Expedition Expedition Navigation Software

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9/25/2020 2:10 am  #1

Polar and drift


(I haven't purchased Expedition Navigation Software yet. I still have some questions.)

When I watch some polars diagrams , I see few lines on a same graph, depending TWA/TWS/SOG (hypothesis there is no tide). OK, but ....

For each TWA/TWS, the difference between COG and HEADING is not the same : drift.
(drift more important if wind (AWA) from side than TWA=170°... but not only...)
And TWA depend of the axis of the boat (so TWA depend of HEADING).
So, when Heading=0°, COG=6°, TWA=x , TWS=y, Bearing to Waypoint = 0°, SOG=10kts, it's not true to make a polar with SOG=10kts : Approach speed to waypoint (VMC) is a trigonometric result of 10kts (I didn't calculate but it's easy).

My first question : 
Is Expedition Navigation Software calculate polars from VMC or directly to SOG ? I fear ("fear" ??) the drift is not considered, so in this hypothesis only TWA/TWS/SOG/Heading would be considered.... so polars are not exact.

My second question :
If heeling increase too much, drift increase too. So VMC decrease (SOG can be constant or "not enough"). I read that 
Expedition Navigation Software have "advanced polar", is this software considering heeling (so drift, so vmc, so....) ?

My third question :
Can Expedition Navigation Software display (feature) "Beat heel" ? (I think is not very important if the software display "beat COG and Perf%" but maybe yes...)

My fourth question :
(​I almost forgot). Adrena software can calculate polars in realtime (I don't know regarding my two previous questions). Is Expedition Navigation software calculate and improve polars in realtime ?

Waves (height and direction) and tides are another subject (ok important for diagrams polars but another subject).

Thanks for your answers.

(sorry again for my bad English)


9/25/2020 9:18 am  #2

Re: Polar and drift

By drift I presume you mean Leeway.

It is up to you and your instrument systems what TWA is referenced to - Course or Heading. See https://www.fieldyachting.com/question-and-answer/calibration/q-from-customer-twd-and-leeway and https://www.fieldyachting.com/question-and-answer/calibration/hi-guys-how-much-twa-angle-correction-is-normal

SOG and COG are GPS variables.

The leeway component has nothing to do with the polar - it is an instrument variable, so the more heel you have the more leeway your instruments will calculate.

Yes, you can load a heel polar to Exp for displaying target heel and polar heel etc.

No Expedition will not automatically update your polars, deliberately. How does your software know if an improvement is performance is not due to a poor BSP, TWS or TWA calibration, or wind sheer, or an inverted gradient? Polar data should only be modified deliberately after proper analysis usign Expeditions own analysis features or other third party sw such as KNDsailingperformance.

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

9/25/2020 12:16 pm  #3

Re: Polar and drift

Exactly as Campbell says.

I would add that the TWA in VPPs, polars and most instrument systems includes leeway.

Modifying polars is necessarily a considered thing. People use Expedition, KND or Sail Njord for this. In Expedition, you an save representative data periods as Tests in stripchart. You can also import the day's sailing data into the graphical polar editor for analysis or export to a file for analysis in Excel etc.


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