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!

Monday, May 15, 2017

The Fastest Way To Get Faster

Some people who saw me driving around today looked at me in a funny way. Was it because I had my arm out of the window in 54ºF (12ºC)? Or maybe because I had 6 GPS units strapped to my arm?
In case you're wondering: I was wearing two yellow GW-52 units on top; two GW-60 PGS watches on the wrist, one facing up and one facing down; and a GT-31 plus a third GW-52 at the bottom.

As fascinated as I am with GPS speedsurfing, even I need a good reason to strap more than $1000 in GPS devices on my arm and go for a drive. I had one! I wanted to reproduce what Boro had accomplished recently. Boro wanted to start using his GW-60 watch, and compared it to his trusty old GT-31 GPS units. He got substantially higher speeds from the GW-60 watch:
  • 2 seconds:
    37.99 knots GW-60, 36.29 knots GT-31
  • 5x10 seconds:
    33.71 knots GW-60, 33.20 knots GT-31
That's 1.7 knots more for 2 seconds! It takes me (and others) years to improve my speed by that much! In many other tests, the speed of the watch had been much closer to the speed of other GPS units - was his watch broken, or was there another cause?

Looking at Boro's GPS files, it was obvious that his speed data had unusually high error margins of +- 1 to 2 knots; that's at least 2 times higher than usually, and about 5 times higher than what can be achieved with good GPS satellite reception. The error margins were about 2x higher when he was going the direction of his speed runs than when he was sailing back.

The first suspicion was that he was using an undergrip, with his watch hand closer to the mast, on his speed runs, and Boro quickly confirmed that this had indeed been the case. The human body is a very good absorber of GPS signals, and the GPS gurus had been very concerned about this very scenario - watches faces down towards the water, with an arm between them and the GPS satellites. I quickly did a little test walk with a couple of GW-60 watches on my wrist, one facing up and one down, and was able to reproduce the roughly 2-fold increase in error estimates if the watch is facing down.

But could such an increase in speed errors cause the observed difference in speed? There are some theoretical arguments against that: if the error is random, and we collect data at 5 Hz, we get 10 data points for 2 seconds, and 50 for 10 second runs. Average error should sometimes go up, sometimes go down, so that the total error should go down the more points we have. With 10 points, we'd expect the error to go down about 3-fold; with 50 points, about 7-fold. With data like Boro's that have about 1.5 knots error estimates, the expected error for 2 seconds is just 0.5 knots; for 10 seconds, it's closer to 0.2 knots; for 5x10 seconds, even lower.

So, something just is not right here. Instead of boring you to death with more theory and statistics, let's get back to my little drive. The idea was simple: let's just see what actually happens if we go for a drive with a bunch of GPS units, some facing up towards the satellite, some facing down. Somewhere in the middle of the drive, turn the arm around by 180 degrees and see what happens! And, most importantly - what's the effect on the top speed?

Let's start with a look at the speeds:
This is about 15 minutes worth of data from 6 GPS units - a bit too much information to see much. I deleted a few data points in the middle, where I turned my arm around. I also split the tracks into 2 segments for each device, before and after I turned the arm. So each device was pointing up for one segment, and down for the other.

Let's jump straight to the 2 second speed results from the first segment:
Four units show top speeds close to 28 knots: all the units that were facing up, and the GT-31. But the GW-60 watch and the GW-52 unit that were facing down show speeds that are almost one respectively 2 knots higher! Let's look at the data in detail:

The top graph shows the doppler speed, the bottom graph shows the error estimates. What stands out is:
  • Two speed curves are substantially higher than the others: the black (GW-52 #3) and the red (GW-60 #2). 
  • The same two GPS units have error estimates in this region that are about 2x higher than the other units,
  • The observed error is not random. Both the red and black speed curves stay about 2 knots above the other curves for 8-10 seconds (40-50 data points). If the error would be completely random, this would be extremely unlikely to happen by chance; winning the jackpot in a lottery is more likely.
I can only speculate why the error becomes non-random at high SDOP values, but there's a likely guess: when the signal quality gets too low, the firmware starts to rely more on "dead reckoning". GPS chips sometimes use dead reckoning when no GPS signal is available, for example in tunnels. A sensible firmware implementation would start using increasing amounts of dead reckoning as the signal quality decreases, rather than a single "all-or-nothing" threshold. There are several indications that something like dead reckoning is causing the observed error characteristics, but that probably deserves another post.

Another question that arose was whether the problems are specific to the GW-60 watch, since it apparently has a GPS antenna that is smaller than the antenna in the GT-31, and thus possibly inferior. Let's look at the error estimate graphs from the "up" and "down" segments to get an idea. I color-coded the graphs so that blue indicates "up", and red indicates "down". Here's the GW-60 watch:
The SDOP values roughly doubled when I turned my arm so that the GPS was facing down. Here's the same graph for a GW-52:
Pretty similar, again with a marked increase when the GPS unit was faces down under my lower arm. Finally, let's look at the GT-31:
The GT-31 was facing down during the first segment, and up in the second segment. Again, we see much better data accuracy with the GPS facing up towards the satellites. Note that I sometimes had to take my hand into the car to turn, and that I may not always have had the arm oriented perfectly up or down, but the overall trend is clear: all three devices show the same trend towards much degraded accuracy of the GPS is worn facing to the ground.

Here are some conclusions and suggestions from this analysis (some of these points just re-iterate what others have stated many time before):
  • When using a GPS watch, make sure that the watch is facing up during your speed runs! If you use an undergrip on your front hand, wear the watch on the inside or (even better) on your back hand. Or get an armband extended and wear the watch around your bizeps - it will get better reception there. For the best GPS reception and the most accurate data, consider mounting it on your helmet (if you wear one).
  • The GPS analysis software may need to be modified to deal with poor quality data better. For example, the default SDOP cutoff of 3.0 that GPSResults uses for 5 Hz data seems too high, as the example above and Boro's initial experience show.
  • While the examples I have shown here focus on speed over-estimates, it's just as possible that the error goes the other direction, and the speed is under-estimated. Here's an example:
The red track is from a downward-facing watch, and the speed happens to be several knots too low just around the maximum speed for this region. So wearing the watch facing downward during speed runs may not just be the quickest way to pick up a knot or two - it may also be the quickest way to loose a knot. So - keep it up!

Monday, May 8, 2017

Fun With Eddie's Pictures

We had a couple of nice windsurf sessions at the "Kennedy Slicks" in Hyannis Port Harbor yesterday and today. Eddie took some pictures both days - yesterday from Hyannis Port, today from Kalmus, about a mile away. Here is a picture from yesterday:
I'm about 200 feet away from the pier in this picture. The wind had dropped a bit when the picture was taken, so I'm hanging really low. I had wanted to stop a few minutes earlier, but went out again when I saw that Eddie was just setting up to take pictures.

For comparison, here is the picture from today:
In this picture, either I have shrunken a lot since yesterday, or the wall is noticeably taller. Some shrinking happened - I was on a 7.0 today and a 7.8 yesterday, so the mast length was about 460 cm compared to 480. But mostly, the wall was taller.

Some of you may think I'm telling stories. Some may grow concerned about walls in the water suddenly growing. Some may astutely observe it's not the same section of wall in the two pictures. But those of you who have windsurfed at the Kennedy Slicks will remember that the spot is quite tide dependent. We don't have a lot of tide in the Nantucket Sound - the different between high and low tide today was only about 3 feet. According to the the tide tables, the tide was about 1 to 1.5 ft lower when today's picture was taken.

Since we've got the pictures, know the mast lengths, and have too much time on our hands, we can calculate the height of the wall above the water. Yesterday, it was about 140 cm; today, it was a bit above 200 cm. Most of the difference was from lower tide levels; the remainder could be from some small differences in height between the different sections, or from slight chances in wind direction that pushed more water towards shore yesterday compared to today (it was SSW-SW yesterday, compared to WSW-SW today).

Why does it matter, you ask? Well, thanks for asking! You just gave me an excuse to post today's GPS tracks:
I got my top speed of the day in the second run. All top 5 speeds were in the first 30 minutes; after that, I could not break 30 knots again, no matter how hard I tried. But the readings from the nearby iWindsurf meter show that the wind actually was a knot or two stronger in the second half of the session. So I should have been able to go faster after the first 30 minutes!

From the beginning to the end of the session, the tide dropped by about 0.8 ft, according to these tide graphs. That's not a lot, but it made quite a difference in the wind quality close to the wall - the wind became a lot gustier and weaker, at least in the really flat section within a 100 or 200 feet of the pier. Nina, who was freestyling and playing with waves, verified that this was a localized effect - when she sailed away further from the wall, the wind became steadier and stronger.

That a higher wall due to a lower tide will impede the wind more is not exactly rocket science, but what surprised me was that even a change of less than one foot made such a noticeable difference. When I stopped sailing, the predicted tide level was at 1.55 ft. At more than 2 feet, the wind gets a lot steadier; at 2.5 ft or higher, the disturbances from the wall are quite small, even when sailing within 150 ft of the wall. The higher tide levels come with a slight drawback: a lot of water can gush through the big holes that are in the first third of the wall close to shore, which creates "general unrest" in the water in the near-shore section. But then, I need to practice speedsurfing in chop, so I'll borrow a quote from Coach Ned: "It's all good".

Friday, May 5, 2017

May May Be Good

Some will probably say I'm jinxing it. My answer? Listen to the great Stevie Wonder! Superstition ain't the way! So I'll just say it: May may be good.
The start has been very good - we have windsurfed 3 of the first 5 days. My kind of sailing, too: flat water! We started at Kalmus twice, but I escaped the bumps by sailing to the Kennedy Slicks one day, and to Egg Island the other day. Why, you ask? Check out this short movie from the Egg Island day:

There are very few things I like more than going full speed into a jibe on perfectly flat water. Amazingly enough, I'm still improving my jibes, even after tens of thousands of tries. Yes, that's right - tens of thousands. A good windsurf session has 50 to more than 100 jibes. Multiply that with 150 sessions per year. Repeat for a decade or three, and you might discover the actual number may be larger than 100,000. Do you need any further proof that I'm a slow learner? But it's fun!

The major thing I have been focusing on recently was to switch the feet as soon as the clew moves forward. It's only six years ago Matt Pritchard told me I should do: "imagine a line connecting your foot and the clew - when the clew moves, your foot moves". In my defense, I have been playing around with not switching the feet at all during the jibe as a way to get into switch stance. That's fun, but not the best way to cure "slow feet". Even in the first jibe in the video above, which is pretty decent, I'm still stepping just a little bit too late. But as the Beatles said - "it's getting better all the time".

The thing that made the recent flat water excursion real fun was that Nina joined me every time. When I sailed up to the Kennedy Slicks, she just followed. That confused the wind a bit, so it dropped soon after. But the next time at Kalmus, it was actually Nina who suggested that we'd sail to Egg Island. Twist my arm!  I got so excited that I did not even think of switching back to my slalom board that was on the beach, because the typical WSW bumps were a bit too large for this wannabe slalom sailor.

Today had some strong wind in the forecast - easterlies in the mid-20s. The rain that easterlies always brings even held of until after noon, so we set out early. After checking out West Dennis and finding it (a) empty and (b) not looking attractive at all, we drove back to East Bay. Nina almost jumped out of the van when we drove close to our home - a mix of temperatures near 50ºF (10ºC), no sun, and previous bad experiences at East Bay had reduced her motivation to windsurf today to almost zero. But somehow, we both ended up at East Bay, where not a single white cap was to be seen inside the bay. But there were some on the ocean, so we decided to rig, anyway - 6.0 for Nina and 7.8 for me, with 99 l and 117 l slalom boards.

To cut a long story (semi-)short, we had a blast. The wind started out good and got better. The water was flat in the middle, and near shore even flatter. So what if it started raining towards the end? We were getting tired, anyway. Thanks to the perfect conditions for laying down jibes, both Nina and I ended up with new personal bests for alpha 500 (that's a 500 m run with a jibe in the middle). Nice!

For the curious, here are my GPS tracks from today: