Our Hatteras trip is coming to an end, after a final great low-wind session. I saw an amazing number of stingrays, and finally got a new sail throw I had been working on. But now, it's time to focus on the next event: the Buzzards Bay Crossing on the May 25/26 weekend in Falmouth.
Fast Eddie deserves the credit for bringing the event back to life, with help from Dan Weiss. Some of the initial ideas did not fly well - a hefty $180 price tag for racing and "gourmet box lunches" and BBQ? But the organizers quickly responded to feedback, and reduced the registration fee for 2 days racing to $80 ($40 for one day), without the food (which will still be available at the Sea Crest Beach Hotel where the event takes place).
This will be a windsurf and SUP race event where FUN is the goal. Anyone who has been to the East Coast Windsurfing Festival (ECWF) in Long Island in recent years can attest to how much fun such an event can be! Unlike the ECWF, the Buzzards Bay Crossing will apparently not include a freestyle competition, but I'd say that's mostly because this is a first-time event (after a long break). I'd suggest to local freestylers to simply show up and do an "exhibition" - maybe we can then get freestyle added next year.
When working on improving your jibes, a GPS can be a useful tool. Looking at the GPS tracks in GPS Action Replay, simply click on "Jibe analysis", and you get a nice table that summarizes your jibes:
I have sorted the jibes by "minimum speed" to quickly see my best jibe of the day. In this example, I kept a speed of 14.3 knots or more through the entire jibe - 70% of the entry speed (the "score"). The next-best jibe was pretty close, and even the average for the best 10 jibes is not so far behind. Great! Indeed, my jibes that day had felt good - for once, I had planed out of the most of my jibes.
However, the results looked a lot worse when I changed one analysis parameter by selecting the "Doppler" checkbox, and re-computing the results. Here is a direct comparison:
Suddenly, my jibes did not look nearly as good anymore - according to the Doppler analysis, I lost more than half of my entry speed in even my best jibes; for some jibes, like the first jibe that's highlighted in blue, the minimum speed dropped to less than half! What is going on?
Let me start by quickly reviewing the difference between "positional" and "doppler" analysis (not all my readers are GPS speedsurfers - if you are, perhaps just skip to the next paragraph). All GPS units, regardless if you use them for directions in your car or for measuring your speed while windsurfing, determine your position from the measured distance to several GPS satellites (typically 6-8). The accuracy of the position is limited by a number of factors to about 3 meters. My GPS, the Navi GT-31, measures the position once a second; the difference in two positions gives you the speed. When windsurfing at 20 knots, you move about 10 meters in one second. An error of 3 meters would then give you a 30% error in the speed, a rather large error! But fortunately, there is another way top measure the speed using the "Doppler Effect". Doppler-based measurements tend to be much more accurate, with reported speed accuracies of 0.1 knots. This is why GPS-based speed events generally require the use of a GPS that logs Doppler speed, like the GT-31 that I used.
So, if Doppler speeds are much more accurate, does this also mean that the Doppler-based jibe analysis is more accurate? Not so fast! Let's look at the GoPro video of the jibe #1, where the position-based speed was 13.8 knots, but the Doppler-analysis thinks it's just 6.2 knots:
The jibe was nicely planed through, which typically requires a minimum speed of more than 10 knots. For comparison, here is a section from the same session where I was moving at about 7 knots (and positional and doppler speeds were very close):
Nina was also wearing a GPS, so I could use her tracks together with mine to accurately define this 8-second period where I was sailing at 7 knots (I was slightly slower at the very beginning and end of the movie).
Obviously, the board speed during the jibe was significantly higher than 7 knots, so the Doppler-based jibe analysis does not seem to be accurate. Let's take a close look at the track and speeds for another jibe (#12 in the table above):
In the middle of the jibe, the direction of the board changes by about 30-40 degrees per second for about 4 seconds. The speed tracks show something interesting, though: the Doppler appears to lag behind the positional speed by about 3 seconds! According to the Doppler data, the minimum speed is after the board started going pretty straight again; the upper (positional) speed graph shows rapid acceleration at this point. Looking at the GoPro video, the positional speed seems correct, since I hung down and pumped to regain my speed right after the jibe. Here is the video:
This jibe (#12) was a bit wider than jibe #1, and the discrepancy between positional and Doppler-based minimum speed was smaller. This was a recurring theme in all the jibes I looked at: the tighter the jibe, the more the minimum speed would drop in the Doppler analysis. I was working on laydown jibes, so I was cranking them relatively tight; Nina's jibes that day were wider, and the speed differences between Doppler and non-Doppler analysis were much smaller.
To some extend, the 1 Hz sampling rate will cause an underestimate of the speed when the board changes direction rapidly. Even in the positional analysis, we measure the distance along a straight line, but the board had sailed a longer arc. In the Doppler analysis, a similar error would occur; however, it should also be of similar magnitude, and not substantially larger. But we need to keep in mind that even the binary data files do not contain "raw" data, but rather the result of a complex mathematical analysis. From the time difference in the speed curves, it appears that the Doppler data include a filter with something like a 3-4 second time constant. In 4 seconds, the board changes direction by perhaps 120 degrees in the jibes - that's very far away from a straight line! I think this could explain why the Doppler speeds in the jibes appear to be too low, and that things get worse the tighter the turns are.
This entire thing is not just a theoretical thing. If I want to know how good my jibe was, it makes a big difference if I kept 70% or just 44% of my speed. Keep in mind that the Doppler-based analysis keeps estimating lower speeds for several seconds in a row, without ever "compensating" for it with higher speeds. If you'd draw two tracks, one based on positional and one based on Doppler data, the "Doppler surfer" would fall behind in every jibe!
This has a significant effect on alpha 500 results. For the top jibes in the tables above, the lower mid-jibe Doppler speeds added about 2 seconds to the time to reach 500 meters, dropping the alpha speed by 1-2 knots! Since alpha team rankings are usually quite tight, even just one knot would make a significant difference. However, I do not think that going to non-Doppler alphas is the solution, since the non-Doppler data errors add up over the 45-50 seconds that a 20-knot alpha 500 run requires.
For now, the solution for alpha rankings is to go with wider jibes with a very even carve. In the future, we can only hope that better GPS units with a higher sampling frequency become available. Several 10 Hz GPS trackers are commercially available already, but it is not clear if any of them provide the Doppler data required for highly accurate speed measurements - and if such Doppler data do not suffer the same shortcomings shown here for the GT-31.
East winds today brought a nice setup for some speed sailing in the properly name East Bay, 10 minutes from our house. This time, my lovely wife decided to join me to sail her Falcon 89. She had bought the board just a couple of months ago from Dani. But while she loved this and other Falcons when she sailed them the past two years, the first couple of sessions on her own Falcon had been frustrating. So we played it conservatively today, and rigged her 5.7 m wave sail that is similar to the sails she usually uses for freestyle.
The wind was in the low 20s, gusting into mid-20s, and the water in East Bay was nice and flat. Nina ended up having a blast - here are a couple of pictures:
Flying the Falcon
Despite the moderate wind, she clocked her second-fastest session ever. After sailing freestyle boards almost exclusively for many months, jibing the slalom board needed some adjustments, and she was not happy with her jibes. Nevertheless, she set a new personal best for alpha 500 - nice!
I mostly just enjoyed the sailing on my XFire 90 / Matrix 7.0 combo. I have been working on different aspects of the jibe in recent sessions, and was starting to get a bit frustrated with the lack of visible improvements. But the flat and shallow water in East Bay creates the perfect jibing playground, and I finally got the impression that all that work is starting to pay off. Here's a mid-jibe GoPro picture:
I think it helped to have Nina on the water - I really wanted to push the sail down and out of the way to see what she was doing. So my arms are like they should be, with the front arm extended and the back arm bent to oversheet. The mast is pointed about 45 degrees towards the water, keeping nice pressure on the carving rail. My posture is better than in most jibes, with a straight back, chin up, and looking into the turn; but my knees should be bent a lot more. The GoPro showed that I still tend to fall back into other bad habits, too, like not sliding the hand to the mast - but there's definite progress. The non-doppler jibe analysis gave my top jibe a score of 70%, with a minimum speed of 14.5 knots, and 12.9 knots min speed average for the top 10 jibes. The XFire 90 does do tight turns quite well, so most of my turns were reasonably tight, which means the results using doppler analysis are a lot worse; but that's a topic for another post in the future.
I'm a bit tired after 3 days of windsurfing in a row. Maybe it's Friday's fault - with Dean having plenty of wind in Hatteras, I tried to do my part in improving our Fogland Speed Surfer team ranking on the GPS Team Challenge and sailed 100 km. We went to Kalmus, where low tide, south winds, and warm temperatures attracted a crowd - Nina, Ron, Marty, Dani, Drew, and Bruce were all to out there. Nice to have plenty of company on the water again! It was, however, rather foggy:
I sailed a new fin for the first time: a 35 cm Type S from Black Project fins. As a speed fin, it's not supposed to go upwind really well, but I still was a bit disappointed. I ended up practicing a lot of tacks just to stay upwind; but perhaps the fog was also to blame, since picking landmarks for orientation was not possible most of the time. Besides seeing lots of friends on the water, the great thing about Friday was that I finally was able to sail without a hood and gloves.
The forecast for the weekend had not been great, with only 14 mph SW wind for Saturday. However, the fog was gone, and we saw a little bit of sun from time to time. The air was a bit colder, but still warmer than the water, so we got a nice little sea breeze enhancement in the afternoon that kicked the winds up above 20 knots. A lot of the same sailors came back to Kalmus, now also joined by Steve. Nina did not sail; instead, she rode her bicycle down to the beach, and took a few pictures:
Martin doing a one-handed downwind 360
Dani going slow. Most of the time, only his fin was in the water.
Sailing the good old Hawk again
Steve
Another 360 by Martin
Besides being a bit colder, another big difference was the chop. With SW-WSW winds, there was a lot more of it than the day before, when the straight south wind has created nice and orderly little rollers. I tried to escape the chop by cruising up to the Kennedy Slicks, but timed it so that I arrived during and extended lull, where even my 7.0 and Dani's excellent 38 cm Vector Volt fin did not alway get me planing. When the wind picked up a bit later, I took out my Hawk to try a new 30 cm MUF Slalom Weed fin. This was quite an amazing fin - it provided plenty of lift, something I rarely get from weed fins, and was almost impossible to spin out. But it was also the first power box fin I ever used that did not fit the box without sanding. It's quite normal that tuttle box fins need some sanding to fit, but all of the perhaps 10-15 other power box fins I have fit without sanding. I went out without bothering to fit the fin, only to discover that it actually was able to move a bit from side to side. Even more amazing that it worked so well, then!
For today, the forecast predicted some N-NW winds in the upper teens for the morning. Since Dani was still in town, and the tides were timed perfectly, I convinced him to try a new spot: Barnstable Harbor. We had a lovely 100 minute session there, starting just as the tide dropped low enough for the marsh islands to come out, and stopping when the wind dropped too low. Dani sailed without gloves and needed to take a bunch of breaks to warm up his fingers, since air temps were only around 42ºF (6ºC). Nevertheless, we had tons of fun, and Dani absolutely loved the spot. Sailing full speed on water where the "waves" are about one inch high is just a wonderful thing. We were a bit underpowered most of the time, but still managed top speeds around 25-27 knots. The nearest wind meter in Chapin showed averages around 12 knots, and gusts up to 18 knots, but we probably did get a few knots more where we sailed.
With three sessions in the last 3 days, I have now windsurfed 11 of 21 days in April. The forecast for the next 2 days also looks promising, and Nina is eager to sail her Falcon tomorrow. With sun predicted for tomorrow, but rain on Tuesday, we might actually not go sailing on Tuesday...
I've been windsurfing for more than 30 years, although this includes several multi-year periods where I did not windsurf at all. I got really hooked again a few years ago, after getting married to my lovely windsurfing wife, and starting to take ABK clinics. We mainly surf on Cape Cod and in Rhode Island, with occasional trips to the Caribbean.
