True Wind without speed transducer

[a64pilot]

Originally Posted by Kelkara

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.

I can only laugh at that, it may be correct in sailing terms, but in other vocations, true wind is the actual wind direction, apparent wind is corrected for vehicle movement. I’m aviation we call apparent “relative” wind, because it’s wind relative to the aircrafts speed and direction.

The vehicle whether it be an aircraft or sailboat only “knows” or “sees” apparent wind.

Actual true wind is what I guess sailors call ground wind. I can only theorize it’s because they didn’t have that, until GPS came along, before then they only knew speed through the water.

I can see where knowing true wind is good for planing, knowing that for instance you can only sail to 30 degrees off the wind would allow you to take a bearing to that point of land or whatever and see if you can make it.

I have I believe a Raymarine i60, it will display GPS speed, and I bekeve when GPS is selected then GPS is the speed source for wind computations.
I guess I need to remove my paddle wheel and see if when underway and GPS is selected if GPS is the speed source.
Of course that requires the I60 to be connected to the NMEA backbone, and a GPS speed present on the backbone.

[tanglewood]

Originally Posted by StuM

You don't need course to determine TWS and TWA. Just AWS,AWA and STW.


(Obviously you do need it to determine Ground Wind Speed and Direction.)

Terminology error on my part. I believe its heading that you need. You need your speed through water, but you also need to know the direction of that travel.

[StuM]

Originally Posted by tanglewood

Terminology error on my part. I believe its heading that you need. You need your speed through water, but you also need to know the direction of that travel.

Only for True Wind Direction, not for True Wind Angle.

[RaymondR]

Originally Posted by tanglewood

Terminology error on my part. I believe its heading that you need. You need your speed through water, but you also need to know the direction of that travel.

I think you may be right on the heading thing. Unless the windicator thing has it's own compass it's going to provide a direction relative to the ships head. Consequently to provide sufficient data to accurately calculate geographically referenced wind direction (two separate frames of reference, ship and geographical consequently conversion required)

The little twerly gig up the top is going to provide an indication of the passing wind speed which will be of the apparent wind (actual over ground plus or minus boat speed over ground) so the "wind over ground" or actual wind speed and direction will be the derived functions (resultant of two vectors maybe)

[noelex 77]

“Ground wind” is poorly named. Really it should be called “True wind” and true wind should be called “water wind” or “current wind”.

Once you understand what this data is conveying, referencing ground wind relative to the boat (GWA) is useful. On a cruising boat it is invariably much more accurate than (TWA). TWA would be preferable at least in theory, but the inaccuracies in STW mean the reading is often in error. GWA uses SOG, which is provided by the GPS, requiring no calibration or possibility of drift with time, errors on different tacks etc.

Of course GWD is also very nice to have, but requires more instrument input.

Try switching TWA for GWA (if this is possible), especially if you sail areas with relatively small currents. Most cruising boats find the change a distinct improvement.

[wingssail]

Originally Posted by a64pilot

...I’m not sure why true wind direction matters, the sails only know apparent wind.

Sails know only apparent wind but sailors, some sailors, prefer to know true wind. I know that I do, even when I'm not racing. We don't display apparent wind at all, haven't for decades. It is just a preference thing. However some of the best racers I know, from the old days, don't need that information displayed on the instruments, in fact often don't use the instruments at all, they instantly figure it in their heads by observation (seat of the pants). I can't. I like my True Wind display and I keep my paddle wheel clean, usually by removing it when in port, so that my true wind is calculated correctly.

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, (and this is a terminology thing left over from a time when GPS wasn't around) TWA and TWS is apparent wind corrected by boat speed through the water. We are slaved to the precedence of TWS terminology from years ago (the 1970's). Like you mentioned, in aircraft TWS would mean relative to the ground, not to the moving water. They have it right. "Ground wind speed" is a work around

BTW since the 1970's B&G has calculated TWD relative to the ground if a electronic compass was present in the system, otherwise TWA is relative to the boat's head. I do not know if they calculate wind speed relative to the ground using GPS.

Originally Posted by barnakiel

...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.

I don't think that polar data is required to calculate TWS and TWA. My ancient B&G calculated them in 1979, without polar data, although that was available on the system and helpful in calculating target speed, but yes, it was a package meant to meet the needs of racers. These values are more common nowadays even on non-racing systems.

Originally Posted by barnab

..We had a NMEA to WiFi dongle (from YachtDevices) anyway and a Raspberry Pi so I programmed it to relabel a VTG sentence (speed and course over ground) as VHW and ever since we have a true wind display. I know VHW and VTG are not the same, but I'm totally happy with the setup.

Now this is interesting, you are intercepting the NMEA speed data and modifying it before it gets to the instrument cluster? What instruments are you using?

[barnakiel]

Originally Posted by wingssail

(...)


I don't think that polar data is required to calculate TWS and TWA. My ancient B&G calculated them in 1979, without polar data, although that was available on the system and helpful in calculating target speed, but yes, it was a package meant to meet the needs of racers. These values are more common nowadays even on non-racing systems.

(...)

OK. I did not express myself very accuartely, did I.


What I should have said was something more along (TWS / TWA) AND polar data ...


Frankly, I have never thought about how the calculation is exactly done as it seems an obvious vectors thing. And I never care about TWS and only in specific occasions about TWA.


You are right today some systems can throw nearly any data at the driver. But how much of this data is in good use and how much is rather a distraction?


Also, btw, the water based angles and speeds, I see them just as an intermediate step while gps was not available. To me the, relative (boat angles) and the true (true to the bottom=ground) data counts. Why should I care about water based data?


Long story short, I am in the AWA / AWS club with some use for ground (bottom) based data - to see the wind veer or back and either judge the weather facts from this or else use this ground data to time my gybes (ot tacks).


But ask 100 people on the dock, veer and back may already be beyond their scope, so ...


;-)


barnakiel

[wingssail]

Originally Posted by barnakiel

OK. I did not express myself very accuartely, did I.

What I should have said was something more along (TWS / TWA) AND polar data...

Frankly, I have never thought about how the calculation is exactly done as it seems an obvious vectors thing. And I never care about TWS and only in specific occasions about TWA...

Don't mean to be pedantic Barnakiel but the calculation for:

TWA= uses only BSP (Boat Speed), AWS (Apparent Wind Speed) and AWA (Apparent Wind Angle)

TWs=uses only BSP (Boat Speed), AWS (Apparent Wind Speed) and AWA (Apparent Wind Angle)

To obtain Ground Speed you use SOG instead of BSP and you apply COG to the boat direction, and adjust everything.

For downwind, You'll have to imagine it, I'm bored with drawing these things right now.

It's all trigonometry

[79Endeavour32]

I really have to shake my head listening to fully grown people ponder an instrument that shows where the wind is coming from when sitting in the open cockpit steering a sailboat and trimming the sails.

[barnakiel]

Originally Posted by wingssail

Don't mean to be pedantic Barnakiel but the calculation for:

TWA= uses only BSP (Boat Speed), AWS (Apparent Wind Speed) and AWA (Apparent Wind Angle)

TWs=uses only BSP (Boat Speed), AWS (Apparent Wind Speed) and AWA (Apparent Wind Angle)

To obtain Ground Speed you use SOG instead of BSP and you apply COG to the boat direction, and adjust everything.

For downwind, You'll have to imagine it, I'm bored with drawing these things right now.

It's all trigonometry

+1!


Pedantic is fine in my books. Thank you!



b.

[barnakiel]

The image reminds me of a conversation I had with a friend long time ago. So long ago that the friend is already expired.


Namely.


Since some say the boat speed is (to be) read off the log rather than off the gps, we can easily imagine the boat moving at said 5 knots against a 5 knots current, thus the boat remaining stationary vs. land.


Now with the drawing correct, why is the TWA different from AWA ??? (Or is it not now?)



That's why I simplify (as you said wrongly) the theory and call 'true' what is true to the ground, not what is relative to the water.


So here you know how I fail.


I admit I was trained in navigation (properly, at a maritime college) but I have built a number of simplified routines once I went into sailing a small boat with hardly any instrumentation. Not to say I do not enjoy refreshing the theory and reminding myself (and getting reminded) of the fact that a simplification is just that, a crutch that may be good in one moment but lethal in another.


Yours,
b.

[barnakiel]

Originally Posted by 79Endeavour32

I really have to shake my head listening to fully grown people ponder an instrument that shows where the wind is coming from when sitting in the open cockpit steering a sailboat and trimming the sails.

You ARE right.


Except that at other times you are not sailing a dinghy on a lake or a daysailer on a sunny day passage but rather crossing a wider body of water and what the wind is doing relative to the clock (judged by landmarks while closer to land - but lacking such guides further offshore) may bear on your safety and / or comfort (or, in fewer cases, performance).


Wind veering, backing, gaining (true, not apparent) force or not, is a good proxy for weather systems passing over you.


b.

[wingssail]

Originally Posted by 79Endeavour32

I really have to shake my head listening to fully grown people ponder an instrument that shows where the wind is coming from when sitting in the open cockpit steering a sailboat and trimming the sails.

79Endeavour, Come on, you are not using your imagination.

Lets say we are crossing a body of water where we cannot clearly see the other side. We are sailing close hauled, feeling the wind in our face and steering to the telltales and trimming as needed (which you and I both love to do).

Unfortunately the destination is upwind to we are on a tack, suppose it is starboard tack.

Now, say the wind gradually shifts 10 degrees to the left. Since we are sailing to the tell tales and steering, the wind feels exactly the same to our faces. And since we can't see the other side, or our destination, we might not even know that we've changed course.

BUT, the compass will tell us! And a True Wind Direction will tell us.

So we know that with the new wind we should tack to port.

[CarinaPDX]

The compass tells us that, and we all should have a good compass, (somewhat) recently adjusted. If seconds matter then get the toy (true wind calculator). Otherwise a masthead wind indicator is all that is needed for sailing to the wind, although a few telltales are also useful. And we shouldn't let things get too out of hand because we are checking our progress with the GPS.

[rslifkin]

Originally Posted by barnakiel

Since some say the boat speed is (to be) read off the log rather than off the gps, we can easily imagine the boat moving at said 5 knots against a 5 knots current, thus the boat remaining stationary vs. land.

I have always wondered why most sailors focus heavily on speed through water and ignore the GPS speed over ground readout. Coming from a powerboating background, in my mind, what really matters in the end is "how fast am I moving and in what direction?" So SoG and CoG from the GPS tell me that. It doesn't really matter if I'm moving a different direction than I'm pointing, or if I'm moving over ground slower or faster than through the water. That's cool to know occasionally, but is of no use for getting me to my destination.