Wednesday, August 16, 2017

Surprise Fun

The wind has been pretty light this summer, so when I saw gusts above 20 mph today, I just had to go sailing. Despite a few light wind freestyle and longboard sessions, my withdrawal symptoms were strong enough to overlook a few minor issues. Kalmus was the only spot showing halfway decent wind, but the wind was from the northwest - offshore and not a good direction for Kalmus. According to the iWindsurf meter, it was rather gusty:
Gusty, offshore, best wind probably in Lewis Bay: a longboard day! When I got to the beach, I could see lots of white caps a few hundred meters from shore, so I rigged the Loft SwitchBlade 7.8. That's usually not my preferred longboard sail, but 3 cams and tons of stability meant I did not have to worry about strong gusts. What followed was more than 2 hours of fun in what felt like my most-powered longboard session ever. Here are the tracks:
At times, I was in both back foot straps on holding on with all I had, 10 feet of board fully out of the water in front of me. At times, I briefly thought that a slalom board might have been fun too, but almost every time, the wind would drop to sub-planing within a minute or two. The speed graph shows the variability quite well - I was mostly just going back and forth on a beam reach, where the board speed is typically close to the wind speed. On a slalom board with the same sail, I probably would have slogged often, which is no fun; on the longboard, I felt lightning fast most of the time. In gusts, the wind was strong enough for deep downwind runs, but the chop was about a foot high, which slowed the longboard down with every little wave I crossed. Threading the chop on a course close to a beam reach ended up a lot faster, and less scary.

As much fun as that was, I am looking forward to September, when the crowds are gone, all beaches are open for windsurfing again, and the wind (hopefully!) returns. Just three more weeks! Lots of fun action in September, too:

  • The ABK Camp Hyannis from September 8-10. I hope all you local windsurfers have signed up already - the camp has sold out every year in the last 4 or 5 years!
  • The East Coast Windsurfing Festival Cape Cod on September 16-17. Fun races, freestyle competition, GPS racing, and probably some cool demo gear - sign up and join the fun! We'll start this year with a "Beer Social" at Kelly's on Main Street in Hyannis on Friday, September 15, at 7 pm.

Sunday, July 30, 2017

Borderline Breaks

The things we can learn from board repairs ... for example where you should not put your feet. Let's start with the end:
This is my blue Skate 110 which I bought to replace the white Skate 110 that I wrote about in my previous post. The picture shows the end result of my latest repair attempt. If you click on the image to see the larger picture, you'll notice a slightly different blue in the repaired area, which extents from the back of the pad to the edge of the board at the "F", and extends forward to the "N".

I discovered a soft spot towards the rear of this area last fall. Upon exploring, I discovered that the glass and the PVC foam underneath had a break. I dug out the core foam all the way to the sandwich at the bottom of the board, filled the void with polyurethane (PU) foam, and glassed it over. The PU foam is harder than the original EPS core, forming a hard "plug" that connects the bottom and the top - good enough, I though.

When putting the fiber glass on top, I noticed bubbles forming: the new structure had several little "pinholes". The inside of the board was still under a bit of pressure, and the air coming out pretty much kept the little holes open. I tried to fix this with an extra coat of epoxy while the board was cooling off, hoping this would pull the epoxy in and close the pin hole in the process. However, this did not work as expected, and I saw bubbles forming after a few sessions.

The next repair attempt including removing the glass around the pin holes, and glassing the entire area over again. This repair held for a couple of month, but then I noted a new soft spot right next to the initial repair. I used some Solarez since it was freestyle time, but that barely held for one session. So .. time to try again.

When I opened the board up, I saw that the initial repair was still intact; however, the top glass fiber layers were broken right at the edge of the repair. When I made the initial repair, I did not rebuild the PVC sandwich layer because I had neither divinicell nor the vacuum setup. I figured that having the PU plug extend to the bottom would make it stable enough; but what I had not considered was that I would have a hard plug right next to the somewhat softer, slightly elastic sandwich construction. This create a "Sollbruchstelle", which then indeed created a break.

This time, I tried to do things right. The first step was to open up a relative large area that included the previous repair area, and digging out all the soft EPS foam. I then hooked up the area to a vacuum pump to see if I could draw any moisture. This removed some slight dampness from the top of the EPS foam, but the vacuum trap remained empty after several hours. Fortunately, very little water had gotten into the board.

The next step was rebuilding the foam core with PU foam:
That was followed by sanding the excess foam off, and removing additional foam to allow re-building of the PVC sandwich. After some filling, glassing, and sanding, here's what the repair area looked like:
When you look closely, you'll notice a slightly different color along the top and right side. This is the edge of wood veneer reinforcement. It extends about two thirds of the way to the edge  near the front of the repair area, but not nearly as far at the back. The original soft spot that had developed had definitely not been covered by wood.

Using wood veneer as reinforcement under the rear footstraps makes a lot of sense, since wood has excellent mechanical properties to absorb and distribute the impact after jumps. Extending the wood past the padding makes sense, too: the padding under the foot pads is not very wide, and my size 12 feet need to be quite deep in the foot straps for my heel to be on the padding. Most of the time, however, I sail with just my toes in the foot straps, so my heel rests on the board next to the foot straps. I must say that I find it somewhat peculiar that the wood reinforcement did not extend further back. There are several possible reasons for this, like to much deck curvature, or "interference" by the fin box. Whatever the reasons were, it seems my heels were often positioned right next to the reinforcements, which eventually led to damage. It's no surprise that the damage was on the right side of the board, either: my home spot Kalmus had starboard jumps most of the time. Even when not jumping, just going over the chop there would put a lot of stress on the back heel, which does not even have any padding underneath.

I am quite sure that the reinforcements in my older Skate 110 were quite different. I use the old Skate a lot more often than the new one, and often with bigger fins, which lead to more outboard foot placements and more back foot pressure, but the old Skate never got soft in the back. The old Skate seemed to have a lot of carbon reinforcement, while the new Skate seems to have wood veneer instead (although the areas I repaired on the two Skates are different). But mainly, the old Skate 110 actually had two sets of footstrap positions, including a more outside one that's missing on the new Skate; and the padding on the old Skate extends further to the rails. So placing the back foot more outside on the old Skates was not a problem, but it appears to be a problem on the new Skate, at least when sailing in heavy chop or jumping a lot. Maybe I just need to go to tiny little freestyle fins to enforce a more centered foot placement and less back foot pressure...

Thursday, July 27, 2017

Fixing My Favorite Board

In 2010, I got a white Fanatic Skate 110, and it has been my favorite board ever since. But last year, I discovered that the deck near the front foot straps was getting soft. My favorite board repair guy, Donnie in Hatteras, did not want to repair it, so I figured I'd use it to learn board repairs on. The first installment was a big nose job last fall. A while later, bubbles started to come out near the soft area, so it was time to learn new stuff - soft deck and sandwich repair! Most of the things I did are based on the excellent instructions on "The Board Lady" web site.

The first step was to take the padding under the front foot steps of:
That took a while. I started to understand why Donnie did not want to do the repair.

A small exploratory cut was next:
You can see the sandwich construction: a few layers of glass fiber on top, then a thin (~4 mm) layer of high density foam, a single layer of glass, and the EPS foam core. The sandwich looks good, but there's a big air gap that should not be there. It seems the EPS foam got compressed just too many times and eventually tore apart.

I enlarged the cut-out area, but the soft area was quite large, so I decided to treat most of the affected area with expanding polyurethane foam, introduced through many little holes that are covered with blue tape in the next picture:

You can see the foam expanding all the way into the cut-out area. In the cut-out area, I removed some of the damaged EPS foam, so I had to build the area back up, also with marine polyurethane foam:
I just love playing with the pour foam. You mix together a couple of liquids, and then you have about a minute to inject it before it expands about 50-fold. Fun!

After sanding down the PU foam, the next step was to rebuild the sandwich layer. I used one layer of glass and two layers of thin divinycell. To press everything together, a vacuum setup is needed, as shown in the next picture:
Once you have the stuff and know how to use it, this is easy. The PVC foam must be covered with three layers of different plastics: perforated release film, then "breather" material, and finally vacuum bagging film. This was the first time I did this, so I made one small mistake: I put the sticky stuff that connects the vacuum bagging film to the board to close to the treated area. That made it a pain to take off later!

I also had to close all the little holes I had drilled to get the PU foam into the board to fill the air spaces. That required some sanding first, and then some epoxy and lots of small little glass fiber discs:
Once the epoxy had hardened, each of the little holes needed to be sanded again to smooth things out and to get a good feathering connection between the new and the old glass.

The divinycell also needed to be sanded down and glassed over. I used two layers of glass, but no carbon. The original construction appears to have a layer of carbon cloth on top, but it's very thin; you can see in the next image that I accidentally sanded through it at a few spots.

The board came with two sets of foot strap positions, inner and outer. I never used the outer positions, so I simply filled the holes in the outer plugs with some epoxy mixed with low density filler (the brown stuff). The next step will be to add one more layer of glass over the entire center area; some sanding and filling; putting new padding down; and painting over all the little holes (more to protect the epoxy from UV than for cosmetic purposes).

This repair has already taken quite a few hours, and still needs more work. Someone who repairs boards professionally would have to charge at least $400-500 for such a job, and even that assumes he'd be a lot faster than I was, and that he'd charge an hourly rate much lower than what I have to pay a local car repair shop. How long the repair will last is an open question, so I can definitely see why Donnie did not want to do this.

For me, however, this was fun, and a useful learning experience. The Skate in the pictures above is not the "Team Edition" model, but this repair clearly shows that it still contained quite a bit of carbon - it looks like at least one thing layer of carbon cloth over most or all of the top. In contrast, the 2015 Fanatic Skate that I got as a replacement did not seem to have carbon reinforcements, at least not at the places I looked so far; instead, the 5 years newer models appears to use wood for reinforcing (more about that in a future post). I always had the impression that my blue 2015 Skate was not quite as lively as my white 2010 model - but the newer models seemed a bit easier to sail in very choppy conditions. I now think that this is because only the older model has the extra layer of carbon, which gives it a bit more stiffness. So I'll be very glad when I can sail my white Skate again on the lighter wind days!

Wednesday, July 26, 2017

Reality Check

I'm venting. You've been warned.

Surfer 1 says to surfer 2: "I'm a much better surfer than you! I bet I was at least 3 knots faster than you!"

It's pretty funny someone would say such a thing, but this did indeed happen. Not only is surfer 1 faster than surfer 2 - no, almost every single day, he is a lot faster than anyone else at the beach! He must be a better windsurfer than all of them!

We could argue that surfer 1 is usually the only one on slalom gear on the beach, or that he sails more than 95% of the other windsurfers, but let's not go there. For the last 2 years, surfer 1 has used a GPS to record his sessions - so how well is he doing? Easy to find out on the GPS Team Challenge web site, which has rankings for 6 different speed disciplines:
Surfer 1 is highlighted in red, so we'll simply call him Chris M. from now on. Surfer 2 is highlighted in yellow. The astute observer might notice that this appears to be the writer of this blog - congratulations!

I'll readily admit that I am not a good speedsurfer. In the international rankings on the GPS Team Challenge, there are somewhere between 840 and 900 speedsurfers who are faster than I am. On our team, there are several guys who sail a lot less than I do, but have much higher top speeds, and will usually be several knots faster when we sail together, even if we are on similar gear. But I don't care much - I do my "slow speedsurfing" when it's fun, and my (equally poor) old school freestyle / freeriding when I want to fall a lot, or it's just too bumpy for my taste. After more than 3 decades of windsurfing, just going back and forth at moderate speeds still is utterly fascinating to me. I don't think I'll ever loose this fascination - fellow local Bruce has not, and he's 20 years older than I am.

But back to the rankings. As bad as I am as a speedsurfer, I still managed to leave Chris M. several spots behind me in 5 of the 6 categories. If I'm not good, what does it make him? The guy who is always on slalom gear? Let's check the overall ranking:
9th overall in a field of 27 (which includes a bunch of freestylers who "volunteered" to take a GPS so that we'd get a second ranking for the team). He barely manages to edge out the lovely Nina, who'll be on freestyle gear at least 9 out of 10 days, hacking away at Flakas, Vulcans, Switch Konos, and Switch Vulcans. I have no problem admitting that Nina has become a way better windsurfer than I am; the same is true for the guys ahead of me in the rankings, and for a few that place behind me (including Bart, Martin, and Graham, who is playing in a whole different league). But Chris M.? Now that's a rather funny thought. Maybe if he shows me a few planing upwind and downwind 360s, and beats me in the rankings. Not that that will ever happen - in the past two years, he has not been able to figure out install GPS analysis software, or use, or even to use the software after I installed it on his laptop.

Looking at the rankings reminds me of another funny story. Less than half a mile from Kalmus is Egg Island, which is a very nice speedsurfing strip. I've been there several times with different guys on from our team. The first sessions were with Dean, who did his usual complaining half the time, but still racked up several 35-knot sessions. Boro sailed there just a couple of times in lighter winds, but managed to set a 37-knot spot record the very first time he sailed there. Chris M., you ask? I've seen him there twice. The second time, the wind picked up a few knots, so when I go got to Egg Island a few minutes after him, he was standing on shore, complaining that he could not sail in "this shit", and that he was so far away from his next-smaller sail. He did a wonderful job at defining the opposite of impressive! When the guard came to tell him that he was standing on a private beach, he sailed back to Kalmus, never to be seen at Egg Island again.

Some of my regular readers may wonder about this post, since I usually try to write about positive or at least interesting things (which some non-geeks may, admittedly, debate). Well, I must admit that I have a very low tolerance for stupidity, arrogance, and selfishness. I'm do not really have a problem if someone is not intelligent or well educated. I have worked with mentally handicapped people, and they were some of the nicest people I ever met. I have also met many nice and good people who had little or no education. But people who are ignorant but think and proclaim they know it all? Like "real estate developers" who "know" more about climate science than thousands of scientists? Keep them away from me! The same goes for self-taught windsurfers who never in decades of windsurfing bothered to learn the rules, and think "oh, it's just like when getting out of the elevator". Really? How stupid do you have to be to think that's a good way when two people on windsurf gear approach each other at a combined speed of 50 mph or more? How stupid and arrogant do you have to be to yell at someone "there are no rules in windsurfing" because you don't know them? Someone who not only learned the rules when he learned windsurfing, but also is a certified windsurfing instructor with a rather good understanding of what US Sailing, US Windsurfing, the UK sailing association, the VDWS, and the top US windsurfing instructors have to say about this issue? Really, really, really stupid and arrogant.

But believe it or not, this entire thing was not really about rules - it was about attitude. I realize that there are windsurfers out there who don't know or understand what the rules are that apply when they are on a collision course. Even windsurfers who know the the rules and play by them sometimes mess us. Usually, we still figure out how to stay out of each others way. In the rare case of a near-accident or a forced fall, things can usually be sorted out with a few friendly words or an apology. But things are a lot easier and more predictable if you know the "right of way" rules, and play by them. There are plenty of windsurfers on the beach who can explain them - including anyone who ever did an ABK camp (and paid attention in the very first lecture).

But if you happen to be the "give-way" windsurfer in what could become a collision, and you force the other person to jump of the board to avoid the collision, you've got a problem. If you think you are a good windsurfer and the other person is not, and you force them off the board, something is wrong with you. If the other windsurfer is a (slightly) older lady, other choice words come to mind.

And still, everyone does stupid things sometimes, and a simple apology can fix things. If someone who feels you treated them wrong when you forced them off the board approaches you, a simple "I'm sorry" would do. Or if your understanding of what should have happened differs, you could discuss it, and perhaps ask others for feedback. But being dismissive and calling the other person a "bad windsurfer" instead? Or later calling the women that you forced of her board a "cranky old fucker" to their friends? That shows some serious moral and mental deficits.

And still, this all would have ended after my last post. I had revised my assessment of Chris M. from "a bit rough on the edges" to "ignorant sociopath", and decided to avoid him - there are plenty of nicer people to talk to on the beach. I am certain that my intentions were very clear, but Chris M. deemed them unacceptable. When we happend to both sail at Duxbury in a NE wind a few days ago, he just had to approach me to ask if we were "friends or enemies". My explanation that I will not be friends with someone who calls other friends "cranky old fuckers" had him come to the conclusion that we now must be enemies, and he proceeded to explain to me that "there are no rules in windsurfing", that "rules are just for races", that "it's just like when you get out of the elevator", and that "only bad windsurfers complain to me".  Given my low tolerance to ignorant idiocy, you can imagine the rest. Even the Duxbury Harbor Master stopped by to make sure we were not fighting; unfortunately, he had no desire to explain the sailing rules to Chris, either (smart man!).

Well, enough venting. I'll end this with a citation from the "Start Windsurfing Right!" book from US Sailing (2nd Edition, p. 103):
"There is also an unspoken rule among sailors which should be considered part of your sailor's code. When two boards meet, it is common courtesy for the more experience person to maneuver around the less experienced one".

Sunday, July 9, 2017

Stand On or Give Way?

The (finally!) warmer temperatures mean it's getting more crowded on the water. Twice during the last week, someone who had been on a collision course with another windsurfer asked me "What should I do?". In both cases, the windsurfer who asked the question was on starboard, the other windsurfer on port. All windsurfers involved were experienced windsurfers. I'll describe the two incidents a bit further below, but first, let me answer the question.

If you are windsurfing and find yourself on a possible collision course with another windsurfer:

  1. Determine whether you are the "stand-on" windsurfer or the "give-way" windsurfer.
  2. If you are the "stand-on" windsurfer, keep your course
  3. If you are the "give-way" windsurfer, adjust your course so that you are not on a collision course anymore.
These rules are simple and clear. Note that there is no "right of way" here! The rules spell out clearly what each party must do. 

How do you determine if you are the "stand-on" or the "give-way" windsurfer? There are a few simple rules:
  • Opposite tacks: If two windsurfers are on opposite tacks, the starboard windsurfer is the "stand-on" windsurfer, and the port windsurfer is the "give-way" windsurfer.  Even simpler: if your right hand is closer to the mast, you "stand on". If your left hand is closer to the mast, you give way. (This assumes that you are sailing in a normal stance, not on the leeward side or back to the sail).
  • Passing: If you are overtaking another windsurfer, you "give-way".
  • Same tack: The windward (upwind) windsurfer is "give-way", the leeward (downwind) windsurfer is "hold-on".  In other words, the windsurfer who pinches higher against the wind has the right of way.
These rules are universal, but it is possible to find an occasional web page where they are explained in a confusing or incorrect manner. For a nice explanation with diagrams, check this "Rules of the Road for Sailboats" page. You can find the same rules at plenty of other places and web sites, for example the American Sailing Association, the Maritime College, boating instruction sites,  the US Sailing Windsurfing Instructor manual, or (in German) at the VDWS.

There are a few other rules and conventions worth noting. 
  • Sailboats (including windsurfers) are  usually "hold-on" when on a collision course with powerboats. Exceptions large boats like ferries and other commercial boats, and against boats with restricted to maneuver (for example in shipping channels). But note that different rules apply for powerboat vs. powerboat situations, and that many powerboat users are not aware that there are different rules for sailboats (and windsurfers)!
  • Stationary objects are by definition "hold-on", so sail around them. That includes windsurfer trying to waterstart.
  • It is a common courtesy to give beginners plenty of room, even if they are the "give-way" windsurfers. Similarly, planing windsurfers typically avoid windsurfers unable to plane. If courtesy is not your thing, you can consider the other windsurfer as "restricted in the ability to maneuver", which would make you the "give-way" vehicle.
  • If you are the "give-way" windsurfer, you need to adjust your course sufficiently to pass in a "safe distance" from the other windsurfer. That is generally at least a mast length away, but if you are passing in front of another windsurfer on the opposite tack, it should be substantially more. That applies even if you are a great windsurfer - even PWA Pros have spinouts and the occasional unexpected crash.
One thing that is often seen when windsurfers on opposing tacks are on a collision course is that the "hold-on" windsurfer changes the course slightly to point higher into the wind. That is usually a courtesy which allows the "give-way" windsurfer  to stay closer to his original course, without having to go too far downwind. 

So, now back to the two recent "Stand On or Give Way?" incidents. In the first one, I was on port tack, and another windsurfer was coming towards me on starboard tack. We were both planing, and at least 200 meter feet apart when I first saw him. It was clear that we needed to adjust course, but with plenty of time, I waited a couple of seconds to see if he'd pinch higher. He did not - instead, he signaled with his front hand to his windward side. I assumed that meant he planned to go there, and adjusted course downwind - only to discover that he did exactly the same thing! Apparently, his hand signal was supposed to mean "you go there". When we both saw that we had both adjust course downwind, he both re-adjusted upwind .. and were still on a collision course! After the two adjustments, we were now getting uncomfortably close, and both ended up in the water. I don't think we ever got within a board length of each other, and we were at least 50 meters apart after crashing, but it was still a bit scary, and neither of us liked it too much. We exchanged a few shouts on the water, including his question "What should I do?". I answered "keep your course if you are on starboard".  I think this is a very clear illustration why we have the "rules of the (liquid) road" - exactly to avoid this scenario.

The second case was a bit less dramatic, since both windsurfers were slogging, but ended up with a bit more drama. The windsurfer on starboard was a women in her 50s; the windsurfer on port a guy in his 50s who often behaves like a teenager. I did not see the incident. I was sitting on the beach next to friends, watching Nina have fun on my slalom gear, when Heidi came and asked "When I'm on starboard and someone comes at me on port, what should I do - hold my course, right?". We all confirmed that yes, that's what she should do. She had tried that, but ended up having to jump into the water to avoid a collision when the other windsurfer also held his course.

Heidi was clearly upset about the incident, and went to talk to the guy when he came of the water a while later. While we were on the beach discussing the issue, another female windsurfer also stated that he had used her as a jibe mark, and come uncomfortably close. There was a lot of nodding on the beach - most of us had had similar experiences. The guy in question apparently thinks that he is such a great windsurfer that it's perfectly safe to pass someone very closely. Well, most of the time he looks good, but he also has quite spectacular blowups almost every session - often, but not always, in jibes. Others on the beach shared other things that had bothered them about the same guy - things like peeing right in the parking lot, without even bothering to take a few steps in the woods, or dropping banana peels in the parking lot after eating bananas.

The talk happened too far away for us to hear, but it was clear that it did not go well - we could see the "I don't have time for this sh*t" gestures quite clearly. Heidi came back to report that that's what he had said, along with things like "you're just a bad sailor". That, now, pissed me off. I can understand if someone has misconceptions about the "rules of the road" - it happens. But being wrong, being arrogant about it, and being rude towards women - that's three strikes against you.

Up until then, I had thought to be somewhat friends with the guy - we even had invited him to share our house in Hatteras next year, despite some warnings from others who had shared a house with him this year. So I went to talk to him, and started by asking if he knew that the right-of-way rules apply regardless of windsurfing skills. He got upset right away  - "you were not there, you don't know what happened", so I asked him for his side of the story. He gave it, starting with "of course I knew she was on starboard", and "I was waiting for her to pick a course, either upwind or downwind". When I tried to point out that the port sailor (he) had to adjust course, I quickly became the target of his yells and curses, which included slurs about my nationality. At that point, it was rather obvious that this was not a guy I would want to share a house with for 2 weeks.

This is not really an isolated incident. Complaints about him sailing way too close to others are rather common, but since he is quite tall and likely to curse, most refrain from talking to him directly. He recently came back from a 2-week trip to Maui, and reported similar incidents there - again calling the other windsurfer(s) there that had complained to him "bad windsurfers" (in somewhat more colorful words).

The story ends on the parking lot a while later. I don't like to hold grudges, so when he derigged, I walked up to him and tried to make up. We actually should hands with "I got too excited" and "you know how I am" sentences exchanged, but could not quite refrain from talking about the incident. He stated that this was none of my business - which is where I disagreed. A better windsurfer bullying a women into the water, followed by being rude and arrogant to her, is not something that I will just let go. It was clear-cut before: he was wrong, and he was being an ass about it. But he made it even clearer what to think of him when he blamed the entire thing on Heidi having a bad day, and proceeded to call her "cranky old f*cker". Sorry, buddy - you are about as old as she is, and you certainly are the more cranky one here. And using terms like that about someone's friends - well, that's just sociopathic.

But on the bright side, we now have one more room in our house in Avon for the OBX Long Distance Race next April - 2 weeks sound front in one of the nicer houses there!

Tuesday, July 4, 2017


My windsurfing talent is limited. Sure, I may get planing 360s every now and then, but learning these moves took me many times longer than others - even other guys my age. Until recently, I thought that the one thing I am good at was sailing for a really long time. But now, I discovered that I have another talent: destroying windsurf gear.

Waves don't count - everyone can destroy windsurf gear in waves. No, I'm talking about just sailing back and forth, and soft little crashes. What - you don't believe me? Check out this picture of my Skate 110:
I am not talking about the big hole that exposes the styrofoam on the inside, either - I am talking about the air gap between the foam and the sandwich layer above it. The area had been soft for a while, but Hatteras board repair guru Donnie did not want to repair it - he said I should just keep surfing it. So I did, until I saw bubbles coming out of the foot screw holes. Then I got curious and looked inside.

On the cutout pieces, it's easy to see that the top layer of the styrofoam bubbles has been compressed. This is the area around the front foot straps, so the likely cause is slapping into the famous voodoo chop at Kalmus too many times. I sailed this board more than 250 sessions and more than 10,000 km (6,000 miles for non-metric thinkers, or the equivalent of 250 marathons for runners). That's not that many sessions for one board, and there is no visible fault in the construction - I obviously have destroyer talents! Since Donnie called the board a hopeless case, I'll keep using it to practice board repair skills. I ordered PVC foam and a vacuum pump to do a proper sandwich repair. However, the soft area has gotten rather large, so I'll also do some pour foam injection in the surrounding areas, and  I may have to also practice putting foot strap inserts in. I think this will keep me busy for a while!

But the board is not what convinced me that I must have extraordinary destructive talents - no, it's the sails. Just a week or so ago, I very gently fell into my favorite sail, a second-hand North Idol 5.6. Here's the result:
There's a huge rip going through the largest panel and into the lower batten pocket. The two pieces of sail repair tape from previous repairs are a sign that this panel was getting close to dying. However, the repair tape was on just one side of the sail, and the rip includes a previous largish rip. It's quite possible that this might not have happened if I had just remembered to also tape the other side of the sail when I got home... always tape both sides, and be generous with the repair tape!

On the bright side, this little mishap forced me to finally build a sail drying rack in our backyard that my lovely wife had requested, so that her precious sewing machine will not get exposed to salt from unwashed sails. That was today's little project:
Some of you will still doubt my destructive talents - perhaps rightfully so, since the sail was pre-damaged. But only yesterday, I succeeded in damaging another sail, with even less force! I fell on top of my sail at the end of a 360 try - very gently and softly, absolutely no force involved! But when I just touched the sail with my finger,  the monofilm split - a brand new 5 inch long tear. For once, this was not a  Karate move, more a gentle stroke. The sail was less than 2 years old, used fewer than 40 times, and looked like new .. well, until yesterday. Now that's destructive talent!

Thursday, June 22, 2017

Ducks Fly Fast

Ducks can fly - why can't I? It's not for lack of trying:
Maybe I need to flap harder? Or maybe I need to grow a pair .. of wings, that is. Probably won't happen.

The picture is from a session two days ago at Kalmus. After sailing back and forth for 9 hours the day before (I call that "distance sailing"), it was time to do something different - a bit of old school freestyle. I'll refrain from doing any freestyle as long as I have any excuse - "the water is cold" or "I did not sail much recently" being two of my favorites. So I still count the Duck Jibe as a freestyle trick. It's tricky to catch the sail after throwing it! Works better than trying to fly, though.

The crash in the picture above was harmless - perhaps all that flapping helped, after all. A little while later (and a little more tired), I had a better crash. If you've learned the Duck Jibe, maybe you remember the "Plank Walk": when your mast hits the water, and your gear suddenly stops, but you keep going ... straight off the nose of your board. That's pretty much what I did. Here's the GoPro footage:
That was fun! As a little bonus, I set my top speed of the day during my brief flight, according to my GPS:
I was three knots faster than in my second-fastest run! If you believe the GPS, that is. I really don't think that I was airborne for longer than 2 seconds - I would have flown about a 100 feet in that time! So let's look at the data more closely:

Here is the narrative to what happened:

  • I slowed down a bit during the sail duck, from 22 knots to 18.5 knots
  • When the mast hit the water, the board came to a sudden stop. Not so my body! Since my foot was still in the strap, the upper body did not just keep the speed, but actually accelerated from 18.5 to 26 knots within 0.4 seconds
  • I hit the water shortly thereafter. As soon as my hand was deeper than about 2-4 inches, the GPS watch lost reception, since water absorbs the GPS signal. We can see that point #24220 is not the expected 0.2 seconds after the previous point, but rather 1.6 seconds. So 1.4 seconds worth of data are missing.
  • GPS chips are made for cars, not for windsurfing. Cars often loose GPS reception, for example in tunnels or under bridges; but usually, they keep going at roughly the same speed and direction as before.  So the GPS chips go into "dead reckoning" mode, assuming that the "car" is still traveling at the same speed, and predicting the speed and new position.
Well, I may be many things, but a car I am not. I did stop when I hit the water. Speedsurfers traveling twice as fast as I did may bounce a few times before sinking, but I did not. So the GPS guessed wrong about how far and fast I traveled when I had no GPS reception. It corrected it's mistake very shortly thereafter: in point #24221, it adjusted the position by about 20 meters, roughly the same amount that it guessed wrong in the first place. The two points with large distances easy to see in the tracks:
For this particular file, only one of the three GPS speedsurfing programs gives the correct 2-second top speed. That is GPSResults, which refuses to use data points that have longer-than-expected time gaps. Both GPS Action Replay Pro and use such data points, and therefore give an incorrect maximum 2-second speed that is "dead reckoning inflated" by about 3 knots.

Using accuracy estimates (SDoP filtering) to identify the "guessed wrong" data points could have worked, but would have required a threshold of 3.0 knots; the current threshold of 4.0 used by GPSResults would have let the bad points through. An "average SDoP" filter with a threshold of 1.5 that I had proposed earlier would have easily identified the bad points, and avoids the potential issues with invalidating larger stretches of data if single points are missing.

Now if I can only convince myself to duck jibe at the end of speed runs, maybe I'll finally get a 35 knot top speed...

Wednesday, June 21, 2017

Fun at Kalmus

It finally felt like summer today! Not that I'm complaining about windsurfing 5 out of the last 6 days, but fog in June just feels wrong. Today was sunny!!
After doing a bit of distance sailing two days ago (206 km, almost the distance of 5 marathons :-), I was a bit tired yesterday - but today was just perfect, with SSW wind and low tide. The 5.6 m freestyle sail felt so easy after 6 and 7 m freeride and 7.8 m slalom sails! Here's a short video from today:

Friday, June 16, 2017

Bump and Bay

Southeast wind all day - where do we go to play today?
Our home spot Kalmus is a S-WSW spot. In SE, it gets wind shadow from Great Island - the little island that extends down from Lewis Bay.
The usual SE playground is West Dennis (marked with an "A") in the map. But besides the $20 fee and a drive that seems to take forever during tourist season, the birds have taken over - the usual windsurf spot is blocked off.
Next to Monomoy Island (marked with a "B") on the map might also be a great spot to find some flat and shallow water. But you'd definitely also find a lot of seals, and the occasional Great White Shark. And someone who does not want to drive to West Dennis now certainly would not want to drive twice as far!
So it was time to check out a closer spot: Oregon Beach in Cotuit. I had tried to sail there a few times before, but the wind never cooperated. But let me zoom in a bit to show you why I had not given up on it yet:
That long sandbar is called Popponesset Island. It's about 800 meters long and just a few feet high - potential speedsurfing territory! I had checked it out once on a longboard, and it looked promising near high tide. The other thing you may notice is the color of the water on the right: green instead of blue means shallow! Nautical maps show depths of 1-3 feet to almost 1/2 mile out. Freestylers like shallow since they fall a lot. Speedsurfers like shallow because it reduces chop.
We started windsurfing shortly past low tide. As usual, Nina was on freestyle gear (90/4.7), so I also chose my Skate 110 and a 5.6 m freestyle sail. It was surprisingly bumpy in front, but after the first 30 minutes, the wind filled in nicely. I had fun doing mile-long runs over to Dead Neck. But when I checked with Nina, she said she did not like it much - the chop was a bit disorderly, the wind a bit gusty close to shore where she wanted to practice freestyle, and she even had found a few deep spots where she could not touch ground.
So now I had a perfect reason to check out Popponesset Bay! The sandbar in front would reduce the chop, and the inside should be shallow enough to stand - so off I went. Getting there from Oregon Beach (where you need a Barnstable resident sticker for parking during the season) was easy, and the bay was shallow, chop-free, and windy! I turned around to tell Nina, but discovered that she had followed me, and was waiting on the ocean side beach near the tip of the island.
We spent the next hour sailing inside Popponesset Bay - here are today's GPS tracks:
Let's zoom in to have a closer look at the inside sailing:
The Google Earth image shows a shallow section in the middle of the bay. When we started sailing, a lot of this area covered by a foot or less of water; at some spot, it was just ankle-deep. In view of a great wind forecast for the next week, I had no desire to once again prove my skills in discovering shallow spots at full speed, so I stayed a bit further to the inside of the bay. The runs there were about 450 m long, about the same as at Fogland inside the bay. Closer to high tide, similar runs further up would also be possible. With a slightly more easterly wind direction, runs right next to the Popponesset Island would beckon .. we left that for another day.
We sailed back when it started raining. Overall, the spot was not quite as good as I had hoped for, but it was still plenty of fun. There's definite speed potential inside the bay, ideally during ESE and within 2 hours of high tide. However, runs are a bit on the short side, and getting there on a really windy day would require braving some not-so-small chop on speed gear. On summer weekends, boat traffic might also be an issue, but today, we only saw two or three boats coming into the bay. Now if someone could just turn off the rain that usually accompanies easterly winds...

Tuesday, June 13, 2017

Longer Is Funner

You'd think watching coupling and decoupling should be fun. It's not, at least if it's the wind. I did that yesterday, so I know. We got ready to leave several times when the wind meter went up to 18 mph, only to see the wind drop again 15 minutes later.

Today was a similar hot day. When the wind meter started playing the same up-down-up-down games again, I had enough. I put my F2 Lightning in the van, drove to Kalmus, and went out with my 8.5 m V8. Here's what happened:
Simply put: lots of fun! The wind was very iffy near the shore; a few guys who tried to go out on shortboards got very frustrated. The longboard, though, was fun from the very start. I toured a mile upwind to the Kennedy Slicks, where the wind seemed more consistent; then I crossed over to the point. The wind was great again on further away from Kalmus; however, I had to go really slow close to the point, because the many rocks there were barely visible at high tide. The swell on the outside was quite big, but I was out there all alone, and did not want to push my luck.

After about 90 minutes, the wind picked up to averages around 23 mph, and became more steady. Blowing sand was a clear indication that gusts were above 25, and my 8.5 was starting to feel a tad big. I also regretted that I had put in only the three rear-most footstraps - I could get in, but I could not find a nicely balanced position, and constantly had to push the nose of the board downwind. Maybe the Lightning thought it was a shortboard? More likely, though, the fin was not right for the conditions. I briefly thought about rigging smaller, but clouds were coming in, so I did not think the wind would stay up long - and indeed, it went back to its nasty up-down-up-down ways before I made it home. But I'm not complaining - I had two hours of most excellent longboard fun. Fully planing in the rear footstraps, with what feels like 10 feet of board in front of you and in the air, is an almost surreal experience. So, playing with coupling and decoupling winds is fun - you just have to bring the right toys.

Monday, June 12, 2017

Too Hot

The wind has been sparse this spring on Cape Cod. So when the forecast for yesterday started to look good a full week in advance, and then stayed good the entire week, we got excited. But unfortunately, the temperatures also started to climb - to the mid-80s (30ºC) even on Cape Cod, and into the 90s inland. With the water temperatures still in the 60s, the dreaded decoupling threatened. And it came:
iWindsurf wind meter readings for Kalmus 6/11/2017
For comparison, here are the meter reading for Hatch Beach, where the wind was sideshore / offshore:

We made it to the water early enough to catch the best half hour of the morning wind, and stopped as soon as the wind dropped, hoping for a second session in the afternoon after it cooled off. The short pickup at 4 had us return to the beach; but by the time we arrived, the wind had dropped down to 13. We spent the next 2 hours talking to friends in the parking lot, and rigged the next time the wind picked up. It was another false alarm! But this time, we waited it out on the water, and were rewarded after half an hour of shlogging. It got better and better during the next hour, until it got dark and the sand was blowing. Everyone (about 5 windsurfers were left) came of the water tired but happy.

While the heat was to definitely to blame for the lack of wind in the middle of the day, it also has warmed up the water quite nicely. A 3 mm short-sleeved wetsuit is plenty warm, and requires frequent falls to avoid overheating - summer is freestyle time! Nina is now seriously thinking about the loop, so I practiced a few chop-hop exercises that might help. One is jumping far rather than high, and keeping speed; the other one is trying to kick yourself in the butt with the back foot, while keeping the front leg at least somewhat extended. I'm pretty sure some teaching gurus have said that both of these exercises help with the loop - but regardless of that, they are also fun. For once, neither my knees nor my ankles complained about the bit of jumping around, but it may have helped that the total time on the water was less than 2 hours. I also got my first Carve 360s of the summer season, so I was pretty happy. Here are the GPS tracks (a bit shifted to reduce the long break between the sessions):
Today was even hotter than yesterday. The wind tried to come up a few times, but it never lasted more than 15 or 30 minutes. There is some hope in the forecast for the end of the week ... there's always hope.

Wednesday, May 31, 2017

Reader Comments about Filters

In response to my post about the "60 Knot New Speedrecord", a reader had a bunch of comments. I answered the first comment in the comment section of the original post, but he came back with a whole bunch more. Mathew wrote:
"Thanks for writing up a follow up. It should be pointed out that the acceleration filter shouldn't "per point" as highlighted in the follow-up... it should apply from last-good-point until the next point which doesn't breach the threshold. Also - and this example highlights it - once a filter has been breached in the middle of a run, the remainder of the run must be discarded [ most of the time ]. The problem here isn't that we dont understand what type of filtering is required - we already do know -> the problem is that the auto-filtering of the software analysis, doesn't match how humans do it. ie: if your data isn't clear, then then the data from that run shouldn't be used. [ Which is why we use two, three or four GPS's.... ] ... In the RealSpeed example, with 3ms-2 (5.5 kns-2) it takes about 3 seconds for the threshold to not be exceeded [ (20.8-4.5) / 5.5]. Since a new threshold is breached within 3 second window, the new window needs to be calculated... and so on. Thus about 7 seconds or so should be dropped.
A standard practice in all sciences, is to drop data that is completely bogus -> thus all "simple filtering" should be applied first, then apply SDoP filters last. [ Caveat: big-data analysis doesn't work this way generally speaking, as the algorithms are sometimes more accurate using bogus data. ] It would be useful to implement the filters accurately, then apply it to these examples vs. applying a simple SDoP filter.
... the point is -> SDoP/SDoS isn't a panacea - it doesn't replace simple physical constraints of the sport. Indeed humans naturally apply a few more physical constraints, ie: even though 3ms-2 is the threshold, it is completely unlikely to that you will sustain 1/3G acceleration for more than a few seconds simply because the drag component goes up very quickly [ vs terminal velocity of about 30m/s ]. "

This is pretty long, so let me just restate what I think he is saying in simple terms:
  1. We know that acceleration in speedsurfing is limited to less than 5.5 knots/second
  2. When looking at GPS tracks, that allows us to identify artifacts without any doubt. If the speed shoots up from 5 knots to 20 knots between 2 data points, we know something is wrong.
  3. If software could implement this "physical constraint" in filters, that would allow the identification of artifacts without SDoP filters.
  4. However, just throwing out single points where the acceleration is above the threshold is not good enough (as I had illustrated in response to his previous comment).
Before I go into a detailed analysis about these points, let me say that I mostly agree with Mathew. For example, the typical acceleration in speed runs is less than 2 knots per second. For example, check out one of Boro's recent traces where he got close to 40 knots:
Let's zoom in on the speed run in the middle:
I selected the region where the acceleration was highest, or close to it. Within about 4 seconds, he went from 31 knots to 36 knots - that's an acceleration of about 1.25 knots/second. So if we accept that this is a typical track, then a threshold of 5.5 knots/second seems rather safe. But is it?

Let's look at what the current default in the most accurate GPS analysis software is, GPSResults. For 5 Hz GPS data, the acceleration filter defaults to 8 m/second(squared) - that's 15.55 knot. That's about three times as high as what we said! Why does GPSResults not use the 3 m/s2 that Mathew suggests?

Well, theories are all nice and good, but it's always better to look at real data. I went through a test set of about 200 GPS files from GW-60 watches and GW-52 units to see how many of these files had points with an acceleration above 3 m/s2. The answer: 125 of 197. That's about 2 out of every 3 files! If we'd implement a filter with this threshold, it would remove good runs from the majority of files.

So what threshold would work? In the test set, the highest acceleration within the top five 10-second runs was 6.35 m/s2. If you're wondering what the data look like, here's a screen shot from my analysis software:
At the second point in the table, the doppler speed jumped from 25.562 knots to 28.03 knots - that's about 2.5 knots faster in 0.2 seconds! Looking at the track, the spike is quite obvious. It looks like random noise added on top of a much smaller real acceleration; in the next data points, the speed goes down again. Overall, though, this run looks good - there's no reason to discard it.

So even for this small set, we'd need a threshold close to 7. If we'd look at more test data, we'd probably find a few tracks with even higher numbers, so the threshold of 8.0 that GPSResults uses for 5 Hz data seems quite reasonable. Unfortunately, having to use a higher threshold throws off Mathew's argument a bit. 

The higher threshold is needed because the higher frequency data from the GW-60 and GW-52 devices has a lot more noise. That's not a problem - since we have a lot more data points, we can average out the noise. But trying to implement this in a filter will be quite a bit more complicated than it seems at first glance.

If we examine the current versions of 4 different programs to analyze GPS data (GPSResults, GPS Action Replay Pro,, and RealSpeed), it appears that only one of the four has a reasonably implementation all filters that are known to work, and uses them by default when calculating speeds (I am talking about HDoP, SDoP, minimum number of satellites, and acceleration filters). Three out of the four programs are missing one or more of these filters, and/or require a separate manual step to apply them. This is for very straightforward filters: "if the value is above (or below) the threshold, mark the point as bad, and do not use it to calculate speeds". It would be a great improvement to see these simple known filters implemented in all four GPS analysis programs.

But even the most accurate program, GPSResults, currently fails to identify some artifacts that can occur when the data quality is marginal (but not obviously bad).  For this particular problem, better filters would be needed - for example filters that use average SDoP errors for short-term speed (or standard error estimates, which also consider all error estimates in a region, not just single points). For errors that result from "dead reckoning"-type overstatements, acceleration filters would not help at all - the very cause of the error is that the software inside the GPS tries to keep speed relative constant when the GPS signal is bad. So far, though, the level of interest in implementing better filters for such artifacts seems darn close to zero.

Let me finish with a comment on Mathew's pointers towards scientific and "big data" analysis. There are some interesting similarities between large-scale DNA sequence analysis and GPS analysis. I worked in large scale DNA sequencing at the beginning of the Human Genome Project (HGP), when the data were analyzed very much like GPS data are still analyzed for records: with some computer help, but also a lot of human checks at any problem regions. During the first years of the project, progress was very slow, and many scientists doubted that the project would finish on time. The first big breakthrough came when someone developed accurate "quality scores" - a measure how likely the sequence data points were to be correct, which is very similar to SDoP values. As soon as these quality scores were widely used, many analysis bottle necks were removed; the accuracy of the "final" results increased dramatically; and throughput jumped. The quality scores were one essential part that enabled further dramatic improvements which allowed the Human Genome Project to finish ahead of its deadline. Early on, I did a little bit of research to evaluate both the accuracy and the usefulness of the "SDoP-equivalent"; the results surprised me positively on both counts. 

In the early phases of the HGP, we did something that is very similar to the wearing of "2, 3, or 4" GPS units that Mathew mentioned - we'd also generate data several times over. We had lots of complex rules and highly educates scientists to check on any discrepancies; but in the end, we ended up with "finished" data that had orders of magnitude more errors that what late could be done completely automatically, using accuracy estimates (and proper filters and analysis algorithms).

So yes, I may be somewhat biased towards seeing the usefulness of accurate error estimates.

Saturday, May 27, 2017

Overstating Speed by 4 Knots

This is a geeks only post. You've been warned.

I love my GW-60 GPS watch, and it has worked great for me. However, recent problems with accuracy that both Boro and Denis on our team had seen made me wonder how often such issues arise. I had never seen them in my data, and I often looked closely enough that I would discover them. But many speedsurfers just quickly analyze the data to get their speeds, and upload those speeds, trusting whatever the software tells them.

So I went on a little fishing expedition on I searched for downloadable GPS sessions from May 15, 2017 onwards, and downloaded data files that were from GW-60 watches. Then, I looked at the tracks in detail in GPS Action Replay Pro and GPSResults, searching for examples where the 2 second speed was overstated.

After checking 15 files, I found one with a problem:  the top speed was just a few seconds of 25-knot speeds, in the middle of what looked like a longer swim. That's not how top speeds are reached! Sure enough, the GPS analysis software came to different results: and GPSResults gave a top speed of 25.3 knots, but GPSResults gave only 21.37 knots - 4 knots less! Let's look at the data:
This windsurfer had set up his watch to record only if the minimum speed exceeded 5 knots. The middle graph shows the speed: it was below 5 knots for about 4 minutes, with a few seconds where he supposedly sailed a lot faster than during the test of his session. The tracks on top show that this was at the very end of a run, after turning around - very suspicious! The accuracy graph at the bottom confirms the suspicions: the watch new that it did not have good data, and gave the data near the spike a much higher error estimate than the test of the track. Which, by the way, is why GPSResults did not include this region when it looked for top speeds.

Here are the data points in this region:
All data points in this regions have +- numbers between 3.2 and 4.957 - in other words, they are junk and should be ignored. For this screen shot, I forced GPSResults to behave badly by turning the filters off. The two other analysis programs, GPS Action Replay and, always behave badly with data like this - they do not have SDoP filters.

In both this and the previous example of speed artifacts, the errors occurred during prolonged times where the windsurfer apparently was in the water, probably swimming, trying to waterstart, or resting. In such situations, GPS watches are more prone to artifacts because of where they are typically worn: on the wrist (instead of on the upper arm or helmet like other GPS devices). That means the watch will be under water much more than a GT-31 or GW-52 GPS, not getting any GPS reception because water absorbs the GPS signal. But at times, the watch may be close to or just above the water, and get a signal for a short time. At that point, the GPS chip has to re-acquire the satellites, and calculate the current position from scratch - which can introduce big errors, especially if the reception is marginal. In the tracks and the non-doppler speed graphs, there are often big jumps and spikes when this happens. Usually, that's not a problem with doppler speeds (which is exactly why we use doppler speeds for GPS speedsurfing!): single speed spikes from position adjustments just disappear in the doppler speeds. But when swimming with the watch close to the surface, we sometime see multiple spikes in the speed graphs. When that happens, the error can "go through" to the calculated doppler speeds, which is what we see in the graphs above.

Many speedsurfers may never see issues like this one because they sail in shallow locations, nail their jibes, and/or waterstart quickly very time they crash. But if you have a session where you spend a considerable amount of time in the water, then the chances of errors like the one described here are significant. Sometimes, it's obvious - Denis knew he had not set a new world record. Other times, it's not; in the example above, the 2 second speed seems reasonably close to the 10 second speeds, at least at first glance, so the result was posted to the GPS Team Challenge. In this case, it did not affect anything; but similar problems could happen during faster sessions that count for the monthly ranking.

While this particular problem here may be specific to GPS watches, and maybe even the GW-60 watch, I have seen similar artifacts in GPS data from all kinds of devices, including the long-time "gold standard" GT-31. Typically, it's easy to recognize such problems if you look at the your GPS traces, and examine your doppler speed graphs for the top speeds. In GPS Action Replay, I also find the "SDoP" graph quite useful to identify problem areas. I don't find GPSResults quite as intuitive, but then, GPSResults does a much better job at filtering out problem areas automatically, so the results are more likely to be accurate in the first place. Hopefully, we'll also see similar automatic filtering soon on and in GPS Action Replay Pro.

Thursday, May 25, 2017

60 Knots - A New Speedrecord?

Short answer: no. Before I get to the long answer, let me warn you: this is another one of my geeky posts. I reckon it may be of interest to about 5 or 10 windsurfers in the whole world! You've been warned.

This all started when our "United Speedsailors of America" team member Denis posted results from a recent session. He had uploaded his results to, and it showed that he had reached 2 second speed of 59.88 knots:
Denis does around 40 knots on a regular basis, so he is wicked fast. But he also knew that he definitely had not broken the world record for 2 second top speed (55.23 knots by Antoine Albeau on the Lüderitz Speed Channel). Even the ka72 results hinted in that direction: his best 10 second speed was just 30.1 knots.
Furthermore, Denis often sails with Boro, who just recently had his own problems with GPS accuracy,  so he was very skeptical about what his watch tried to tell him. He had also worn his old trusted GT-31 GPS unit, and that had reported a top speed of only around 30 knots.
World-famous speed surfer and GPS guru Roo (yes, he is from Australia!) immediately looked at the tracks, noticed that during the time of the supposed record speeds the "sat count dropped to 6/7", and came to the conclusion that due to "the surrounding mountains the sats that were just above the horizon produced poor signals which caused the anomoly".

Well, this seems to make sense - Denis and Boro are windsurfing on a mountain lake, so the mountains around it must be too high! Since almost everyone else is windsurfing in flat terrain, maybe we can just ignore their problems? That's simple! Simple is great! Right?

Not so fast, cowboy! Sure, there are 10,000 ft (3,000 m) mountains around Washoe Lake. But the lake itself is at 5000 ft elevation, measures 2 x 4 km, and is in a valley that extends more than 10 km from each end of the lake. So a lot of the sky and the GPS satellites will be visible. So let's look at the tracks:

The doppler speed graph from his GW-60 watch is shown on the top in red; for comparison, the GT-31 data are shown in blue at the bottom. The GW-60 data show 3 spikes: 2 above 50 knots, and one above 70 knots! Before we jump to any hasty conclusions, let's zoom in on the largest spike:

We can see that the blue (GT-31) graph shows a speed close to zero around the time of the spike. In other words, Denis was most likely in the water at that time. There are few things that kill GPS reception as well as a few inches of water! We can also bet that Denis wore his watch on his wrist, and the GT-31 on his upper arm, so at times, the watch will have been deeper under water. Sure, the watch should not show a spike like this .. but I have seen similar spikes in GT-31 data in the past (and do not usually see them in GW-60 data, even when I fall a lot). As Forest Gump said: "It happens".

But let's not stop there - let's look at the actual data points:
Pay attention to the "+/-" column, which shows error estimates for the speed (also called "SDoP", for "Speed Dilution of Precision"). The SDoP values start at around 2, but then quickly jump up to 4.957, which is where they stay for a while. At the first line of the highlighted area, we can also notice that the time between data points jumped from 0.2 seconds to 16.6 seconds - for more than 16 seconds, the GPS did not have enough reception to calculate speed. Denis, where was your hand?

If you wondered why the +/- numbers never got higher than 4.957, give yourself a geek gold star! That's simply because the SDoP numbers are stored as a single byte in the .sbp files, and the highest number possible corresponds to 4.957 knots. In plain English, an SDoP of 4 or higher indicates complete junk.

So, the watch actually knows that these data points are junk! Any analysis software should now, too! As guru Roo correctlu pointed out, GPSResults completely ignores the problematic data points, and gives top speed results that are practically identical to the results from the GT-31 unit. GPSResults has an SDoP filter, and by default refuses to include any regions with SDoP values above 3.0 (for 5 Hz data). The big question remains: what about other software?

Many Australian speed surfers analyze their results on, and Denis did. However, it appears that either does not use SDoP filters, or does not use them correctly. If we assume that Denis' problem had more to do with putting the watch under water than with being surrounded by mountains (which was not a problem when he was not swimming), then this problem might affect many other speed surfers, and needs to be addressed (there's also a problem with the alpha 500 numbers being wrong for this track).

Now, how about my favorite GPS analysis software, GPS Action Replay Pro (GPSAR, which I used to create all the screen shots above)? Unfortunately, the results are not pretty:
GPSAR calculates a top speed of 64.55 knots! Based on the settings shown in the speed dialog, GPSAR does not have any SDoP filtering at all, and thus arrives at incorrect results.

So, here's the bottom line:
  • Accuracy estimates are fantastic to automatically identify bad data can lead to very large over-estimates - even fake "world records"
  • and GPS Action Replay Pro do not appear to have functioning SDoP filters at this point in time; any data analyzed with either software should be carefully checked!
  • GPSResults has SDoP filters that can correctly identify artifacts such as the ones described here; however, as I have shown in a previous blog entry, the default settings may not be adequate to catch smaller errors, which can overstate speeds by a knot or two
As mentioned above, Denis's data files are available on I also made zip files with the data from my previous post about GW-60 accuracy for Boro's data and for the 6-GPS test drive.
Added May 28, 2017:
In a comment to this post made today, Mathew made the bold statement "You dont need SDoP - just have a sensible acceleration filter". Well, acceleration filter can indeed be helpful to identify some problems, but they can not identify all problems that SDoP filters can find. The program GPS Action Replay does have an acceleration filter, but using it (which is an extra step that most users won't do) does not fix the problem. Applying a 3 m/s2 acceleration filter in the current version of GPSAction Replay (5.23) removed 801 points from Denis' track:
But afterwards, the speed table still shows a top 2 second speed that is 17 knots to high:
That's a little better than the 64.55 knots before the filter, but still wrong. Perhaps this is a bug in GPS Action Replay that could be fixed - but acceleration filters can, by definition, only identify some artifacts: those with too much acceleration. In case of artifacts that remain high for a few seconds, acceleration filters will fail. Here's an example, using RealSpeed and the problem trace I described in my next post:
Here, the speed increases from less than 1 knot to 20.5 knots in 0.2 seconds, and then to 24.7 knots. After that, the speed remains around 23-25 knots for more than 2 seconds. The acceleration filter identifies the first 2 points, and also a few points at the end of the artifact (shown in red); but the points in the middle have low acceleration and look fine to the filter, which is why they are shown in green. Even after applying the acceleration filter, this data set still gives a 2 second "top speed" that is from an artifact, and 4 knots higher than the real top speed in this track. But the SDoP values in this region, which are all above 3, make it very clear that the data cannot be trusted and should be discarded.

Another example of errors which acceleration filters cannot catch, but SDoP filter can, are the more subtle 1-2 knot errors that can result from using an underhand grip in speed runs, with the watch facing downward and the lower arm blocking the GPS signal. It remains to be seen, however, what the best thresholds and implementation details for SDoP filters are that can catch such artifacts, without throwing out "good" data where just a single data point has higher-than-usual error values. That requires looking at a bunch of GPS files in detail to see how to separate the good from the bad, and will take a while.

Wednesday, May 24, 2017

Lightning in my Harbor

Lightning is the only thing that gets windsurfers off the water. But sometimes, Lightning may be the only thing that gets windsurfers on the water. Obviously, I'm talking about two different things.

The thing that got me on the water today is shown in this picture:
It's an F2 Lightning from 1987. I got it a few months back for $100. What a steal! Of course, it needed some work - a few little holes needed patching; the footstraps were falling apart and needed to be taken off; the mast track needed an hour of my attention before I figured out how to make it work again; and the daggerboard gasket needed to be replaced (with a few layers of sail repair tape for now). The picture above is from its brief maiden voyage at Fogland last week. Since then, I started to work on putting some footstraps on, and got the mast track working again, so I was dying to test it. When the Chapin wind meter showed averages around 14 mph today, with gusts to 17, I just had to go! The GPS tracks tell the story:
For comparison, here are tracks from an earlier session on a 117 l slalom board with the same sail:
The slalom session had quite a bit more wind (gusts up to 24 mph), but the tracks are more the typical back-and-forth session (I had to use a weed fin, which did not help). In comparison, going upwind on the F2 Lightning was not only 10 times easier, it also was a lot more fun - the longboard railed up very nicely! I played around with the upwind stance Andy Brandt had shown me last year - front foot sideways on the daggerboard knob, with the lower leg lying on the board, and the body far out over the water - that worked amazingly well! The top speed on the Lightning also was quite good - almost 24 mph, compared to 28 mph on the slalom board - but again, gusts were 7 mph on the slalom day! It really helped that going upwind on the longboard was so wicked easy; that made long, deep downwind runs easy, too. But the biggest difference was when the wind dropped down to 12 mph during the last third of the session. On the slalom board, that would have meant boredom and pain - there's simply no fun to be had on slalom gear unless it's planing. The Lightning, in contrast, was still fun, slowing down proportionally to the wind strength, instead of dropping suddenly from 20 mph to 6 mph as the slalom board would have done. The $100 I spent for the F2 Lightning were the best $100 I ever spent on windsurf gear!
Some of the readers with excellent memory may remember the title of this post, and wonder why I called Barnstable Harbor "my harbor". Well, that's simple: I had it all for myself! There were a couple of fishing boats out - maybe one every few of square miles. But otherwise, the harbor was all mine - as it is most of the time when I windsurf there. Not that I'd mind sharing - it's big enough, and one of my all-time favorite windsurfing places!