Calculating True Wind Direction

[noelex]

Pretty sure he said he had relative True, (I.e. 60 degrees from the starboard Bow) but was looking for a true wind bearing.

I think you could be right. Sorry I completely misread it as not having any true wind.
I don’t think the ST60 will do wind bearing, but its been a few years since I used one. More sophisticated instuments will like the B&G H3000. It even draws a nice graph if you want.

 

[noelex]

I'd think the true on the wind instruments is simply calculated by interface of log and wind and the calculation is based on the log speed . Apparently there is a way to feed the SOG rather than log speed

see
http://taketwosailing.com/blog/post/2010/10/09/SeaTalk-NMEA-Bridge.aspx

Thanks for the link its good to know there is a way even if involves purchasing a third party product it would be a worthwhile option for many.
I think you would probably loose all calculated tide information however as I presume it works by renaming the STW sentence to SOG.

 

[RichardS]

I'd think the true on the wind instruments is simply calculated by interface of log and wind and the calculation is based on the log speed . Apparently there is a way to feed the SOG rather than log speed

see
http://taketwosailing.com/blog/post/2010/10/09/SeaTalk-NMEA-Bridge.aspx

That's interesting as I also don't have a paddle wheel and can't display true wind on the ST60.

Thanks

Richard

 

[john_morris_uk]

I think that what I was trying to say was that its the true wind direction relative to the boat - in other words wind relative to the water that you actually need.

We have a Raymarine Compass and gyro aa part of the Autopilot that sends data to the bus, and both the multi display and the GPS tells us a 'true wind' angle - but its wind direction calculated as seen by a drifting boat on the tidal stream - not a boat anchored.

It also assumes that the paddle wheel is clean - but I usually give a it a scrub everytime we go sailing so no problem.

 

[photodog]

The key to this is taking off your hat, and looking at the water.

True wind is at 90' to the tiny little waves being made by the wind on the surface...

Apparent wind is what you feel on your face when you take your hat off.

 

[jimi]

The key to this is taking off your hat, and looking at the water.

True wind is at 90' to the tiny little waves being made by the wind on the surface...

Apparent wind is what you feel on your face when you take your hat off.

Wanna bet i could produce a valid argument to prove you're wrong on both counts ;-)

Pint of Doombar?

 

[photodog]

Wanna bet i could produce a valid argument to prove you're wrong on both counts ;-)

Pint of Doombar?

No doubt..

(As long as the reason isnt "Cause the RYA says its wrong." )

Shall we discuss it in the Bridge? That way I can compensate you instantly....

 

[jimi]

http://www.oceansail.co.uk/Articles/WindTwistArticle.html


apparent wind in yer face is slightly different to the apparent wind on the sails cos they're at a different height

 

[photodog]

Of course in the Southern hemisphere things are the other way around.

This is one of the reasons why boats sometimes sail better on one tack than the other.





I have to say, I have never sailed so long on one tack that I changed hemispheres...

 

[citlab]

My ST60+ calculates true wind from boatspeed & apparent. No need for sog.
However, the S60 series remains with an impressive record of unfixed bugs. In my view nothing is impossible. This is quite amazing considering how mature the product should be by now.

One of the annoying bugs is its inability to set wins angle alarms when set up for a metric scale (yes, I from the baltic). The instrument applies the same multiplier for angles as it does for the knot-to-m/s math. This has been the case with both the ST60 and the later ST60+ series.

Last season, I failed to calibrate my wind instrument despite every effort. I finally found out that the analogue hand had become 15degrees off from the digital reading. Has anyone else encountered the same?

 

[trapezeartist]

Thank you, all. Especially Flaming and Angele who worked out the question I was trying to ask. Just to confirm, I have no problem getting a true wind speed and true wind direction relative to the boat (calculated using speed throught the water). What I am after is true wind direction relative to the world. Two main reasons spring to mind, although I'm sure there are others:
1. So I can write wind speed and direction in the log without having to switch on my brain first.
2. So I can decide in advance which side to rig the boom preventer and whisker pole when setting off on a run. (A few degrees error could mean sailing by the lee, or having to gybe and re-rig everything.

So the conclusion is that what I am looking for may be lurking inside the chart plotter somewhere. I'll get out the instruction book and report back.

On a secondary note, using STW to calculate true wind is notably inaccurate when sailing in the Bristol Channel. Last time out, just before we gave up and turned on the donk, we were doing 1.2 knots through the water and 6.2 knots over the ground. Imagine the different sort of answers you would get for true wind in that situation, depending on which flavour of boat speed you were using.

 

[goboatingnow]

Ive yet to hear a convincing reason why we need true wind as opposed to ground wind displayed or calculated on a boat.

Dave

 

[trapezeartist]

I just had a look in the C70 manual (should have done that first). I can turn on a "wind vector" that will show the wind graphically. As I always run the plotter north-up, that will be fine for the hourly log.

In the data bar section it says that there is a wind group covering true wind, apparent wind and ground wind. In the absence of anything specific I'm assuming that they is speed and direction. So it looks like I just have to play around with what data is displayed on the data bar and I should be able to get what I want in numeric form. The only snag is that I don't want to lose any of the data I currently display and the screen on the C70 is not very big. Oh well, you can't have everything.

 

[Angele]

I just had a look in the C70 manual (should have done that first). I can turn on a "wind vector" that will show the wind graphically. As I always run the plotter north-up, that will be fine for the hourly log.

That's the one. While you are at it, turn on the tide one too. Even more useful IMO!

For the sceptics out there who wonder what the use of this is, I have to say that it makes filling in the logbook easy for even the most junior crew member - my 10 year old son, for example - he can see the arrow on the chartplotter when down below and put an entry in the logbook without having to have Photodog's inate ability to read the waves and have an internal compass so that you can intuitively know at all times which way is north! Sure, I'm all for teaching wind awareness and all. But, this is also a good way for junior to learn and practice points of the compass too.

In the data bar section it says that there is a wind group covering true wind, apparent wind and ground wind. In the absence of anything specific I'm assuming that they is speed and direction.

Yes. That is it. However, mine reads "starboard 50 degrees" or "port 120 degrees" at the mo - i.e. relative to the boat's heading. Not sure if you can change it to actual compass direction. Have a play and report back.

 

[flaming]

Ive yet to hear a convincing reason why we need true wind as opposed to ground wind displayed or calculated on a boat.

Dave

Because you sail in true wind, not ground wind.

If you had ground wind calculated (which lets not forget would require heading input to be compared with COG) then you would miss effects that you might want to use, for example tide induced wind shifts and tide induced changes in wind velocity.

Sure, these things should still be evident in the course you are steering (shifts) and the amount of heeling (velocity) but if you have gone to the trouble of having a set of electronics calculate a figure - why not have one that you can actually use?

For example, if I use the wind speed to help decide when to reef, especially downwind, then I might get caught out. (note - obviously a good sailor should use more than just a number, but I suspect most people here would admit to using them as a guide at least)

Imagine I am running in a 15 knot ground wind, and with a 3 knot tide behind me. My boat speed is 5 knots.

I feel 15 - 5 - 3 = 7knots. My true windspeed reading is 12 knots, my ground windpeed reading is 15 knots.

Now the tide changes to 3 knots against me. I feel 15 - 5 + 3 = 12 knots. My true windspeed is 18, but my ground windspeed is still 15.

Before the tide changed, if I was relying on my instruments (set to ground windspeed) to decide the sailplan I would have set sail for 15 knots, but might actually be a little underpowered as I'm actually sailing in 12.
After the tide changed I still have sail set for 15 knots - but I've now got 18.

My instruments are giving the same reading - but I've gone from sailing in 12 knots to 18.

Is that convincing?

 

[goboatingnow]

Because you sail in true wind, not ground wind.

i dont know about you,but ive always sailed in the apparent wind.

.....
After the tide changed I still have sail set for 15 knots - but I've now got 18.

My instruments are giving the same reading - but I've gone from sailing in 12 knots to 18.

Is that convincing?

Not really, firstly true or ground wind calulations are quite unstable numbers on a small boat, so worrying about 2-3 knots makes no odds. secondly , I dont know about you but such tidal shifts take time and one feels the wind anyway. In practice this is somewhat of an academic discussion anyway. I do think that Ray ST60 should use SOG if it doesnt have STW, rather then not calulating the wind at all.

dave

 

[flaming]

i dont know about you,but ive always sailed in the apparent wind.

Which you can reference to the true wind, but not the ground wind.

Not really, firstly true or ground wind calulations are quite unstable numbers on a small boat, so worrying about 2-3 knots makes no odds. secondly , I dont know about you but such tidal shifts take time and one feels the wind anyway. In practice this is somewhat of an academic discussion anyway. I do think that Ray ST60 should use SOG if it doesnt have STW, rather then not calulating the wind at all.

dave

Tide does take a while to change of course, but my point is that for the same ground wind you can need very different sail plans. But the true wind would always reflect the actual conditions that you are sailing in.

Knowing the ground wind would be interesting, as it might make predicting a tidal wind change easier, but don't forget that unless you are heading straight into, or away from the wind your heading and COG will be different, so the calculation is not as simple as using SOG instead of boatspeed.

 

[noelex]

but don't forget that unless you are heading straight into, or away from the wind your heading and COG will be different, so the calculation is not as simple as using SOG instead of boatspeed.

This is true of true as well. most instument make no alowance for leway.

 

[noelex]

True wind certainly does have its uses, but so does “ground wind”. Changes in ground wind both in speed and direction are helpful deciding what the wind will be doing in future. It also is the best indicator of the likely speed and direction of wind when anchored helping decide the best spot to drop the hook.
I think if you asked most yachtsmen which they would rather have, assuming identical accuracy, racing sailors would go for true wind and cruising sailors would go for ground wind.
The problem is that on most yachts other than carefully maintained racing yachts the true wind reading is significantly less accurate than ground wind.
It should be pointed out that unless you have one of the new 5HZ GPS units the update rate for ground wind will be a bit slower, which can be important for fast accelerating racing machines.

It would at least be nice if all instrument manufacturers gave us a choice.

 

[flaming]

True wind certainly does have its uses, but so does “ground wind”. Changes in ground wind both in speed and direction are helpful deciding what the wind will be doing in future. It also is the best indicator of the likely speed and direction of wind when anchored helping decide the best spot to drop the hook.

I can't think of a single possible scenario where the difference between true and ground wind would influence where you dropped the hook. This would impy significant tidal flow, which I'm sure would be far higher up my list of things to consider than the <10 degree difference between the two, don't you?

I think if you asked most yachtsmen which they would rather have, assuming identical accuracy, racing sailors would go for true wind and cruising sailors would go for ground wind.

I don't.
Ground wind is only of intelectual interest to cruising sailors, as the amount of sail you can set, or if you can pinch past that headland are all to do with true, and NOT ground wind.

It is of interest to racing sailors, as we need to be able to predict shifts as we sail into or out of tidal streams, but we have tide charts to help us with that - as we're more interested in the difference between over here and over there, than what over here actually is. But we're much more interested in TWS, as we know how fast our boat should go and how high it should point for a given TWS, and strive to achieve that. And when we approach a mark we want to know what sail to use on the next leg - again a function of true wind, not ground wind.

It would at least be nice if all instrument manufacturers gave us a choice.

I do agree with this - as an integrated system with compass, wind, speed and GPS could give each, it does seem lazy not to.