PolarCOM “True” Wind Calculation

When used with “relative” (apparent) wind data source, PolarCOM provides two modes of “true” wind calculation. These modes are labeled “STW” (Speed Through Water) and SOG in the configuration. STW is a “traditional” method, used by most hardware wind instruments. It needs no GPS input and requires STW and HDG (your vessel heading) to calculate “true” wind. It also results in an approximation of “true” wind.

SOG mode provdes more precise “true” wind calculation, but it also requres COG as well as HDG. That means you still need a heading sensor to calculate “true” wind – having only GPS input without heading is not sufficient.

If you are interested in explanation as to why that is, read on:

Let’s say you have the following -
1. Relative apparent wind (from VWR/MWV NMEA sentence) as delivered by windvane
2. GPS information including COG/SOG.
3. nothing else

Consider this – your vessel is heading in some direction which is most likely NOT aligned with COG. Relative wind is a vector that you can calculate based on a coordinate system that has boat heading as one axis (and another axis perpendicular to it).

On the other hand your SOG (and SOG induced “wind”) is a vector that by itself can only be considered in a system with one axis along the COG (and another perpendicular).

Without heading there is nothing to reconcile these two systems and no way to do any reasonable operations on these vectors. You need heading to convert one of these vectors to the coordinate system of the other.

Here is a practical example using PolarCOM. The first set of dials (img 1) shows COG and heading that differ by 20 degrees. The true wind is calculated here based on SOG (and using both COG and HDG to adjust vectors). You can see true wind both relative to the boat (second dial from the left) and as an absolute direction (last dial on right). There is no STW (as the heading dial shows).

Img 1

On the second image (img 2) I adjusted heading to match COG. This is what would effectively happen if you used SOG for true wind calculation but did not have an adjustment angle (and assumed direction of travel matching heading).

It is quite clear that true wind resulting from such calculation is not the same as the one that results from properly accounting for COG/HDG difference. And the difference is not insignificant – 30 degrees and about 40% stronger.

Img 2

Incidentally, third picture (img 3) shows calculations using STW and heading. I set STW to be equal to SOG, which is not necessarily true, but it helps make an example simpler. As you can see it is also incorrect, but the result is essentially the same as when using SOG alone without proper heading. I.e. there is no gain in calculation precision.

Img 3


6 Responses to PolarCOM “True” Wind Calculation

  1. isail says:

    I think you’re confusing matters when you say your using COG and HDG in the correct calculation. You don’t really need COG at all. You calculate TWA based on AWA, AWS and SOG and then you calculate wind direction by HDG+TWA. COG is never used. Sure, you could calculate TWD as COG+TWA+(HDG-COG), i.e. use an adjustment for the diff between COG and HDG, but it should be apparent that COG cancels out in that equation.

    On another note, have you considered an iOS client app based on PolarCom (that would just be able to take data over IP)?

    • polarnavy says:

      I am not sure I agree. In short, if you have AWA and AWS related to vessel heading and SOG related to vessel course – these are two un-related vector systems. In the correct calculation, you simply can’t say anything about relationship of these vectors. I.e. – there is no way to meaningfully connect AWA/AWS and SOG.

      PolarCOM does include (now) a mode where COG is being ignored (essentially assumed to be equal to heading) – this mode of operation is only approximately correct when heading and course are close.

      As far as iOS is concerned – probably not. First, there is already a product that provides the same functionality for iOS – I doubt there is a need for another one. In fact, there is a good chance that PolarCOM will be rolled into PolarView at some point. As much as I like the concept, there is a non-trivial number of users who keep having trouble with an idea of separate instruments and navigation, something that needs to be solved.

  2. Noel Guevara says:

    Good day,

    Greetings! Just wanna clarify what is the difference between the TRUE WIND AND THE TRUE RELATIVE WIND? Many thanks in advance.

    Noel

    • polarnavy says:

      As far as I know there is “true wind” which is wind as it is experienced by a stationary object, in degrees from true North, and relative wind – which is wind as it is experienced by a vessel, in degrees from vessel heading. Not sure what “true relative wind” is.

      • Rick Macdonald says:

        Not sure I’ve heard of “true relative wind”, but if anything I’d guess it’s the true wind measured from the vessel heading instead of measured from true north. You could experience it by slamming on the breaks and stopping the boat’s motion. Not sure what interest it is. It’s not a new vector but just true wind adjusted from north by your heading angle.

  3. Bill Kinney says:

    True Relative Wind is what a wind instrument calculates without a heading input, only a AWA, AWS and STW. It is calculated relative the the vessel’s bow, not relative to geographic north. If the vessel’s bow is pointed North, then True Relative Wind and True Geographic Wind are the same. It is an approximation, for all the reasons that are described above.

    It could also be done using SOG, making the (sometimes poor) assumption that the SOG is parallel to the vessel’s heading

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