurah, where Fangy's Weed Farm is located. If you want to find this place on the map, look for Coodanup and Mandurah Bay in Western Australia - but the map won't tell you what makes this place special. Neither does the picture of the launch site:

The main sailing area is about half a mile to the right, hidden behind the trees. It's about knee-deep, sometimes shallower, with thick seaweed growing right to the surface. In some areas, the water surface looks more like a lawn than anything else - and you can sail right through this "magic carpet". You do, however, need a specialized fin - a regular weed fin won't make it through! Fins need to have a rake of at least 50 degrees to shed the weed, and they need to be quite short - 22 cm or less is the norm.

To get us on the water required the help of two local sailors, Mike and Ross, better known as "Decrepit" and "Fangman". Fangman had developed his own fins especially for the local conditions which work a lot better that most other high rake fins, and he had prepared three fins especially for us. Mike, with whom I had had a bunch of email exchanges about GPS prototypes, came of the water when we arrived and drove to Fangy's house to pick the fins up, since Ross was up in Lancelin. When we discovered that the Fangy Fins did not fit quite right, Mike gave Nina one of his fins (also self made, with a stainless steel front edge), and let me use his board. Another local sailor lent us a couple of fin screws that somehow had not made the trip.

The sailing was perhaps the most unusual windsurfing I have ever done. We sailed straight through weed beds which would have meant instant catapults with regular slalom setups, without slowing down, except for an occasional tug when you hit an unusually dense patch. If you managed to hit one of the channels where the weed had been pushed down, you'd accelerate, since the water was mirror-flat in 25 knot wind. A bit behind the thicker weed patches, the surface of the water would clear, and the "chop" would build up to a centimeter or two.

Still slightly jet-lagged due to the 13 hour time difference, we kept the session short, but came off the water with big, fat grins. Nina had set four new personal bests, I had one (plus the second-fastest session ever, in a lot less wind than in the fastest session!). When we returned to the same spot two days later, Nina again improved her top speeds, setting two new PBs.

Our third session was yesterday in Safety Bay. The spot was quite nice, somewhat similar to Bonaire in the setup with a large shallow section next to a deeper section (but the launch is in the deep section). That was a short session - Nina's mast broke in the middle of her third run, and she had to swim back for 20 minutes. But in the evening, she picked up two freeride sails and a mast for free from a local freestyler. It is quite incredible how many nice and helpful people we have encountered in our first week here! It has already restored my faith in people - wonderful in these times where consideration for others seems to be out of fashion in many parts of the world.

Nina in Perth

Christmas kangoroos

That's Why

Some of my readers might have been wondering why I want to go to Australia to speedsail in very weedy water. Well, it was windy in Western Australia yesterday, and the speedsurfers around Perth made a very good argument for visiting! Some highlights:

• 26 speedsurfers from 3 teams posting sessions on GPSTC
• 19 new personal bests
• Top speed 41.6 knots
• 19 sailors posting 2 second top speeds above 34 knots (9 above 36 knots, 3 above 40)
• A 38.7 knot nautical mile, with 3 posts above 35 knots 
• 8 alpha 500s above 24 knots, including 2 above 26 knots

Conditions must have been fantastic - 5 of the windsurfers sailed more than 100 km. The sail sizes used were mostly in the 5.8 - 7.0 meter range, with 6.2 m the most common size. Since most of these guys are "recreational" speedsurfers, it's safe to assume the wind was strong, but not extreme. The closest wind meter I could find (in Rockingham) showed wind averages of about 30 knots:

Most of this happened in Mandurah Bay, about 60 km south of Perth, WA. Here's another video from this spot:

The Shaking Is Real

I got a few runs today at the Kennedy Slicks before the wind turned too westerly. Here are GPS speeds from on run:

The red line shows the acceleration (the difference in speed from point to point). It's noisy! It all that shaking real? Well, I might have given the answer away in the title, but I owe you an explanation.

First the parts of the graph: it starts with a short break, then some walking into the water, some slogging, sailing through choppy water at about 22 knots, then in smooth water right next to the pier at 28-30 knots. A jibe is next, then back to the the start, pinching upwind. Note that the biggest acceleration changes are in the choppy approach to the wall.

I had my phone right below the GPS in an armband, and recording the acceleration data (at 10 Hz, just like the speed above):

This is a bit more confusing since we measure the three dimensions separately, and the phone changes orientation. But note that the overall pattern is quite similar to that from the GPS! Here are a couple of zooms:

I also had a GPS on the head, here's a comparison of the jibe region:

Note that we get a larger acceleration with the GPS on the arm - that right when I flipped the sail! The arm went forward first, reducing measured speed, and then back, increasing measure speed. The GPS picked that up nicely! The accelerometer peak at this region confirms that this is a real movement.

The head GPS has a smaller acceleration at this spot, but it's still above the 8 m/s2 filter threshold in GPSResults. With default settings, GPSResults would not give me an alpha for this run! Silly. If we measure at higher resolution, filters need to be adjusted accordingly!

Lanes Through Weeds

Weeds and speedsurfing? Yes! Check this video from Fangy's Weed Farm down under:


Only a few more weeks, and we'll be there, desperately trying to find the lanes. The promise of 37 knots in 20-25 knots of wind certainly makes spending a day or two on a plane worthwhile!

Accelerometer Fun

If the image above makes you curious, keep reading. Otherwise, stop now! This is a slightly geeky and (for most windsurfers) irrelevant post.

In recent discussions about GPS prototypes, some issues about acceleration arose. Current GPS analysis software has "acceleration filters" that aim to automatically identify artifacts by looking at the speed change from one point to the next. We know how fast we usually accelerate on windsurfers - something like 1-2 knots per second is typical. Much more than that, and it's probably an artifact (or perhaps a humongous catapult). Points with high acceleration are excluded from further analysis.

For 1 Hz data from the good old GT-31 GPS units, a threshold of 4 meters per second squared (4 m/s2) worked well. For the newer 5 Hz units, GPSResults automatically raises the threshold to 8 m/s2.  However, when analyzing 10 Hz data from prototype GPS units, we often noted discrepancies to 5 Hz units, especially in the nautical mile runs, and often tracked the acceleration filter down as the culprit: if a single data point in the middle of a nautical mile run had an acceleration above 8.0 m/s2, then the entire run would go missing, since runs cannot contain filtered points in GPSResults (with the default settings).

Why would 10 Hz data have more "problem points" with a high acceleration? Here are two possible causes:

1. Noise and simple math: acceleration is speed difference divided by time. If we go from 5 Hz to 10 Hz, the   number we use for time goes down from 200 ms to 100 ms. To illustrate this, let's assume that we have a "noise peak" of 2 knots (1 m/s) between two points. At 5 Hz, this gives at acceleration of 5 m/s2; at 10 Hz, we get an acceleration of 10 m/s2 which would trigger the filter.
2. Actual movement of the GPS: we have seen above that a speed difference of just 1 m/s is enough to trigger the filter if we measure at 10 Hz. That corresponds to just 10 cm (4 inches) of (extra) movement. With a GPS on top of a helmet, just having the head move suddenly when hitting a piece of chop could cause such an extra movement; similarly, the wrist might move when the rig is hit by a big gust.

Which one of these is the culprit? Well, if it's an actual movement, we should be able to measure it, right? How about using an accelerometer? Every smart phone has one, and there are plenty of apps on Google Play to read it and store the acceleration numbers. Unfortunately, that raises the next question - how good are the accelerometers in smart phones? 

Fortunately, the cold temperatures and lack of wind gave me time to play around a bit to get some answers. I'll switch to a question-and-answer format for the rest of the post to describe what I did.

Q: What were the questions?

1. How does the accuracy of a smartphone accelerometer compare to the acceleration calculated from GPS speeds?
2. Can we use a smartphone accelerometer to differentiate between random noise and actual movements in GPS data?

Q: What's the setup for the experiment to get answers?

The basic idea was to use a phone and a GPS in a controlled setting with regular acceleration changes, and to compare the data from the two devices. I used a pendulum setup: as the pendulum goes back and forth, it goes from zero speed to maximum speed at the center, and back to zero speed at the top on the other side. The changes are gradual and smooth, basically creating sinus curves for both speed and acceleration. Here's a picture of the setup I used:

It's a dustpan with a stick, and a plastic case to hold an Android phone (Samsung Galaxy J1) and a u-blox 8 / Openlog GPS prototype. The case is attached by velcro. I stepped out onto the balcony, held the stick at the top, and let everything swing back and forth, doing roughly a half circle (180 degrees) with every swing.

Q: How did you log the data?

The GPS was set to log at 10 Hz (NAV-PVT only), after letting it warm up for 20-30 minutes. On the Android phone, I installed the app "Sensor Record", and logged Accelerometer data at 10 Hz (100 ms delay). 

Q: How did you analyze the data?

I used software I wrote to read the GPS files, select the regions I wanted to use, and copy and paste the data into LibreOffice. I also copied and pasted the accelerometer data (from the .csv files the app made), and calculated a measure of total acceleration as:

a = sqrt(x*x + y*y + z*z) -9.81

where x, y, and z were the sensor reading for the three dimension, and 9.81 is the standard gravity (which the z sensor shows without movement if the phone is lying flat).

For the GPS data, acceleration was calculated as the difference in doppler speed to the previous point, and converted to m/s2. 

To align the data from the two devices, I used the time stamps as a rough start, then created a line graph, and deleted cells in one column to create the exact alignment.

Q: What are the results?

Let's start with a graph that shows the measured GPS speed and acceleration:

So this is roughly as expected, except that it's a bit noisy. All the movements and speeds were about the same, so all the peaks should be more similar than they are. The next graph compares the acceleration from the GPS (in red) to the acceleration from the phone accelerometer (in blue):

The acceleration measured by the phone was much smoother, and much closer to the actual motion of the pendulum (for a better look, check the first picture in this post, which is a plot of only the first section in this graph). So that's quite promising!

Basic physics tells us that the changes in speed for a pendulum should be smooth and continuous, generating a sinus curve. Therefore, changes in acceleration should also be smooth (remember that the derivate of a sinus curve still looks like a sinus curve, just offset a bit?). So let's look at the changes in measured acceleration in the accelerometer data:

The solid curve shows the changes in acceleration; the broken line shows the acceleration for comparison. You may notice that lines going down are a bit steeper than the lines going up - that's because I pushed the pendulum a bit from the right to the left so that the height would remain the same for every swing. 

Now let's look at the changes in acceleration in the GPS data - we'll just add a read line to the graph:

This is much more chaotic! All the extra spikes in the data are from random errors in the GPS speeds. They are just much easier to see in this graph (which is typical for second derivative graphs).

Q: What are the answers to the initial questions?

1. How does the accuracy of a smartphone accelerometer compare to the acceleration calculated from GPS speeds? The smartphone accelerometer is much more accurate (at least in this test). That's no big surprise, since it was designed to measure acceleration, while GPS units are primarily designed to determine location.
2. Can we use a smartphone accelerometer to differentiate between random noise and actual movements in GPS data? Yes, it seems that the smartphone accelerometer can be a useful tool to answer this question. Note that in this experiment, the GPS was a bit more challenged than when windsurfing, because it was done right next to a house, and because the GPS antenna changed its orientation to the sky by 180 degrees during each run, ending up sideways to the sky. 

Q: Why can't we just use accelerometers to calculate speed?

If the acceleration numbers are so accurate, it would seem logical that simply integrating them should give us speed numbers - would they not also be accurate? Only in theory (and perhaps in the absence of gravity .. but that's also a theoretical scenario). In practice and in the presence of gravity, we would have to know the exact orientation of the sensors at all times to calculate accurate speeds. Any small errors would quickly accumulate. According to a device manufacturer, even relatively small "angle error" of two degrees would lead to a velocity error of almost 2 knots within 10 seconds - much worse than what we can get from a GPS.

If you take a close look at the graphs above, you can see some obvious indications of the "angle problem". The graphs from the accelerometer acceleration has much higher positive than negative values. If we'd integrate that to get the final speed, we'd end up with a final speed in the 100 knot range! 

That is a result of the way I treated earth's gravity in the calculations, subtracting it after combining the three dimensions. Theoretically, gravity should be subtracted from the z-dimension before adding the acceleration vectors to calculate net acceleration. That's easy enough if the phone orientation is (a) known and (b) constant. But in the experiment above, the phone changed orientation relative to the earth: at the ends of the arcs, it was standing on its side, while in the middle, if was flat. So at the ends, the y-sensor would have measured gravity, while in the middle, it was the z-sensor; and in between, both of them at varying degrees. In theory, it would be possible to do a more accurate calculation by measuring the orientation at the points of zero speed, and calculating the intermediate positions; but that is rather complicated, and not required for this experiment.

Fun at the Slicks

More pictures, less words:

The forecast was 22 mph WSW, sunny, and warm. No surprise we got low 30s! We started sailing just before noon to catch the high tide at the Kennedy Slicks. GPS tracks:

Falcon 99, Loft RacingBlade 6.3, BP Weedspeed "38". Top speed (2 sec) 32.4 knots. My fastest 5x10 second average ever on a slalom board; I was faster only 3x on a 72 l speed board. Nina did freestyle, overpowered on 4.2. A bit too gusty there for freestyle, speed is more fun!

A video from one of the runs:

Racing Lessons

One of the great things about racing is that it shows us where we need to improve. Sometimes, these are things that are not obvious during the typical back-and-forth sailing, but the prospect of more races in the future can motivate us to improve. In races that are mostly beam reach or slightly downwind, like the recent ECWF Hatteras races, jibes are very important. Here's a list of what to learn:

1. Jibe dry on any equipment you may use in racing.
2. Jibe dry in chop, with distractions, and at any spot - not just in nice flat water where nobody is near.
3. Learn to adjust the radius in the middle of the jibe to avoid obstacles.
4. Get back to full speed quickly after a jibe.
5. Plane through jibes.
6. Pick your jibe path so your competitors end up behind you.

Most of these points seem quite self-explanatory, perhaps even obvious. If you fall in a jibe, you'll loose a lot of ground. If you usually jibe dry, but never jibe around people or jibe marks, the distractions and extra chop may make you fall. If you're in the middle of the pack, you often have to adjust your jibe radius because a sailor in front of you crashes or comes to a dead stop. 

However, I had never realized the importance of #4 - getting back to full speed quickly after a jibe. I had often worked on #5, planing through a jibe. But whenever I come off the plane in a jibe, I'd usually take my time and wait for the next gust or swell to push me back up on a plane. On the second day of the ECWF races, when we had planing conditions for four races, I learned the error of my lazy ways ... 12 times in a row (in 4 races with 3 jibe marks). On the straights, I had at least similar, and often better, speed than the two guys (Andy and Keith) who finished ahead of me in most races. In the jibes, I came off the plane most of the time, but so did Keith. But it took me about 25 seconds to get back up to full speed, much longer than Keith, so he usually gained at least 100 meters at every jibe mark. 

I'm pretty sure Keith does not train much for races, so why did he get going so much faster? Perhaps the reason is that he usually sails in waves, which I (almost) never do. Wave sailing at Hatteras often includes a lot of slogging and pumping practice - be it to catch a wave, or to get enough speed to make it over the shore break. Lazy sailors get pummeled and don't catch waves! Nor do they get to beat wave sailors in races :-(.

Point #5, planing through jibes, is really just a logical consequence of #4. However, chances that you'll plane through a jibe in racing are always lower than in free sailing, since the jibe mark dictates where you jibe; other sailors create chop and may disturb the wind; and the jibe radius is often chosen to keep others at bay, or to sneak around them, which can make it hard to plane through the jibe. Even PWA slalom pros often don't plane through jibes! But you can always see them pump like crazy to get back up to speed.

The last point about picking your path is how Andy managed to win 2 of the 4 races, despite being on slower gear. Andy was usually third at the first jibe mark, but always had the highest approach, which allowed him to see where both Keith and I were jibing. He could then come in between and end up before us. At that point, it did not matter much if he planed through the jibe or not, since he blocking us. This approach requires quite a bit of experience, confidence, and skills - perhaps more experience than can be gained by attending one or two race events per year. Some of the top level slalom sailors now train slalom on Tenerife, with up to 20 races per day, often for several weeks in a row - that can amount to hundreds of training races! So I'll put this one on the back burner for now, and concentrate on regaining speed after a jibe. 

Too Much

The forecast called for wind in the 30s (mph, that is). That's what we had when I started rigging - a 4.7, since the wind cannot be trusted. The 4.7 never got wet. Neither did the 4.0 that I rigged afterwards - I switched to the 3.4 Nina had rigged, since the wind had picked up:

I got onto the water when the wind averages where in the mid-40s, gusting into the mid-50s. I took advantage of this rare opportunity to sail in a lot of wind. A lot. Enough for the Red Bull Storm Chase. More than I had ever sailed in before. At one point, the averages where 49 mph, gusting to 59 mph. For those used to other units: 59 mph is 95 km/h, or 10 Beaufort - Windstärke 10, "Schwerer Sturm".
That was a bit too much for me, even on the 3.4. I pretty much had to waterstart in both straps; sailed out of the harness half of the time; had the sail barely sheeted in, and was nevertheless fully planing on my small FSW board. The wind was onshore, the tide was low, and I was wearing a helmet, so there never was any real danger. But the fun-factor was somewhat limited, and when gusts hit, I had a really hard time to keep the board on the water. Back on shore, it was not just blowing sand - it was blowing shells! Even getting the gear back to the parking lot safely was a 2-person job.

Usually, low tide and SW wind at Kalmus is flat and smooth, but not today. Eddie caught Nina trying a Shove It (when the wind was "only" around 25-40 mph):

A few minutes later, it looked a bit windier:

It still does not look that dramatic on the picture, but she came in a few minutes later, too overpowered on the 3.4. When the wind picked up a bit more in the next half hour, we had smoke on the water. At one point, I thought it was getting flatter again, because the wind was flattening out the little waves.

Well, finally getting a session on a 3.4 was all nice and good, but can we now please go back to being comfortably powered on 4.0 or larger?

ECWF Hatteras Day 2

Yesterday saw the second day of the ECWF Hatteras, with freestyle before noon and racing in the early afternoon. Results will be announced tonight at a party at the Mad Crabber in Avon, but it seems quite certain that Andy Brandt and Nina won the freestyle competition.

We had about 18-20 knots for the racing. Here's the course from my GPS tracks:

Total course length was 3 km (just shy of 2 miles). My top speed was around 29 mph. I was behind Keith and Andy in three of the four races; in one race, I managed to come in second after Keith. Keith had the best starts and was first at the first jibe mark in at least three of the races; Andy had the better jibes and won two races. My jibes were dry, but mostly quite poor, and my re-acceleration after the jibes was much slower than Keith's. I shortened the distance between us on most legs thanks to higher top speed, but could never get ahead of him. The speed tracks above show that it took me almost 30 seconds to get back to speed after most jibes - way to slow. Now I know what I have to work on for next year!

In the first race, I started with a beach start right at the pin end, but I forgot to properly orient the board in time. When I straightened it out during the last seconds before the start, I ended up over the start line.  Unfortunately, I could not see the lower mark because of several sailors in between, and neither could I hear the announcements from the boat, so it was not until I crossed the finish line before I heard that I was over early.  I was not worried too much about it because I assumed that we'd have at least one discard with 8 races. That later turned out to perhaps be overly optimistic .. which would have meant that Nina (or perhaps somebody else) would have pushed me off the third place, even though I finished second or third in the other seven races. Now I'll have to wait until tomorrow evening before the final results are announced...

ECWF Hatteras Day 1

Race start at ECWF Hatteras. Picture by Outer Banks Kiting

Yesterday was the first day of the East Coast Windsurfing Festival Hatteras, organized by Mike Burns. After a skippers meeting in the morning, we started with light wind freestyle. Seven guys and three women competed in two sets of heats. They put on a great show that included just about any light wind trick you'll find in the Tricktionary, including Jaw Breakers, Ankle Biters, head dips, splits, back-to-back, rail rides, and all variations of turns and 360s. Things got "heated" for a minute when Keith pulled my clew on a close encounter and made me fall off - but fortunately, I was carrying a few balls for juggling, and threw one of them at him that scored a lucky hit. Swift punishment!

The afternoon saw 4 races in 10-12 mph. The course was mostly reaching or slight downwind, with an upwind leg at the start to spread out the crowd. Nina killed it on her Ultra Cat with her "new" (15 year old!) 7.5 m AeroLite race sail from Gonzalo, coming in first in three of the 4 races and second in one race. Andy raced a Flyer 280 foil board with a foil on an 18 in mast with a 7.8 Loft Switchblade. Despite not enough wind and too much see grass to get up on the foil, he was unreachable for anyone except Nina, and scored second places in three races. Since I was on my "new" Equipe 2 XR and (almost) nobody else had race boards, I came in third in most races.

I had been a bit upset the day before when I discovered that my 2-year old carbon boom for the 8.5 m sail I wanted to use was broken, without any crashes that could have caused this (at least not any I remembered). Fortunately, it had not broken completely while I was a mile away on the water, and Ocean Air had a replacement. But all that was forgotten the next day once the competition started. Seeing Nina kill it in both freestyle and racing was fantastic. Coming out ahead of Andy Brandt in freestyle was a very unexpected treat, for which I probably have to thank the very slick rails on the Windsurfer LT that Andy was using.

Here are the prelininary results after day 1:

Open Racing:

1. Nina
2. Andy
3. Peter
4. Keith
5. Jason and David (tandem)
6. Brian
7. Ned
8. Ray

Limited Racing:

1. Tom
2. Gaetan
3. Simon
4. Phil
5. Randy
6. LarrySergey
7. Alan

Women's Racing:

1. Pam
2. Lisa
3. Paula
4. Mary
5. Carole

Women's Freestyle:

1. Nina
2. Pam
3. Lisa

Men's Freestyle:

1. Peter
2. Andy
3. Ned
4. Simon
5. Jason & Keith (tied)
6. David

Things will get mixed up today since we'll have freestyle and racing in 20-30 mph wind!

Everything Breaks

Everything breaks. Two boards (beyond repair). A few sails - one of them with 5 ripped panels. RIP. The whirlpool. The washing machine. The side door of the van. A weird fuel thingy in the van. Something else that gives the same error code (but at least we can still put gas in now). The mast foot on the Mega Cat (at least this one was an easy and cheap repair!). Today: the two year old carbon boom. At least it had the decency to bring me back to shore. I did not even realize that it was broken until I rearranged stuff in the van.

All that was annoying. More bothersome was when Nina was not able to windsurf for three weeks because of neck problems. Fortunately, they were just some muscles acting up, and things eventually got back to normal. Except that she then picked up something that looked like food poisoning. Twice in three or four days. But things seem to be back to normal again - well, mostly. She rocked at a light wind training session for the ECWF Hatteras today, even landing a Matrix and a Clew-pulling Anklebiter for the first time. The light wind freestyle competition tomorrow should be fun! Although a part of me expects something new to break...

Hatteras Videos

As usual, we had a blast at the ABK camp in Hatteras the last 5 days. We had a nice mix of light, medium, and strong wind - here are a few videos, starting with a planing upwind 360:

A bit of light wind freestyle:


On the medium wind days, we got to try the foil demo gear from the ABK van. Fortunately, they had shorter masts that worked well with the water depth at the camp location in Waves. Here's a video where I prove that I can spin out a foil, too:


Nina, of course, did a bit better on the foil:


Plenty of wind in the forecast for tomorrow - mid-30s in the morning, dropping into the 20s after noon. We'll have a skippers meeting at Ocean Air in Avon for the ECWF Hatteras at 10 am - hope to see you there!

Is 5 Hz enough?

This is a geeky post about GPS speedsurfing. You've been warned.

I am currently trying to get a couple of GPS devices that I have developed approved for the GPS Team Challenge. The units use u-blox 8 GPS chips, which are very accurate and provide speed accuracy estimates that can be used to automatically identify "bad" sections, for example artificially high speeds related to crashes.

One of the issues that came up is: at what rate should the data be recorded? Some popular GPS watches record only every few seconds, which is good enough for some uses, but not for speedsurfing. The venerable Locosys GT-31 recorded once per second; current Locosys units record at 5 Hz, every 200 milliseconds. The u-blox chips can record up to 18 Hz, although that limits the chips to using only two global satellite systems; for the highest accuracy, tracking satellites from 3 systems is desirable, which limits recording speed to 10 Hz.

There are some theoretical arguments that higher rates are better, because they give more accurate data. One big part of that is that random measurement errors tend to cancel each other out, and the more data points you have, the lower the remaining error gets.

But there are also some practical issues with higher data rates. The resulting larger files are usually not much of an issue, unless you're traveling to a spot that has a slow internet connection, and want to upload data for analysis to web sites like ka72.com.

Slow analysis and drawing speeds can be more of a pain. Most of the currently developed analysis software was developed for 1 Hz data, and can get quite slow with large 5 Hz files. Some steps appear to be coded inefficiently, showing N-squared time complexity - they take about 25 times longer for 5 Herz data. With 10 Hz data, add another factor of 4, and now we're talking about 100-fold slower.

A bigger issue is that higher data rates can lead to "dropped" points when the logging hardware can't keep up with the amount of data. I recently ran into this issue with my prototypes, and have seen indications of the same problem in data files from a GPS specifically developed for speedsurfing that's currently coming onto the market. But fortunately, the frequency of dropped points in my prototypes is low enough (roughly one point per hour) that the data can still be useful.

To see how much we actually can gain from increasing data acquisition rates to 10 Hz, I did a little experiment. It started with a windsurfing session were I used two prototypes at 10 Hz (for control, I also used 2 or 3 "approved" GPS units). Comparing the data on these units for the fastest ten 2-second and 10-second runs gives a good idea about the actual accuracy of the devices; the data from the other GPS units I used help to confirm that.

Next, I took one of the 10-Hz data files and split it into two 5 Hz files by simply writing one record to one file, the next one to a second file, the third one to the first file again, and so on. This simulates measuring at 5 Hz, but I get two 5-Hz files from the same device.
In the same way, I created a couple of 1-Hz files from the original file, this time selecting every 10th record for the first file, and every 10th record but starting one record later for the second file.

I analyzed the speeds for all these files in GPSResults, put them in a spreadsheet, and calculated the differences. Here are the results:

Looking at the "Average" lines, the observed differences increased from 0.041 knots to 0.074 knots for 2 second runs, and from 0.027 knots to 0.036 knots for 10 second runs. Going down to just one sample per second increased the observed differences more than 2-fold for both 2 and 10 seconds.

The observed differences are close to what would be expected by sampling theory, which predicts that the error is proportional to the square root of the number of samples taken. The expected numbers are shown in the "Theoretical error" line above.

But what error is "good enough"? Let's look at the top 10 teams in monthly ranking for the GPS Team Challenge for September to get an idea:

In the 2-second rankings, teams #7 and 8 are just 0.06 knots apart; in the 5x10 second average, the difference is 0.07 knots. The smallest difference in the 5x10 second category is 0.6 knots between teams ranking 8th and 9th.

Looking back at the observed differences at 5 Hz, we see that the average was just 0.036 knots (note that this is actually the average of the absolute differences).  For a 5x10 ranking, the expected error would be roughly two-fold smaller, or less than 0.02 knots. This seems quite adequate, it would have given the correct ranking in the 5 x 10 second category. Note that errors go down even more for the "longer" categories like nautical mile, 1 hour, and distance. Only in the 2-second category were two teams so close together that the observed average error is similar to the difference in posted speeds. Note, however, that many speedsurfers still use GT-31 devices that record at 1 Hz, and that even the 5-Hz Locosys units tend to have 2- to 3-fold higher errors than the u-blox prototypes I used in this test. It is quite well known that the 2-second category is the most likely to be subject to errors; however, it is still a lot better than the "maximum speed" category that is used in some other GPS competitions. For 5 Hz data, the expected error for single point "maxima" is about 3-fold higher than for 2 seconds!

So, to answer the question in the title: yes, it is!

ECWF Cape Cod: GPS Racing

It's been a couple of days since we completed the sixth annual East Coast Windsurfing Festival Cape Cod, and it's just beginning to appear possible that we might have it again next year. A longer report of the event will follow, but for now, let me talk about the GPS Racing.

The best wind of the weekend was last Friday, so I decided to hand out GPS units to anyone who was registered for the event and would take one. That included several Kalmus regulars (mostly non-speedsurfers), as well as PWA slalom pro Marco Lang. Since we all wanted to go to the event party at Inland Sea in the evening, we all had to stop early, just before the wind picked up, so most of the results were from 19 mph or lower wind averages. Here are the results:

Speeds are in miles per hour.  Everyone went faster than the average wind speed of 19 mph. Marco beat everyone by at least 4.5 mph (4 knots). Check out this picture of Marco flying on the fin:

The next group of three (PR, AR, and CE) all reached about 31.5 mph. Two of them (PR and CE) sailed exactly the same equipment as Marco; one (AR) was on slalom gear with a larger sail. Weight also plays a role in speed, but two of the three weighed in roughly the same as Marco - so the 4 knot speed difference is entirely due to skill.

The next sailor, NS in 5th place, stands our in a group of her own with 28.4 mph. She was also on full slalom gear, sailing happily along with Marco:

The last group of three sailors with speeds around 24-25 mph was on freeride or freestyle gear. Going just a bit of the wind is usually faster, as JSh's speed shows. All of them looked quite fast, and were going about 5 mph faster than the average wind speed.

All the speeds were from the Friday before the event. Since we never got enough wind during the event to run a GPS race, we used to top speeds from Friday for the GPS ranking at the event. Congrats to the winners, and many thanks to all who competed - and to Marco for showing what real speed looks like!

ECWF Cape Cod 2018

We had the 6th East Coast Windsurfing Festival Cape Cod last weekend. 21 windsurfers competed over the course of 2 days in racing, freestyle, SUP racing, and SUP relays. A big "thank you" to all that helped make this event a success, especially:

• Marco Lang for traveling from Austria to participate at the event, answer many questions at the event party and on the beach, explaining the gear, and many helpful tips
• Phil Mann from InlandSea for hosting a great party on Friday evening, organizing demo foils from Naish, and being at the beach both days to set up the demo gear and help anyone wanting to try it
• Jason Meunier from VoilOka for driving the huge trailer with demo gear down from Canada, and setting up the demo gear
• Bruno Robida from 2 Rad Windsurfing for financial support and raffle items
• Vincent Lindauer for his efforts to get demo gear to the event, and for asking Marco to come to the event
• Chris Eldridge, a local windsurfer and Fanatic/Duotone team rider, for help with the demo gear 
• Peter Kimball from AP Kimball Construction for financial support and setting up a canopy for the event 
• Jerry Evans from Chatham Wind and Time for making the trophies and financial support
• Tom Ben-Eliyahu for help with the demo gear and driving the boat during the tow-in foiling
• Barbara Baldwin for helping with registration and running the event
• Joanie Scudder for arranging the sponsorship from Hy-Line cruises
• Everyone who contributed to the Gofundme campaign to pay for Marco's travel expenses

Many of the people listed above had helped similarly in the past, and participated in the races during the event.

It was great to have demo gear from Fanatic, Duotone, and Naish at the event. Many of the competitors and a number of non-competitors took advantage of the opportunity to test new windsurfing and foil gear.

Next, a brief day-by-day description of the event.

Friday:
Phil Mann and Naish hosted a party at Inland Sea in West Dennis in the evening, with a Q&A session with PWA pro Marco Lang. It was well attended and fun, although most windsurfers arrived a bit later since we had a nice southerly breeze at Kalmus. The Fanatic and Duotone gear was already available at the beach, so many windsurfers tried new boards, sails, and foils.
To make sure that we would have a ranking in the GPS Racing category, I handed out GPS loaners to everyone I saw who was registered for the event, and had expressed interest in GPS racing. We had about 8 competitors in total. Marco Lang beat everyone else by about 4 knots, reaching a 2-second speed of 31.55 knots (2 second average) on a Duotone Warp 7.7 and a Fanatic Jag 108.

Saturday:
Side-off northwest wind from 10 to 30 mph created interesting conditions for racing, and challenging conditions for the freestyle competition. With a first race start shortly before 11 am, we managed to do five races and a couple of freestyle heats. Many windsurfers again took the opportunity to test race gear.

Sunday:
The wind forecast for the day was extremely light, and unfortunately came true. In the morning, Marco gave a very nice description of the Duotone sail range, followed by a description of the Fanatic boards by Jonathan when he eventually joined us.

After lunch, we ran SUP races and a SUP team relay, which again was tons of fun. We also used out little inflatable for "tow foiling", which created both astonishing rides and spectacular crashes.

Despite the very light wind, we managed to get one freestyle heat and the freestyle final in, where Mike and Henrikas displayed a variety of 360s, Ankle Biters, and more. When Henrikas failed to get his trade-mark Back-to-back in the very light wind, but Mike landed a clean Matrix, the decision when to Mike. Since Henrikas had won the SUP/shortboard class in racing, this set up a tie for the "King of the Cape" title. We had to dig deep into the tie breaker rules to find out who would be the King of the Cape 2018: Henrikas! Congrats to the new King and Jeanne, who is the 2018 Queen of the Cape.  I plan to post the full results of all disciplines in a separate post - for now, a few pictures:

Nina and Marco speeding on Friday

Racing action

Henrikas, King of the Cape 2018

Chris and Marco on the inflatable Fanatic tandem

Towing Tom

"Best Stoke" winner Spencer

Mike mid-Matrix

Freestyle can be exhausting!

Happy winners Ansel and Martin

Overall, it was again a fun event. Many competitors and demo gear users came to us after the event to thank us (which is definitely appreciated!). The stoke was high even on Sunday, when the wind was very light - we know how to have fun even without wind!

You may want to stop reading here.
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Unfortunately, the light wind also seemed to keep some windsurfers away. The low attendance, despite moving the event by a week to get better wind, and despite demo gear and a PWA pro as the "guest star", makes it questionable whether there will be another ECWF Cape Cod in the future. Organizing the event is quite a bit of work in addition to the event days - just getting the signatures from all necessary town officials can take a day, and that's one of the easier parts. The organization we formed specifically to host events like this, the ECWA, ended up loosing money on this event; unless the ECWF Hatteras in October is a big success, we would need to cut expenses at future events.

Every event will have some unexpected things happen that cause stress for the organizers. Some of these can't be helped, like crashing scoring programs or some random idiot driving over the ladder we use to put longboards on top of our van. But other thing are very demotivating. As most of you know, the event was run by a German couple. Germans expect that if you give a word, you stick to it (well, at least Nina and I do!). If a local shop or the representative of a big windsurfing company says he'd bring or send items for the event raffle, and then does not, that's a big disappointment. For the local shop, we are fortunate enough to have another local shop that has always supported the ECWF (Inland Sea Windsurfing). For the large company, we will try hard to separate the person who apparently forgot his promises from the company (which we love), but it's not easy. And just for future reference: no, it it not ok to come to an event, to not register, to not sign the liability waiver, but to insist on racing on equipment that the event organizers and race director have specifically excluded from the races. You are free to disagree, of course, but just joining a race on equipment that was deemed too dangerous for this event is extremely inconsiderate and arrogant. Stating that you would "stay away from everyone", and then later posting picture where you are right next to several others in the middle of the race? Priceless. (Other words come to mind, but shall remain unwritten)

I run a small company. We have often presented our products at meetings where we had access to a number of potential customers. Every single time, we had to pay for this opportunity - often thousands of dollars. For the ECWF, we did not ask for money, since giving the local windsurfers the opportunity to test gear was more important for us. We did, however, ask for at least a small contribution to the raffle, which gives windsurfers an extra reason to join the event. Despite promises, we did not get anything. We thought that a company might at least contribute to the cost of getting one of their team riders to the event, especially in a year where they have a significantly increased marketing budget since they changed their name. Again, nothing. Well, "Denken ist Glückssache".

I am trying hard to focus on the great people I got to know at the event, or who I have known and who have participated and/or helped again, rather than on the few negative experiences, but it's a bit of a struggle right now. So unlike last year, where we started getting the permit for the next event soon after the event, we won't plan next year's ECWF Cape Cod before our trip to Australia. There, we'll attend a few events and races, and speedsurfing on some of the top spots in the world, and wave sailing in some of the best waves in the world, will hopefully put things into perspective.