True Wind Direction

We have a Nexus NX2 system on our 36ft C/R.  It has been a pretty good system for the 6or so years it has been installed.  We invested in the gwind vertical wand and the fancy multi-axis compass in particular to try and get good quality wind data and spent a fair amount of time in the early days calibrating stuff.  

 
Garbage in, garbage out.  Ground wind vs true wind. Gps speed vs paddlewheel.  We considered, reviewed and educated ourselves on all of it.  We read and re-read.  Or at least we tried.

 
But true wind direction has proved to be a major and persistent disappointment because we never got a TWD number that didn’t wander around by a dozen or so degrees every time we tacked or gybed.  
 
Did anyone ever nail that on a Nexus system?  Are we chasing the impossible?
 
I am very aware of the B&G articles and calibration details for TWD in that system.  The Blur guys have published a formula for figuring out a TWA correction table for inputting it into the system - the equivalent input for which is wholly missing in Nexus.  I’m starting to think it was impossible all along and is beyond the reach of a normal recreational crew - both the money required for a comprehensive H5k system and the energy required to maintain it and calibrate it constantly.


 
Advice welcome.
 
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It is possible.  I don't know about the limitation NX2.  

I'm not a fan of the B&G method of multiple by 0 and add the desired result.

You'll need to ruthlessly characterize each error from the foundation up.  Then eliminate each.  

I don't think you can collect enough data to eliminate the cumulative TWD error at a system level.

(I'm talking about data sets that are greater than the total life of multiple suits of sails)  

So if your not sailing all day every day it can be tough to have enough data.

Can you use COR files with the NX2 system?  (This is first level of error correction for the masthead sensor. )

Focus on collecting data around tacks and jibes after you have the compass and BS spot on.

Have the helmsmen focus on stable target speeds on both side of the tack and jibe.

Don't cloud the data set chasing TWA during this data collection time. 

Post your strip charts if you can.  Data rates > 1 hertz should be fine for the gross work.

Have fun

 

chriggel

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You will need the Garmin / Nexus Race wind sensor, the Nexus server and the "nexus race"-software. The race TWA calibration table in the wind-sensor can be manually edited the same way as you would do it on a H5000. This has worked for me for many years and I have my angles within one degree even after tacks or gybes. Takes some time though, but the B&G manual is a good start. Actually your calibration value is higher to widen the angle and negative to narrow it. Apply half of the seen difference in TWD to your TWA calibration and you are done.   

 

Frogman56

Anarchist
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Sydney
What Henry said above!

But maybe a couple of ideas, all from long time B&G entertainment.

1. The speed calibration is the foundation. Do not calibrate under motor. Calibrate at full heel reaching, as the crossflow and the proximity of the keel leading edge to the paddlewheel changes things. If you can find a place with not much current, sail crosswind (90 TWA) against the gps. At full heel, say in 12 tws, the leeway will be about 1/2 of the upwind leeway. In the B&G, this seems to trigger good activity, and so, in ten minutes, your speed calibration is within +/- 0.1. Just sail for 1 - 2 minutes on each tack...

2. Record this, and just observe as you race. Carefully observations over time might allow you, intuitively, to adjust for the last 0.1. (This part is not, is not in the B&G book.)

3. For your yacht (unique relationship between sensor, keel leading edge, leeway, etc.) you might derive a relationship between speed measurement upright under motor, vs. under sail. So in some emergency (say system component replacements) you can do an emergency quick and dirty calibration.

4. All the above dependent on clean... bottom, foils paddlewheel, minds etc

Good luck anyway!

 
Thank you all for your input.  With this help, and that from a very helpful soul who PMed me on the topic, we have discovered that the correction tables in the Race software are not working for me for some reason.  But they also don’t open - they give an error.  I have an nWind v1.1 running at the masthead, but that needs new bearings anyway… is there a later version, can it be upgraded, is it worth the effort?  I know that these masthead units were not intended to be user/field servicable, so anything I attempt here is a hack.  So i’m pondering whether i spend $1300 on a new masthead wand (gWind) and continue this Nexus adventure or whether I retire the whole system and replace with B&G.  Given the age and unserviceability of the nexus stuff, its probably the latter. 

 

chriggel

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You would need a "gWind race" sensor - it's is the same sensor with just the TWA table enabled in it's software. 

 

slug zitski

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You can calibrate forever … even import some rocket scientists from NASA ….to  chase TWA perfection 

but you will never win

the slightest  bit  of wind sheer will corrupt everything 

tell tails and your eyes never lie 

 

nickjs

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I had the same issue as Dacron.  In the end purchased a Sailmon E4 processor, which is similar price to H5000 processor, but has the advantage that you can re-use all your existing displays and transducers, so was a much more cost effective way to get reliable and accurate wind information.   It also has completely automatic calibration for transducer offset, TWA and TWS, so makes that side of things easy.

 

George Dewey

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The Nexus stuff can be made to work well. In addition to the issue that you reported with the cal table, did you calibrate port and starboard separetly for the log transducer upwind?

 
The Nexus stuff can be made to work well. In addition to the issue that you reported with the cal table, did you calibrate port and starboard separetly for the log transducer upwind?
It doesn’t work.  The server gives me some sort of “this requires a <xyz> transducer.  Changes to this field will not be factored into any calculations” error.  I always assumed - since we had bought the top line wind wand and hpc compass that it was simply broken functionality… seems I maybe didn’t get the latest and greatest even thought the whole systen was new less than a year before Nexus was discontinued by Garmin.

 

Mordoc

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It doesn’t work.  The server gives me some sort of “this requires a <xyz> transducer.  Changes to this field will not be factored into any calculations” error.  I always assumed - since we had bought the top line wind wand and hpc compass that it was simply broken functionality… seems I maybe didn’t get the latest and greatest even thought the whole systen was new less than a year before Nexus was discontinued by Garmin.
Anybody having any experience with the Airmar DST810 Smart?

 

George Dewey

Super Anarchist
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Charleston, SC
It doesn’t work.  The server gives me some sort of “this requires a <xyz> transducer.  Changes to this field will not be factored into any calculations” error.
You get this error when trying to calibrate the log transducer? Do you have the Nexus log transducer with the red paddle wheel wired directly to the server?

 
You can calibrate forever … even import some rocket scientists from NASA ….to  chase TWA perfection 

but you will never win

the slightest  bit  of wind sheer will corrupt everything 

tell tails and your eyes never lie 
I saw this nugget and had to respond.   

Now you know there is shear and how much,  you can use this information to refine your strategy.  

Henry

 
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