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1/09/2016 11:54 am  #1


Questions regarding Magnetic vs. True / Layline Times / TWA

Hello,
I noticed that when switching from 'magnetic' to 'true' mode in Expedition the layline times ("Tm on P", "Tm on S", "Td R Prt", etc) are different than the What-if? numbers.
I'm using Expedition v.10.2.9 together with deckman and dfw2exp.

With deckman in MAGNETIC mode; dfw2exp in magnetic mode ("Heading, twd and cog in degrees true" unchecked) and "Magnetic mode" in expedition/settings/system checked the layline times in the number box and in the What-if? Window are same:


With deckman in TRUE mode; dfw2exp in true mode ("Heading, twd and cog in degrees true" checked)  and "Magnetic mode" in Expedition unchecked the times differ:


I positioned the boat in an area with relatively high magnetic variation so the effect is quite obvious.

--------------------


In the two pictures above the Leeway angle is -38.8°. Could somebody please explain how this angle is calculated? With a Course of 52.1° and a TWD of 10°, I'd expect a TWD of -42.1°. This TW angle is shown in deckman.



In a second example I entered an offset of 3° in Deckman's "adjust wind angle"-correction table for the prevailing wind speed/wind angle. This changes the TWA in deckman to 45° and the TWD from 10° to 7°. But the TWA in Expedition remains almost the same. It differs now more than 6 deg from the TWA in deckman:






 

 

1/12/2016 3:50 pm  #2


Re: Questions regarding Magnetic vs. True / Layline Times / TWA

Not answering your questions, but have you selected expedition to calculate twa, tws, leeway or any other variables?


:::Campbell Field::::::www.fieldyachting.com:::
 

1/14/2016 12:36 pm  #3


Re: Questions regarding Magnetic vs. True / Layline Times / TWA

Layline times and What-if? are now (v.10.2.11) showing the same numbers, also when in TRUE mode.
Just out of curiosity: Which mode (magnetic or true) are you guys using and why?

@Campbell: I didn't find a setting in Expedition where I could enable or disable to calculate any of these variables.
As far as I understand all variables are directly imported from deckman through Dfw2Exp. All variables (TWD, Course, Heel, Leeway,..) show exactly the same values as in deckman. Exception: TWS is alway slightly less in Expedition (up to 0.2kn). TWA in Expedition should be 'Course' minus 'TWD' but that is not the case (in deckman it's correct).

 

     Thread Starter
 

1/15/2016 5:16 am  #4


Re: Questions regarding Magnetic vs. True / Layline Times / TWA

Seems odd, but first of all lets check Exp Calibrations and Calculations. Available in the drop down menu 'Instruments'

[img]https://static.wixstatic.com/media/3a4a49_f338fceeb5204034b17db8fc1ce4edd7.png/v1/fill/w_452,h_392,al_c,usm_0.66_1.00_0.01/3a4a49_f338fceeb5204034b17db8fc1ce4edd7.png[/img]

[img]https://static.wixstatic.com/media/3a4a49_b6365284212b45359dd714a9fbc7f68c.png/v1/fill/w_191,h_296,al_c,usm_0.50_1.20_0.00/3a4a49_b6365284212b45359dd714a9fbc7f68c.png[/img]


:::Campbell Field::::::www.fieldyachting.com:::
 

1/17/2016 9:10 am  #5


Re: Questions regarding Magnetic vs. True / Layline Times / TWA

Campbell,
thanks, that solved it!
I was just aware of "Exp calibrations" which I was always accessing through 'Settings / Racing / Start calibration values'.
Checking now after your hint 'Instrumens / Exp calculations', "Twa &tws'" was selected. Deselecting it solved the issue.

     Thread Starter
 

1/18/2016 12:46 am  #6


Re: Questions regarding Magnetic vs. True / Layline Times / TWA

nav wrote:

Campbell,
thanks, that solved it!
I was just aware of "Exp calibrations" which I was always accessing through 'Settings / Racing / Start calibration values'.
Checking now after your hint 'Instrumens / Exp calculations', "Twa &tws'" was selected. Deselecting it solved the issue.

Great! One of those traps I have fallen into in the past!


:::Campbell Field::::::www.fieldyachting.com:::
 

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