True Wind vs Crazy Speed Transducer

[negrini]

PS: when I called out Raymarine, my suggestion was searching the net for SOG first, then STW after. But remember, calculating True wind from STW is a big mathematical mistake, accepted only during pre-GPS era.

[brak]

Originally Posted by negrini

Brak, this is incorrect. All those sentences are native to any GPS nowadays. But note, there is no need for HDG, nor a compass at all, as useful true wind is relative to boat, so when you calculate the vector, wind direction relative to Earth is no needed. Just like STW, there is no need for COG, just the apparent angle. What I want is a safe true wind speed indicator to decide weather or not raise my spy.

Ah, but you do need HDG to get a correct result. Here is why:
Let's say you have the following -
1) relative apparent wind (VWR) as delivered by windvane
2) GPS information including COG/SOG.
3) nothing else
Now - think about 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 system that has boat heading as one axis (and another axis perpendicular).

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.

I could bring an example but I think if you try the math you will see what the issue is. Now, granted in most situations heading and course over ground are "quite close" and the results are not too far off, but by the same token in most situations STW and SOG are quite close too. And so the error would be about the same in both situations.

[brak]

Here is a practical example. The first set of dials 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).

On the second image 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.

Incidentally, third picture 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.

[negrini]

Yep !! I must correct my statement, you're right indeed. I was refering to a particular case where SOG is aligned to HDG, but your approach is rather more complete. There are 2 factors affecting a correct true wind speed and angle: 1- By using SOG instead STW you remove the stream current factor, and 2- by using COG and HDG you remove the apparent angle incertainty. In fact, NMEA 2000 has solved this (PGN 130306) by adding new True Wind difinition, allowing user to choose from SOG or STW, but both account for COG and HDG for a more precise true calculation. Brak, thanks for it, I enjoyed the discussion a lot ...

[brak]

Originally Posted by negrini

In fact, NMEA 2000 has solved this (PGN 130306) by adding new True Wind difinition, allowing user to choose from SOG or STW, but both account for COG and HDG for a more precise true calculation.

Woo-hoo, so PolarCOM is NMEA2000-"compliant" now

If only there was a hardware adapter that provided NMEA2000 frames to be read directly by a PC (rather than all those monstrosities that "convert" NMEA2000 to 0183). That's one drawback of NMEA2000 - no simple direct interface to computers, unlike its predecessor. I think that helps hardware manufacturers sell more stuff, but certainly not software.

[negrini]

Although NMEA is a .org association, it's managed by people from marine industry and naturally defend their interests. There is no other explanation to not using standard transport protocol like TCP/IP or even UDP, or using standard connections like RJ-45 on dry areas, etc ... That would simplify the entire system and save a lot to us.

[brak]

Originally Posted by negrini

Although NMEA is a .org association, it's managed by people from marine industry and naturally defend their interests. There is no other explanation to not using standard transport protocol like TCP/IP or even UDP, or using standard connections like RJ-45 on dry areas, etc ... That would simplify the entire system and save a lot to us.

The easy solution they could have taken would be to simply encapsulate 0183 in ethernet frames (or any higher level protocol from the standard stack at their choice), add a few routing tags and keep everything and everyone compatible while overcoming performance and connectivity issues. But that's neither here nor there, too late.

I wonder if some of the bigger software makers are represented there. what would they do? go back to incompatible proprietary interfaces? that seems counter-productive. Stick with NMEA0183 - sure, but that means manufacturers will have to continue supporting it. I know no one gives a rat's behind about my product, but what about all those MaxSeas and Navionicses etc? They also need to get data somehow, and I just don't see any provisions for PC connectivity in NMEA 2000.