Thursday, October 25, 2018

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...

Wednesday, October 24, 2018

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!

Monday, October 22, 2018

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...

Saturday, October 20, 2018

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!

Saturday, October 6, 2018

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

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!