Wind Instrument Calibration

[Dockhead]

Originally Posted by wingssail

We use OpenCPN for navigation but not for wind information or tactical information while sailing. For that we use the B&G displays directly and we also have a "Sail Racer" app on an android tablet which receives raw data from the B&G via blue tooth and provides most of the tactical information we use on deck while racing (and I use it while cruising, just to play around). It's a bit fiddly but it does not require a below decks PC like Expedition and costs a lot less.

Hot tip; thanks. I'll check that out.

[wingssail]

Originally Posted by Dockhead

OK, that makes sense.


One of the biggest (of my several) problems I have now is not being able to determine the favored tack when sailing upwind. In a long distance (multi-day) upwind leg, playing the wind shifts correctly is the whole difference between success and failure. And this is true even cruising keenly.


If my mark or waypoint is within 15 or even 20 degrees of the true wind direction, my instruments are useless -- I can only figure it out empircally, by making a series of useless tacks. It gets really old doing this over and over again on a long upwind leg even if my boat is easy to tack. I long for accurate information, even accurate within 10 degrees. I would kill for true wind direction accurate to 5 degrees.


Downwind troubles me less. I have a big assy on a "cheater pole" (from an old TP52) and don't sail that deep anyway. It's so much trouble to gybe that I will not do it for a short term wind shift.



I wouldn't necessarily have to have the accurate true wind data on my instruments (although that would be awfully nice). If I had it at least on my ship's computer, I could get it from there onto a tablet, and this would be enough for making tactical decisions. So perhaps the OpenCPN Tactics plug-in is going to be adequate for my purposes. We shall see. I just hope I don't end up buying an H5000 here in Europe, where it is 1.5x as expensive as it would have been in the States which I just left.

Well, favored tack has a lot of considerations, such as, do we expect a shift? or, is there current at play?, etc. But if you know the rumb line to the mark and the true wind direction you have the basic info you need. Your instruments should give you true wind direction to within 5 degrees. Mine do and I have a very old system.

You don't need several tacks (except while calibrating the system). You know your tacking angle for the wind strength you have. Lets say it is 90 deg. If you can sail 45deg from the rumb mark then neither tack is favored. If you are sailing 55 deg from the mark you are not on the favored tack. If you tak over and it is 55 degs on the other tack, you have current against you or light wind.

BTW I thought the vertical wind spars from B&G were under $USD2000

[Dockhead]

The other reason why I need reasonably accurate True Wind is so that I can correct my polars, and sail to them.


Without corrected polars, my routing is useless. This screws up long upwind legs cruising as well as racing.



If I had some actual polars for my actual boat model and didn't have to make my own, this would not be so critical, but I don't. I've been using polars for a Hylas 54 and they are way off.

[Paul L]

Originally Posted by Dockhead

The other reason why I need reasonably accurate True Wind is so that I can correct my polars, and sail to them.


Without corrected polars, my routing is useless. This screws up long upwind legs cruising as well as racing.



If I had some actual polars for my actual boat model and didn't have to make my own, this would not be so critical, but I don't. I've been using polars for a Hylas 54 and they are way off.

Your being a bit dramatic. If you had perfect polars and a crew that could sail to them, your routing would still have large errors due to the inherent forecast error. No forecast gets wind speed, wind direction, sea height, sea direction accurate to anywhere near the accuracy you are demanding of your true wind calculation. And in practice, especially offshore in seas, you aren't able to sail to the polars. They will always be an aspiration.
If you know the Hylas polar is way off, then you should have the multiple data points to adjust it. Creating your own polar is an iterative process.

[Paul L]

Originally Posted by Dockhead

Well, that's exactly what I do to calibrate for offset -- as described in Post #1 in this thread. That's how I know my sensor is misaligned by 2 degrees. Another question however is how to correct this error in the network. I can do this calibration either in one of my Triton MFDs, or in one of my Zeus MFDs. Unfortunately setting this offset in one of the MFDs is not recognized in the others. This is maddening and one of my jobs next week is to figure out how to make it work across the network. If I can't get it to work I might have to climb the mast an correct the alignment mechanically. Or rewire the CV7 so that I get 0183 data from it which I can correct in the device which converts it to N2K (presently converted to N2K at the masthead with a WindyPlug).




But offset is only one element of calibration. There is also:


1. Linearity
2. Heel
3. Upwash


I can't calibrate any of that with the equipment I have now. Perhaps linearity is already good enough that I can ignore this. It think Heel is the big one, and perhaps it doesn't need calibration either to get reasonably usable true wind. We shall see.

In your first post I got confused when you added step #3 to the process. The error shows up in no wind conditions as soon as you are moving in a straight line, as all the wind is created by the boat and comes from directly ahead, at 0*

[44'cruisingcat]

I just adjust so the boat sails the same close hauled angles and speeds on either tack. The goal is to be able to sail and tack on autopilot without constantly adjusting.

[ronstory]

Just a crazy idea, but LCJ Capteurs has a "universal" interface box called STBG OPTION and it looks to have calibration settings.

Perhaps if you connected it via 0183 to one of the support MFD and let it rebroadcast the info out on the N2K network it could meet your needs.

[wingssail]

Originally Posted by Dockhead

The other reason why I need reasonably accurate True Wind is so that I can correct my polars, and sail to them.

Without corrected polars, my routing is useless. This screws up long upwind legs cruising as well as racing.

If I had some actual polars for my actual boat model and didn't have to make my own, this would not be so critical, but I don't. I've been using polars for a Hylas 54 and they are way off.

MAKING POLARS FOR MOODY 54

I would approach this task by using available polars and modifying them. For instance you could follow the steps below. (If you used the polars available on SailRacer - Tactical app for sailors. Android, iOS, inkDisplay, Watch, assuming you have an account which means you downloaded the app and paid the $40 for the "Premium" version, there are over 8200 keelboat polars you could choose, but none for a Moody 54).

1. Select an appropriate polar, such as Moody 42 DS
2. Copy the entire table into excel (you may have to type in the values unless you can find a way of copying them as an excel sheet)
3. Take another polar, such as a Hylas 54, and find some constant you can apply to the first set which gets you close to some value on the Hylas which you think is correct
4. Apply that constant to the moody 42 DS, which assumably has a similar performance profile as your boat, only slower,

Once you have created a set of numbers which you like you can use them in your instruments or in the Sail Racer App on a tablet on your boat.

The Sail Racer App computes about 40 values including at least 5 which are based on your polar diagram, which is downloadable from the server once you create it for your boat.

Sorry if I seem to be promoting the Sail Racer App. I have some problems with the app and the support, but it really is quite good and I've found nothing else which dsoes what it can do, once you attain competence in it.

PM me if you want me to be your tech support on Sail Racer.

[ronstory]

Here is polar for a Moody 54 DS. Great site for polar info, BTW. ;^)

ORC sailboat data.

[Dockhead]

Originally Posted by wingssail

MAKING POLARS FOR MOODY 54

I would approach this task by using available polars and modifying them. For instance you could follow the steps below. (If you used the polars available on SailRacer - Tactical app for sailors. Android, iOS, inkDisplay, Watch, assuming you have an account which means you downloaded the app and paid the $40 for the "Premium" version, there are over 8200 keelboat polars you could choose, but none for a Moody 54).

1. Select an appropriate polar, such as Moody 42 DS
2. Copy the entire table into excel (you may have to type in the values unless you can find a way of copying them as an excel sheet)
3. Take another polar, such as a Hylas 54, and find some constant you can apply to the first set which gets you close to some value on the Hylas which you think is correct
4. Apply that constant to the moody 42 DS, which assumably has a similar performance profile as your boat, only slower,

Once you have created a set of numbers which you like you can use them in your instruments or in the Sail Racer App on a tablet on your boat.

The Sail Racer App computes about 40 values including at least 5 which are based on your polar diagram, which is downloadable from the server once you create it for your boat.

Sorry if I seem to be promoting the Sail Racer App. I have some problems with the app and the support, but it really is quite good and I've found nothing else which dsoes what it can do, once you attain competence in it.

PM me if you want me to be your tech support on Sail Racer.

Thank you, Fred! This is tremendously helpful