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      Abbreviated rules   07/28/2017

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allene222

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About allene222

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    SF Bay
  1. Anyone measuring tidal current?

    Today I did another cal run. I picked a different location, one picked by looking for an area of constant current in SailFlow. The main thing I learned was don't use SailFlow to find out where the current is constant. Use depth off the chart and find an area of constant depth like I did last time. The previous area was much more constant in both depth and current. That said, the current today was 1.4 knots so about twice what I had in the other area and in the opposite direction. In terms of calibration, I decided to increase the E-W compass calibration by 1.5 degrees. Speed was right on, which was encouraging. I will probably go out again next week in the flood and see if this change holds up in the opposite current condition.
  2. Anyone measuring tidal current?

    I took Papoose out into the cold hazy winter and made 7 runs out in a very consistent current field. The depth varied from 9.5 to 10 meters and the current was constant across the runs. There are two ways to look at the results. One is that it confirmed that I have 1/10 knot accuracy in that all the readings were between .7 and .9 knots and the best estimate of the current is 0.82 knots. (The tide chart said 0.70 knots). The other way to think of the day is to use it as a new baseline with new coefficients which would give very little spread in the current (1/4 of a tenth). That means I must go out again in an ebb tide (today was a flood) and use that run as a verification. My concern is that I had to turn the gain of my knotmeter up about 1.6% to make the data fit the measurements and if I had to guess, I would have said I should turn it down. I am concerned that the knotmeter may not repeat. But the ratio of the two senders was very close to what it was on the last test day so likely that is not an issue. My next run will be the end of the week, all going well.
  3. Anyone measuring tidal current?

    Roughly, it takes a minute to stabilize the readings after a turn and I like to get two minutes of data then another minute to get to the point to start the next run back. That is four minutes per run and at 6 knots is .4 nautical miles. I need to pick a peak current to minimize the change in change in current vs time. My set of runs is about 1/2 hour and the current change across that time is small can I will be able to calibrate it out due to my multiple runs. The current changes fastest at slack so I want to avoid that although there is an argument that I am wrong about that and that what I really want is a linear change in current which would be at slack. These criteria were all found out by having problems with previous runs. If the weather is good, I might go out at 2PM today and give it another try. I should know if I am in a steady current field just by observing my readout of current. If it is steady, each run will show steady numbers, even if wrong. If every run is steady, I have a steady current field. If all the runs are the same, I am calibrated. If all the runs are different, I can adjust my cal factors when I get home and based on the scatter of the data, have a pretty good idea if this is going to work. Right now I think I have pretty good N-S data but my E-W data was in a channel and it is likely the current was different in the channel than it was outside it. My best guess is that I have things off by .05 knots N-S and by some unknown amount E-W, perhaps by as much as 0.25 knots.
  4. Anyone measuring tidal current?

    I am at Sierra Point, about 3 hours from Richmond. One thing to remember is I need an area with a diameter of about .4 miles of constant current, or at least a gradual gradient. I probably want to avoid dredged areas as they create sharp changes in current. We get about a knot peak current so not even close to what you have in the north bay.
  5. Anyone measuring tidal current?

    I have seen really chaotic current in SF Bay many times. You can even see it in the bay model if you ever have a chance to hear Kame's talk. I would hate to try and find a good test area in the north bay. But I don't see that kind of chaos in the south bay especially around where I am planning on doing my next test. Maybe the south bay is like one of those long channels you are talking about. You can see the target area in post 29.
  6. A big project!

    I have seen several projects this ambitious. I just have never seen one finish. People work for years and then just disappear. There was one old wood boat rebuilt around here. It was Freda, an 1885 32 foot yacht. The rebuild took 8 years and cost $500,000. It is the oldest sailing boat on the West Coast. One of the unfinished projects was a boat like mine, an L-36. The plan was to replace all the floors and frames, then replace the planks. At that point it would be hard to call it a restoration. He did replace at least some of the floors and frames and then I never heard from him again. I will be amazed and in awe of this kid if this boat is finished.
  7. Anyone measuring tidal current?

    I need to say something about the 0.1 knot goal. I come from the instrumentation world. In that world you want to have your instruments be at least 5 times as accurate as what you are interested in. So if you are interested in 1/2 knot, you want to have 0.1 knot accuracy. I mean, if the accuracy were 1/2 knot, I could just look at the wake on the channel marker and get that close.
  8. Anyone measuring tidal current?

    Just two units sitting on the window sill on the second story. But they do show the .02 knots speed readings so they are getting some reasonable signal. This kind of error shows up over hour timeframes so I would like to see something close to good enough before driving out to the baylands and spending that kind of time watching grass grow looking for the ultimate accuracy. You are not going to get 2m off the water on a boat. At least not on my boat. These are the MTK3339 v3 chips. Both the same. I have 18 of them right here in my study right now. I can't find specifically a min elevation function. The only thing I can find that might be it as I don't it is Packet Type: 223 PMTK_SET_AL_DEE_CFG. Best I can find for a development board would be this https://drotek.com/shop/en/rtk/794-tiny-rtk-gps-neo-m8p-2.html I see they also list a SMARTNAV that is no longer in stock and looks like it is also based on an Intel Edison. Another sucker who trusted Intel. Sure glad I don't work there any longer :-)
  9. Anyone measuring tidal current?

    I just did an experiment with two normal mode 2 GPS units. Across 1/2 hour the reading of the distance between them, which is one of the readiouts, changed by more than a boat length (14 meters) so that is not going to work without the fancy linked together units. I will check and see if I can program my units to min elevation and run an experiment to see how well that works. $10K each. That is more what I was think, which is of course out of the question. The u-blox says they have different modes. Stationary, moving, and both moving. And of course, stand alone. It is less clear what the accuracy is in the different modes. The u-blox neo-m8p is $160 for just the chip at Digi-key. Not sure I want to be the pioneer on this. I am going to try to calibrate what I have first. I think it will work.
  10. Anyone measuring tidal current?

    So basically you are using GPS receivers that lock to the same constellations. This is what I was describing as receivers costing thousands of dollars but I may be seriously behind the times here. Maybe they are hundreds instead. I Googled and found a "fund me" project to build a low cost RTK receiver and see that they are using an Intel Edison. They are so screwed as Intel obsoleted it with very short notice. I know because I had a product that I had to redesign. I replaced an $80 Edison with a $10 ESP-8266*. I have a ton of Intel Edisons and it is what is in my RaceBox (but I have lots of spares now). As far as the sail goes. This was two years ago so I don't recall the precise numbers. We see 0.02 kt noise in the unit. With the unit mounted on the cabin top we were getting terrible noise, probably at least 5x that, so made the decision to use a remote antenna, which turned out to be a pain. We have since combined the GPS and the processor in a stern mounted unit with and use a remote WiFi displays so no wires to deal with. On the cabin top, the unit was under the boom and the mainsail. On the bow the sail is off to one side so it may not be as big a problem as I was thinking. What RTK receivers do you use? *The ESP-8266 is more like $3 but add charger and memory and it goes up a tad. Point is, Intel can't compete in this market.
  11. Anyone measuring tidal current?

    That is also the algorithm I use. I use k=6 and clip leeway at 15. I understand what you are saying about fitting k and will have to try that. I don't actually recall how I got k = 6. It was probably a wag. I have the option of using measured heel or a heel factor derived from wind. This was derived from VPP on a couple of boats and fit better than using heel. double wind_factor = max(min(1, rawNmea.tws / 14), 4/14); double heel_factor = wind_factor * cal_array[target_heel] * pow((min( 1, ( 180 - abs_val(rawCalc.twa) )/85 )),2); Target heel is 25. You still need to determine "k".
  12. Anyone measuring tidal current?

    That is also the algorithm I use. I use k=6 and clip leeway at 15. I understand what you are saying about fitting k and will have to try that. I don't actually recall how I got k = 6. It was probably a wag. I have the option of using measured heel or a heel factor derived from wind. This was derived from VPP on a couple of boats and fit better than using heel. I will offer it without comment: double wind_factor = max(min(1, rawNmea.tws / 14), 4/14); double heel_factor = wind_factor * cal_array[target_heel] * pow((min( 1, ( 180 - abs_val(rawCalc.twa) )/85 )),2); Target heel is 25. You still need to determine "k".
  13. Anyone measuring tidal current?

    How do you measure leeway? I went back in my log and see that I do have both leeway and updraft correction turned on. However, I do not use leeway in calculating TWD. TWD readings are influenced by leeway and updraft but in opposite ways. So it seemed to me it was better to just put in an updraft term that was perhaps smaller than the actual updraft and not introduce the considerable noise of the leeway number. That approach also reduces the noise of the updraft term so I think it is much better. I do use leeway numbers in the current calculations so I am concerned with the noise. I have two algorithms for leeway. One uses the heel sensor. The other estimates heel based on wind. My research says the wind one is more accurate but that work was based on a very limited set of theoretical boat data so I don't have huge confidence in it. But the match between the data I had and the model was better with the wind model. So maybe I will switch models if the leeway noise is a problem in the current calculations. Problem is that if you think current is hard to verify, think about leeway.
  14. Anyone measuring tidal current?

    Most interesting. I love thinking about novel approaches. I have no shortage of GPS modules or active antennas so implementing it would not be a problem. However, I get about 1/4 of the noise out of my compass compared to the plot you show and that is with the boat moving. There might be some filtering in the compass so perhaps not a fair comparison. I did a quick and dirty simulation by using my GPS locations when I was going 4.5 knots compared to the location 5 readings later. With that I got similar noise to the compass. But if I could get the same noise and not have the linearity errors of the compass that would be great. There are two problems with this. One is a practical problem of finding where to mount the forward antenna. The GPS accuracy falls off when the antenna is under a sail. I have observed that so that is not theoretical. I don't know how much it affects locations as I was looking at speed. It would be very difficult to place an antenna forward that was not under a sail. I would say that is a risk. So I would ask the question, have you done this or is it theoretical? The other probably more significant problem is that this only works if the two receivers are locked to the same constellation of satellites. Doing that moves you from cheap GPS units to thousand dollar ones. I have GPS boxes that can measure the difference in location between them and that moves several meters over time. That is not noise, but drift so it cannot be filtered out. Where this approach might be useful is in calibrating the compass. I could compare my average compass reading to two GPS locations separated by 75 meters and monitor the satellites to make sure I had the same constellation for both readings. Interesting and thanks for posting. I look forward to your other posts.