True Wind without speed transducer

[The Perfessor]

Hi,
I have a Raymarine Axiom 9 chartplotter and an i70 instrument display. I do not have a speed transducer. I use the i70 for my wind information. Is there a way to use the SOG from the chartplotter via the gps to feed the speed to the i70 so that I can get true wind and VMG, etc.? I'm looking to add the speed transducer as it will also give me temp and speed over water, but it is pricey when all is said and done. I'd be ok with just knowing my true wind and VMG. Any help is appreciated. This is for a C&C Landfall 43 sailboat for some context. Thanks! Rich

[barnakiel]

And what do the chartplotter and the wind instrument instruction manuals say about this?

barnakiel

[The Perfessor]

So far, all I can figure out from the manuals is that anything to do with speed comes from a transducer. I'm still digging, though.

Edit: I saw from another post what appears someone doing just what I'm looking to do; set the speed calcs to use the GPS SOG... I'll keep you updated as to what I find out.

[barnakiel]

Imho if i70 does not allow gps sog in SPEED data sources menu, then you can't.


Menu: data sources,
Option: speed,


What does it show when all instruments in the network are ON.


?



b.

[a64pilot]

I think mine can, I know it will display GPS speed as well as paddle wheel speed.
I’m not sure why true wind direction matters, the sails only know apparent wind.

I also believe true wind direction and speed is apparent wind corrected for vessel speed over the ground, that speed thru the water doesn’t play into it.

Think of it this way if your speed thru the water is 5 kts and your in a 5 kt current so that your SOG is zero, then true and apparent are the same

On edit, all my instruments are NMEA 2000 networked, otherwise the i70 couldn’t know GPS speed.

[barnakiel]

Well, clearly it should will display gps sog as this is i70 and these can display everything including ais. The question remains if the wind function will or will not pick up gps sog as the speed to use in true wind angle calculation.


I share your doubt re why people need true wind (speed). If it were about direction, I would guess this has to do with tracking daily patterns to use for timing gybes in offshore sailing, or in judging low pressure movement relative to the boat. But true wind speed is something I only see in specialized racing software as a part of polar data fed into the package.


b.

[StuM]

Knowing TWS and TWA is useful when you are planning a course change especially when going from a downwind to an upwind heading.


A AWS/TWS switch is common on most wind speed indicators I've seen.


The venerable ST60 switches between True and Apparent at the press of a button.

[barnakiel]

Originally Posted by StuM

Knowing TWS and TWA is useful when you are planning a course change especially when going from a downwind to an upwind heading.

Yes. Of course.


But does it need to be read off a screen (I mean, outside of pro racing)?


I think most sailors to whom this counts have 'innate' (gained from plenty of experience) sense for this?



b.

[StuM]

Originally Posted by barnakiel

I think most sailors to whom this counts have 'innate' (gained from plenty of experience) sense for this?

You'd be surprised

[Kelkara]

Originally Posted by a64pilot

I also believe true wind direction and speed is apparent wind corrected for vessel speed over the ground, that speed thru the water doesn’t play into it.

Actually true wind is apparent wind corrected for speed through the water. Correcting for SOG is often called "ground wind" ... at least according to Raymarine.

https://forum.raymarine.com/showthre...d=987&pid=3077

So if your Raymarine device doesn't offer a "ground wind" option then you are going to need a STW speed transducer to get true-wind.

[RaymondR]

I find it fairly confusing to ascertain how hard and from where the wind is blowing in the dark, no problem in daylight.

[barnakiel]

Originally Posted by RaymondR

I find it fairly confusing to ascertain how hard and from where the wind is blowing in the dark, no problem in daylight.

Yes. It seems less obvious when we have less visual feedback.


In our boat, there is that windvane 'wing' behind the boat - think of a huge windex working just behind your stern - and roughly eye-level.


So when caught in a rainy sqall at night, I simply drive by looking at that wing. This may drive me wildly off the original course, but at least I never get a bad Chinese crash gybe anymore!


I think having a similar device in any boat should be very easy - it is just a simple vertical axis vane. It does not need to drive anything, just sit there and show you what the wind is doing.



barnakiel

[barnakiel]

btw if cog is available from gps and awa from the instrument, one can use the data and calculate true parameters with an App.


Data from Ray to phone via wifi. Then deploy any racing App or build your own that shows the awa, twa, etc. It is easy.


b.

[noelex 77]

“True wind “ calculated without STW input is more correctly, as has already been noted, called “ground wind”, although I suspect ground wind is actually what most sailors (incorrectly) understand true wind to be indicating

Personally, when cruising, I find ground wind more useful than true wind. Unless you clean and calibrate your paddlewheel frequently ground wind is more accurate and consistent than true wind. This is a major advantage. I also find the combination of apparent wind and a ground wind display that independent of boat and water movement creates a more complete picture of how the wind is altering.

B&G instruments can be changed quite easily to read ground wind and I would suggest everyone gives this try. My last Raymarine instruments were many years ago and at this stage they could not display ground wind. Hopefully things have changed as it an easy and useful feature to have.

Even if you prefer a true wind display having the option of ground wind means you can still have a “true wind” option if your paddle-wheel becomes fouled in the middle of a passage.

[meirriba]

Originally Posted by StuM

Knowing TWS and TWA is useful when you are planning a course change especially when going from a downwind to an upwind heading.


A AWS/TWS switch is common on most wind speed indicators I've seen.


The venerable ST60 switches between True and Apparent at the press of a button.

It will show TWS ONLY if you have speed through water transducer. If you dont, toggling to TRUE will show APPARENT data...