True wind direction

[Ephesian]

SWMBO is very keen on getting true wind direction on our instruments - i.e. as an absolute number in degrees, not apparent direction relative to the boat. I'm currently repacing some old instruments with a new set up based on Raymarine.

According to the manual, the new instrument (i70) will display true wind direction. Wind data comes onto the SeatalkNG bus via a pod to which the wind transduer is attached. GPS data is provided via a Raymarine e7 plotter.

I'm not clear how the system will derive true wind direction, and specifically whether I need to add a digital compass? Do the wind instruments typically include a compass or can the displays derive the direction from a combination of apparent wind angle and boat heading / COG from the GPS?

 

[FWB]

My ST60 system takes it from the autopilots fluxgate compass to display on graphic display. Shown as say NE Force 4. The true display on the wind instrument head needs boat speed.

 

[Ephesian]

Yes I wondered about that, however I have a ST2000 tiller pilot, so it isn't always connected and certainly not when racing....

 

[john_morris_uk]

The Raymarine on our boat does the vector calculation from the boat speed (taken from the log NOT the GPS!) and from the fluxgate compass built into the autopilot. Mind you are autopilot is always connected as its a wheel steering and the autopilot has its own quadrant onto the rudder stock. (There's an electronic clutch to connect or disconnect.)

I know that its the log not the GPS because if the log gets fouled, then the true wind readings go wonky. If I scroll through the pages on the GPS (we don't have a plotter), it gives the Beaufort wind sped and direction (ENE 4 etc) but it also gives the true wind direction in degrees.

They don't make the GPS unit that we have any more - but I am pretty sure that the same data is available on the Raymarine Multi data display which also plots nice graphs of the wind speed and direction if you press the right buttons..

 

[bbg]

True wind speed (and direction, and angle) needs inputs from the masthead (wind speed and angle) and boat (boat speed and angle) and compass.

With these data, an electronics system can calculate TWS, TWA and TWD. If it is programmed to be able to do so.

Note that using GPS data will not provide True wind speed, but Ground wind speed which is different.

Apparent - is the wind "felt" on the boat
True - is the wind "felt" by an object floating in the water
Ground is the wind "felt" by an object fixed in one location

(Thanks to flaming for explaining the difference in an earlier thread.)

Edit - I can't be sure but I think some systems are capable of taking data from the GPS - I believe mine was - but this won't produce "true" data but "ground" data.

Having said that, this will only have a significant effect where tidal vectors are significant compared with boat speed vectors.

 

[FWB]

Your i70 instrument uses seatalk ng. This is Raymarines version of NMEA 2000.
You will have to get your log readout to talk to the seatalk ng bus. Then you can switch between true/apparent on the i70 display.
You will probably need one of these...
http://www.raymarine.co.uk/view/?id=3338

 

[AntarcticPilot]

If I've understood the system correctly, you don't NEED a heading sensor; as you say, COG and SOG can be derived from the GPS, and that coupled with relative wind direction and speed are all you need.

However, it strikes me that COG will not be very responsive. It depends on GPS positions at 2 second intervals, and has to be filtered over a longer period than that to give reliable results (the error in a single fix is several metres, and that is large in comparison with the distances moved between fixes). So, your wind direction after tacking or other sharp change of direction may take a short while to settle down and be reliable again.

A flux-gate compass unit on the SeaTalk network would seem to be a useful addition.

 

[noelex]

True wind speed (and direction, and angle) needs inputs from the masthead (wind speed and angle) and boat (boat speed and angle) and compass.

With these data, an electronics system can calculate TWS, TWA and TWD. If it is programmed to be able to do so.

Note that using GPS data will not provide True wind speed, but Ground wind speed which is different.

Apparent - is the wind "felt" on the boat
True - is the wind "felt" by an object floating in the water
Ground is the wind "felt" by an object fixed in one location.

That is a good explanation, but for a crusing boat "true wind" derived from the the GPS ( or ground wind ) is more useful and importantly much more accurate than "true wind" derived from the log. It's also what most sailors think is true wind ie the direction and speed of the wind that would be felt on land next to the boat.
True wind derived from the log requires a very calibrated and well set up log, which is rare other than in top racing boats. It's a pity companies like Raymarine do not offer true wind derived from the gps, it would be an easy software change.

 

[theoldsalt]

BBG,

I have always believed that "true wind" is the same as "ground wind" since the effect that produces an "apparent wind" can be either movement through the water or movement of the water (or both).

"True wind" must be refenced to something stationary otherwise it isn't "TRUE".

Anyone willing to agree?

 

[theoldsalt]

Further to my post 9,

If you are on a boat stationary in the water drifting with a 5 knot current with no wind over the land then you would feel an apparent wind of 5 knots not a true wind of 5 knots as the wind is only there due to the current.

 

[maby]

BBG,

I have always believed that "true wind" is the same as "ground wind" since the effect that produces an "apparent wind" can be either movement through the water or movement of the water (or both).

"True wind" must be refenced to something stationary otherwise it isn't "TRUE".

Anyone willing to agree?

I will - I think there is some confusion here - my wind instruments show both "true" and "apparent" wind without any connection to fluxgate compass or GPS - the ususal definition of "true wind" is the speed and bearing of the wind relative to your boat after subtracting the vector for the boat's motion - in other words the speed and angle of the wind to your boat if you were to come to a halt relative to the ground. Strictly speaking, this would require an input from the gps to give speed over ground, but most instruments will take the speed from the log and assume no leeway or currents to come up with a reasonable figure. You can then convert this to a bearing relative to magnetic north by taking a compass reading and adding an offset for the bearing of the boat - I guess that you may be able to set up some fully integrated systems to do the sums for you, but I think most sailors would interpret true wind as the angle of the wind to the boat taking into account the motion of the boat.

 

[Ephesian]

OK, so let's not call it "true" .. let's call it "actual" or something similar.

What I want is the wind direction as if I were standing still on the surface of the earth. So not including the motion of the boat whether by tide or power/sail....

I get the definitions below, but they are angles relative to the boat's bow either taking account of motion through the water or not. To get actual wind direction you need to know which bearing the bow is pointing at so you have something from which to add or subtract your relative true/apparent angle?

 

[FWB]

If you want true readout from your i70 you will have to give it boat speed as well as the feed from the wind transducer. If the boat speed is zero then true and apparent will be the same.
Tidal effects notwithstanding.

 

[Twister_Ken]

Further to my post 9,

If you are on a boat stationary in the water drifting with a 5 knot current with no wind over the land then you would feel an apparent wind of 5 knots not a true wind of 5 knots as the wind is only there due to the current.

If there was a 5kt current I wouldn't be drifting, unless there was a 5 kt wind going the same way.

But I know what you mean!

 

[noelex]

BBG,

I have always believed that "true wind" is the same as "ground wind" since the effect that produces an "apparent wind" can be either movement through the water or movement of the water (or both).

"True wind" must be refenced to something stationary otherwise it isn't "TRUE".

Anyone willing to agree?

I agree this is the most sensible definition of true wind and it's what most sailors mean by true wind, but it is not the definition of true wind.
It would be great to see more instrument companies offering this as an option. If nothing else it's a much more accurate display than true wind derived from the log ( which includes boat movement through even if it's called, incorrectly in my view, true wind) so many yachts have a true wind display that varies from one tack to the other and from upwind to downwind.
I can display both and prefer true wind to be displayed as referenced to something stationary. This could not be calculated in the days before GPS so we are stuck with a screwy definition of "true wind" and this log based wind, is what is displayed on most yachts.

 

[maby]

OK, so let's not call it "true" .. let's call it "actual" or something similar.

What I want is the wind direction as if I were standing still on the surface of the earth. So not including the motion of the boat whether by tide or power/sail....

I get the definitions below, but they are angles relative to the boat's bow either taking account of motion through the water or not. To get actual wind direction you need to know which bearing the bow is pointing at so you have something from which to add or subtract your relative true/apparent angle?

exactly so - but I've never seen any instrument that does that for you. I suppose it has some interest from the meteorological point of view but it is of little value to the helmsman - he (or she) just wants to know where the wind is coming from so they know how to trim the sails and plan the next tack. You obviously will need some input to measure the bearing of the axis of the boat to magnetic north - can be from the GPS if you are moving reasonably fast, but no use if you are stationary or moving slowly - so add in a fluxgate compass - at least £80 and potentially much more!

Personally, if you are sufficiently interested I would recommend taking the true wind bearing from your instruments and adding it to the ship's heading from the steering compass - far cheaper!

 

[bbg]

I agree this is the most sensible definition of true wind and it's what most sailors mean by true wind, but it is not the definition of true wind.

All of the instruments I know of reference true wind to the boat's log.

Like it or not, the phrase "true wind" was developed before GPS, and depended on input from the boat's log.

For most cruising boats "true wind" is more important than "ground wind", which is not enormously relevant.

flaming posted something a while ago to demonstrate the importance of the difference between ground wind and true wind when racing - particularly when racing on a course where the current is not the same across the course.

I'll say it again - whether everyone likes it or not, true wind has come to mean the wind by reference to the surface of the water - not to a fixed point.

 

[maby]

I agree this is the most sensible definition of true wind and it's what most sailors mean by true wind, but it is not the definition of true wind.
It would be great to see more instrument companies offering this as an option. If nothing else it's a much more accurate display than true wind derived from the log ( which includes boat movement through even if it's called, incorrectly in my view, true wind) so many yachts have a true wind display that varies from one tack to the other and from upwind to downwind.
I can display both and prefer true wind to be displayed as referenced to something stationary. This could not be calculated in the days before GPS so we are stuck with a screwy definition of "true wind" and this log based wind, is what is displayed on most yachts.

All true, but I don't think that this definition of true wind is what the OP is looking for either - he wants a "true wind" reading of zero degrees to indicate that the wind is coming from the north rather than directly in over the bows. I don't think any boat instruments offer that - it is of very limited value to the helmsman or navigator.

 

[bbg]

exactly so - but I've never seen any instrument that does that for you. I suppose it has some interest from the meteorological point of view but it is of little value to the helmsman - he (or she) just wants to know where the wind is coming from so they know how to trim the sails and plan the next tack.

Most decent instruments can do it. And it has a lot of interest, even for cruising sailors, precisely so they can plan the next tack (or gybe). If you are anticipating a persistent shift, you might decide that you want to tack "early", in anticipation of the shift continuing. So you want to know, for example, when the wind goes from 260 TWD to 270 TWD.

 

[noelex]

Further to my post 9,

If you are on a boat stationary in the water drifting with a 5 knot current with no wind over the land then you would feel an apparent wind of 5 knots not a true wind of 5 knots as the wind is only there due to the current.

In this case
Apparent wind is 5 knots
" true wind" is 5 knots
So called " ground wind " is 0

I believe most crusing sailors expect their instruments to display 0 in this situation, but Unfortunatly they do not! Unless you set ( and the instruments allow) true wind to displayed from the GPS.