webcraft
Well-Known Member
I have edited my reply above since you posted this to explain that GWD is purely a display function on this particular instrument - it is not a separate value.
- W
Judging true wind direction
- Thread starter snowleopard
- Start date
I have edited my reply above since you posted this to explain that GWD is purely a display function on this particular instrument - it is not a separate value.
- W
The engineer
Well-Known Member
But what does the display function of that (and other instrumnts) indicate in your extensive experience and knowledge?
Lucky Duck
Well-Known Member
In fairness that was the very first result I came upon when I googled it. My experiance of sailing on a boat with high end Raymarine kit (Seaquest 36 - Circus) was that the instruments reported all three metrics in strength and direction.
Edit: Just found this link - http://www.panbo.com/WindVectors2d5.jpg which offers a definition of the various terms
Last edited:
D
Deleted member 36384
Guest
The users of ground wind are coming from the issue back to front. They should have used true wind as it is commonly understood and then introduced names for the other vectors that generate the apparent wind.
We wouldn't be having this conversation if they did as ultimately true wind direction is referenced to true north and all the rest are just conventions used for convenience.
We wouldn't be having this conversation if they did as ultimately true wind direction is referenced to true north and all the rest are just conventions used for convenience.
The engineer
Well-Known Member
So it must have a different value to true wind I would assume.
webcraft
Well-Known Member
I can't disagree with that. Most instruments show true wind referenced to the boat's heading as this is far more useful than indicating it as a compass direction. Raymarine have added a compass rose display for true wind 'because they can' - I doubt many use it regularly. If the GWD did show wind speed however it would indicate the exact same figure as the True Wind display.
This does not alter the fact that the true wind speed and direction is relative to a fixed point above the earth's surface, and is not altered by a tidal vector. Only apparent wind is altered by a boat under way being carried by the tide.
- W
The engineer
Well-Known Member
So would you agree Mr Craft, that Maretron's description is correct:
Ground wind is the speed and direction of the wind referenced to a compass rose, which is different than apparent and true wind which are referenced to the bow of the boat.
Ground wind is the speed and direction of the wind referenced to a compass rose, which is different than apparent and true wind which are referenced to the bow of the boat.
flaming
Well-Known Member
No, no, no and no. I'm sorry, but that is almost entirely incorrect.
GWD and GWS would NOT be the same speed as true wind speed and direction as reported by any of the instruments that are able to do so.
True wind speed, as reported by wind instruments since they have been able to do so, is calculated using the apparent wind as measured by the anemometer, and the speed as measured by the log. As such true wind - as defined by what wind instruments have measured since the 80s - IS NOT relative to a fixed point above the earth's surface - it is relative to the water.
Ground wind is a new feature to wind instruments (or instrument networks really) as it also uses heading and GPS data to calculate the wind relative to a fixed point on the earth's surface.
webcraft
Well-Known Member
I find the above very misleading, as the 'ground wind' speed and the 'true wind' speed will always be the same, whereas the above implies that there will be a difference in the speed. There isn't. The only difference is that one is referenced as a compass direction, one is referenced to the boat's heading.
To put it another way in case there is still misunderstanding . . . the speed value will not change, only the referenced direction. This value can be found by adding or subtracting the true wind angle to or from the ships' heading. All the GWD display on the ST70 is doing is automating this process.
Speed and absolute direction of either 'ground wind' or 'true wind' are identical when measured by an observer stationary over the earth's surface. Ground wind is a term created by instrument manufacturers.
I agree with Flaming that TWS as calculated by older instruments took the boat speed as measured by the mechanical log (speed through the water) and therefore could not take any tidal or current vector into account. As a result these instruments were not measuring True Wind Speed, but it was as close as they could get.
I believe that more modern instruments use the GPS to calculate TWS rather than the mechanical log, but it is possible that the ST70 in fact calculates a value known as true wind speed using the mechanical log. The reason I find this unlikely however is because the ST70 only displays ground wind direction, not speed. If it were measuring two separate values, one using the GPS and one using the mechanical log, then the ground wind speed and the true wind speed would also vary.
I now realise that what Flaming chooses to call 'ground wind speed' is what most of the rest of us call 'true wind speed'. The value Flaming refers to as 'true wind speed' is an approximation of true wind speed that does not take tide or current into account.
The fact remains that the definition of true wind speed is the wind over a stationary point, it is NOT anything to do with a boat under way but not making way. That is a misunderstanding brought about by the limitationsn of systems that take the boat speed from the mechanical log, not the GPS.
As we all have different (or no) instruments using one manufacturer's definition of a term is not helpful and we should stick to the definitions of trure and apparent wind as they ahve been accepted for centurie. True Wind is wind relative to a fixed point on the earth. Apparent wind is wind relative to a moving point.
There is no definition for true wind plus forward motion of the vessel but minus any tidal vector, although this is what the True Wind display on a system using the machanical log for calculation is showing.
- W
Last edited:
NealB
Well-Known Member
I think the instrument manufacturers have introduced 'new' definitions.
My old 'bibles' (Hiscock, Knox-Johnston, Proctor, Cunliffe), all just refer to true, and apparent, wind where 'true' is the wind felt when not moving (ie the meaning that Webcraft uses).
No mention of ground wind.
So...no surprise that there's an element of confusion when young upstarts like Flaming come along, changing definitions that have been used for generations (I hate smilies, but that's to be read with a smile!)
My old 'bibles' (Hiscock, Knox-Johnston, Proctor, Cunliffe), all just refer to true, and apparent, wind where 'true' is the wind felt when not moving (ie the meaning that Webcraft uses).
No mention of ground wind.
So...no surprise that there's an element of confusion when young upstarts like Flaming come along, changing definitions that have been used for generations (I hate smilies, but that's to be read with a smile!)
Last edited:
flaming
Well-Known Member
The problem is that wind instruments have for many years been outputting "true" wind which they calculate using the apparent wind, as measured by the annemometer, and the water speed as measured by the log.
So this has become the definition of true wind. Wind relative to the water.
So to define wind relative to the land - ground wind would seem to fit.
bbg
Well-Known Member
Depends what you want to do with the information.
True wind ANGLE is referenced to the boat's heading. Very useful for sailing deep downwind, as you don't want to get too close to 180 and risk an accidental gybe.
True wind DIRECTION will tell you where on the compass rose the wind is coming from. Very useful in recording as the wind backs or veers, as you might want to tack or gybe as soon as a predicted shift comes through.
I have to say that I hadn't heard of the distinction between true wind and ground wind until reading this thread (although I haven't been in top-level racing for maybe 15 years). I understand the difference that flaming makes, but outside areas with very high currents it probably makes very little difference to most sailors. In most cases the boat speed will be many times (possibly even an order of magnitude or more) the current, so I expect that most of the time the difference between TWS and GWS will be "lost in the noise".
flaming
Well-Known Member
It's not the relationship between boat speed and wind speed though. It's the relationship between wind speed and current speed.
It's a big deal racing in the solent in light winds.
For example, imagine a northerly (ground) wind of 7 knots. Enough to run racing.
When the tide is flooding east, the difference between the current speed inshore in Osbourne bay, and in the main channel can be 2 knots.
So the True (water relative) wind at the buoy in Osbourne bay is the result of a 7 knot wind, and a 1 knot current. So slightly east of North at about 8 knots.
When I get to the main channel I then meet the full force of the current and the true wind is now the product of that same 7 knot northerly and a 3 knot Easterly. So the true wind appears to shift further to the east and increase slightly. I've just been lifted on Starboard, and it was entirely predictable.
pyrojames
Well-Known Member
Which I think explains the confusion. Instrument manufacturers have been claiming to give "true wind" for years by taking the apparent wind and the boat speed to get "true wind" which was not what the general meteorological community considered the "true wind" and so with their "new" feature introduced ground wind, rather than admit they hadn't actually been measuring "true wind" for years.
I maintain that the racing community is a small, or very small community when compared to the rest of the sailing world, aircraft, navy, oceanographic research and climate research. Ground wind would have some relevance to pilots, on the basis of wind shear only and the change in direction/speed with altitude. True wind remains the wind at a geographical location relative to true north.
flaming
Well-Known Member
Just in case there is any misunderstanding this is incorrect.
Consider the extreme case.
I am anchored in a fast flowing river. I am feeling a 10 knot wind from the North. This is the ground wind.
I then break my cable and start drifting with the current. Let's say it's a 10 knot current, and it's running due East. I now feel a 10 knot easterly, in addition to that 10 knot Northerly. So that's going to be be a North Easterly at circa 17 knots.
My boat is stationary in the water. That's the "true" wind I'm feeling. And it is different in strength and direction to the ground wind.
NealB
Well-Known Member
I'd say true wind was 10 knot northerly, apparent wind is around N'Ely whilst I'm drifting, and will change as I start to move through the water.
bbg
Well-Known Member
Got it.
webcraft
Well-Known Member
I agree that this is how true wind speed on an instrument set up as you describe is calculated - indeed, I am just back from a 700 mile trip on a boat with a Raymarine system operating precisely as you describe. However, the instrument in that configuration is NOT measuring true wind speed as usually defined (unless there is no tidal or current vector). It is only an approximation of true wind speed. The value you refer to as 'ground wind speed' is what is normally called 'true wind speed'.
So - what is 'wrong' is your definition of true wind speed as the value measured by a boat instrument set up in a particular way. True wind was a concept understood and defined long before Seatalk was invented!
- W
D
Deleted member 36384
Guest
Your previous point to me about the manufacturers of wind instruments that calculate true wind from the speed log is a good reason why the term ground wind has been introduced.
Your example above though does prove that you are feeling the apparent wind on your drifting boat. However, the racing requirement to understand the vectors, now with the instrument makers term ground wind, is fair enough.
This may lead to unnecessary confusion though if a student of a racing school had been taught how to sail in a sea school. Its entirely manageable and if the student could not grasp the vectors and their implications for tactics due to terminology, then they are unlikely to be a good racing tactician anyway.
Good thread this,
The engineer
Well-Known Member
By jove I believe I have too 
What I used to call true wind in the above scenario would be calculated by knowing you were drifting. That calculation/gps info is now presented to me as ground wind. Very useful indeed. It does involve a bit of re thinking on the way one used to do things.
What I used to call true wind in the above scenario would be calculated by knowing you were drifting. That calculation/gps info is now presented to me as ground wind. Very useful indeed. It does involve a bit of re thinking on the way one used to do things.