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3/30/2016 1:00 am  #1


TWA correction for Leeway

I'm new to Expedition and try to understand the basic setup in combination with my onboard NKE system (and learn from there onwards...).  Questions I have are about the correction of TWA for Leeway.  The Expedition manual states this about it:

"Please note that True wind angle displayed in Expedition and StripChart includes the effects of leeway wherever possible. Polar data from designers generally present TWA in this manner (TWA to the boat's centreline plus leeway equals TWA track) as do Ockam systems. B&G is also likely to migrate to this convention in the future. For systems that do not add in the effect of leeway into TWA (B&G, Silva, etc.), leeway is added to the TWA received from the instrument system wherever possible. There are several benefits to this. For example, calculations of VMG have to include leeway and it makes the optimal routing function easier to use. This is why you may notice that Twa as reported by Expedition may be a few degrees wider than Twa reported by the instruments."

Now I have the following questions:
- Does NKE include Leeway in the TWA calculation? (I will try this myself next time I'm at the boat by creating excessive leeway by entering an very high k-factor in the leeway calculation in the NKE system).
- Does Expedition add Leeway to the TWA it receives from the instruments depending on the brand/type of the instruments configured?    Or does Expedition only add Leeway to the TWA in case you have configured Expedition to calculate TWA from the raw instrument data (AWA, BSP etc) it receives (and wil Expedition than just use the TWA without adding Leeway in case you have configured Expedition NOT to do the TWA calculation). The reason I ask is that you of course want to avoid that Leeway is added twice (first by the instruments and then by Expedition).

Thanks in advance for your help!

Last edited by ZeeZee (3/30/2016 1:01 am)

 

4/04/2016 1:32 am  #2


Re: TWA correction for Leeway

I did some testing over the weekend.  It seems that NKE does NOT add leeway to the TWA.
Can anyone confirm this?

     Thread Starter
 

4/04/2016 1:52 am  #3


Re: TWA correction for Leeway

I don't know about NKE specifically, but it ​sends data in NMEA 0183 format.

The NMEA spec for 0183 and 2000 is relative to the centre line. Exp adds leeway to these if possible. So, the NKE should not include leeway if all is correct.

That part of the help was written when we were still using B&G H2000, so I need to update the B&G comments.

 

4/04/2016 5:57 am  #4


Re: TWA correction for Leeway

Thanks Nick, that indeed seems to be in line with my finding of Leeway not being included in the NKE numbers.

So from what I understand Exp always adds Leeway to TWA, even if you have Exp NOT calculate TWA. Correct?
Then exp needs to know the amount of Leeway and this is NOT in the NMEA output from NKE (afaik there is not even an NMEA sentence for leeway).  Should I now enable Leeway calculation by Exp (in the "Instruments/Exp calculation" menu)?   In that menu, what is the difference in formula used for leeway calculation when I check the "leeway from heel" or "leeway from bsp components"?  When I check both boxes, is leeway then calculated according to  Leeway = k * (heel/bsp^2)  ?
In that case I assume I also need to check "Leeway" under "Instruments/Exp calibration" and specify the K factor in the table  (1 number for a single K factor or an entire table for additional accurancy)?   In case I enter 1 number for the K factor, should the "lineair interpolation" box be checke or doesn't it matter?




 

Last edited by ZeeZee (4/04/2016 6:01 am)

     Thread Starter
 

4/04/2016 11:50 am  #5


Re: TWA correction for Leeway

If the instruments send leeway, Exp will use that unless you tell Exp to estimate it.

​A normal value for k is something of the order of 10-13.

You probably don't need to check the leeway calibration option. I think Exp would just use this if you tell it to calculate leeway. Of course, you also need heel.

 

4/05/2016 5:27 am  #6


Re: TWA correction for Leeway

Thanks again Nick.
When I'm at the boat again this weekend I will experiment with different settings for having Exp calculate leeway and having it included into TWA.  Will report back here in this thread as reference for others.
 

     Thread Starter
 

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