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 ka72.com 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.