Wind speed: relative to water or ground ?

[Dave_S]

This might have a seemingly obvious answer but I've asked worse questions 🙂

When your instruments say TWS & TWD = #, X, Y is that relative to the water or the ground ?

[ItDepends]

It's not a silly question at all...

For systems set up as most serious sailors would want them, TWD and TWS are relative to the surface of the WATER.

If you want wind relative the ground, look for GWS and GWD. Most systems have that data calculated as well.

This assumes your instrument system is set up with a proper water speed measurement.

It's not often it makes a big difference, but it can when sailing in strong currents and light winds.

[noelex 77]

True wind should be relative to water, but on some instruments it is possible to change true wind to ground wind.

If this is done many/most instruments will still display the information as true wind. I am not sure it matters. If you ask most sailors to define true wind they will give you a definition of ground wind not true wind .

This topic is hotly debated on the forum, but personally I prefer to display apparent wind and ground wind (for a cruising yacht) primarily because it eliminates any errors in my paddlewheel affecting the results.

[ItDepends]

Originally Posted by noelex 77

True wind should be relative to water, but on some instruments it is possible to change true wind to ground wind.

If this is done many/most instruments will still display the information as true wind. I am not sure it matters. If you ask most sailors to define true wind they will give you a definition of ground wind not true wind .

This topic is hotly debated on the forum, but personally I prefer ground wind (for a cruising yacht) primarily because it eliminates any errors in my paddlewheel affecting the results.

There is a downside to using GPS as your primary speed reference in most instrument systems. It means that you lose the ability to calculate current. For some places that might not matter much, but in others it can be a big deal.

When I am navigating in and around the Gulf Stream, (with speeds of up to 4.5 knots) I can not imagine making decent progress without knowing the actual speed of the current I am sailing in.

[noelex 77]

Originally Posted by ItDepends

There is a downside to using GPS as your primary speed reference in most instrument systems. It means that you lose the ability to calculate current. For some places that might not matter much, but in others it can be a big deal.

When I am navigating in and around the Gulf Stream, (with speeds of up to 4.5 knots) I can not imagine making decent progress without knowing the actual speed of the current I am sailing in.

Agreed. I am not advocating eliminating STW. Current information is vital in many areas. Ground wind can be chosen (if this is what you prefer) without disregarding STW or failing to calculate current data.

[Dockhead]

You've gotten good answers. But to reinforce the point -- True Wind for sailors is referenced to the WATER. Not indeed to the ground.



That's not an accident. We sail in the sheer between water and air. The ground is not relevant to sailing performance, as we sail in water.


Lack of good STW data makes it impossible to calculate true wind data which is useful for sailing.

[StuM]

It's historical.



Without a means of measuring speed and COG (i.e. GPS), the only one you can easily derive is relative to water. And with strong currents, that is the important one when considering a change of heading.

[Dockhead]

Originally Posted by StuM

It's historical.

Without a means of measuring speed and COG (i.e. GPS), the only one you can easily derive is relative to water. And with strong currents, that is the important one when considering a change of heading.

It's certainly historical, but it persists because it's the true wind which is relevant to sailing.


We have three kinds of wind:


Apparent Wind -- measured directly
True Wind -- Apparent Wind corrected by HDG and STW
Ground Wind -- Apparent Wind corrected by COG and SOG


That's also the way the instrument makers give us the data.


Each kind of wind data has its own use.


Apparent Wind -- for sail trimming, and autopilot uses this in wind mode when wind is forward of the beam.

True Wind -- for tactics, laylines, etc. And autopilot uses this in wind mode when wind is aft of the beam.
Ground Wind -- for predicting changes to True Wind when the tide changes.


Ground Wind is not a substitute for True Wind. You won't get a valid layline from Ground Wind, and your autopilot will sail dangerously wrong if it uses Ground Wind in wind mode.


That's why you need valid STW and accurate HDG. STW is the hardest thing to measure accurately, which is why so many sailors are tempted to substitute SOG. I've been through a bunch of different speed log transducers over the years trying to find the holy grail, and haven't found it yet. I even bought an H5000 Hercules this year for better calibration of the speed log.





All that being said, it's actually funny that I have just sailed several thousand miles with no wind data at all! The guy I hired to replace my wind instrument in June before my summer cruise didn't show up, and I never had time during the cruise to go to the top of the mast myself and deal with it. So we sailed all summer with the Windex and wind in the face. Back to basics -- my first boat, when I was a teenager, had nothing but a Windex. It was actually refreshing in an odd way -- I would say better than having inaccurate wind data, because you are not distracted by the bad data from concentrating on feeling the wind directly.

[Dockhead]

Originally Posted by noelex 77

Agreed. I am not advocating eliminating STW. Current information is vital in many areas. Ground wind can be chosen (if this is what you prefer) without disregarding STW or failing to calculate current data.

It's not just calculating current you need True Wind for -- you cannot calculate a valid layline without it, even in the slightest current. Without valid True Wind, your autopilot can't steer in wind mode with wind aft of the beam, nor can you hand steer safely near dead downwind. Ground Wind is really no substitute.


Note that modern racers don't even really use Apparent Wind much. They put an enormous amount of effort into calculating valid True Wind and they sail to that on most points of sail.



Ground Wind is really no substitute. And in any case, Ground Wind should be expressed in compass direction (as it normally is with our instrument systems), not angle from the bow, where it is profoundly misleading.

[Franziska]

We have a broken paddle wheel sensor at the moment (the paddle wheel is fine, seems to be the sensor or cable).
We have hardly any current here.

I set the wind instrument to use the GPS speed to calculate true wind.
Not for laylines or the like, but just as a helpful indicator/alarm for reefing.
On a catamaran an instrument showing true wind speed is really helpful in this respect.

It's also nice to see windspeed and angle at anchor over time.
Navmon PC has some nice graphics for this.

[noelex 77]

Originally Posted by Franziska

I set the wind instrument to use the GPS speed to calculate true wind.
Not for laylines or the like, but just as a helpful indicator/alarm for reefing.
.

If there is no current true wind=ground wind, with the added advantage that GPS speed is accurate and reliable without any calibration errors on different tacks, with heel etc.

After the erratic results in true wind that can seen on a typical cruising boat (due to paddlewheel error) it is a revalation.

Be aware that apparent wind, true wind and ground wind are different measurements. There are situations where each of these options are preferable. I disagree with some that ground wind is not useful sailing. In some situations it is the best option especially when combined with apparent wind.

Everyone should at least keep this an option in mind and also know how to switch over the display (if your instruments allow this) for situations where the paddlewheel becomes fouled mid passage and it not convenient to remedy the situation.

[Dockhead]

Originally Posted by noelex 77

If there is no current true wind=ground wind, with the added advantage that GPS speed is accurate and reliable without any calibration errors on different tacks, with heel etc.

After the erratic results in true wind that can seen on a typical cruising boat (due to paddlewheel error) it is a revalation.

Be aware that apparent wind, true wind and ground wind are different measurements. There are situations where each of these options are preferable. I disagree with some that ground wind is not useful sailing. In some situations it is the best option especially when combined with apparent wind.

Everyone should at least keep this an option in mind and also know how to switch over the display (if your instruments allow this) for situations where the paddlewheel becomes fouled mid passage and it not convenient to remedy the situation.

There is a point when your STW data is so poor, that you will be better off with Ground Wind, since your True Wind is garbage. Garbage in, garbage out, as the engineers say. So yes -- in an emergency, in an area with little current, I agree it's good to have this in your bag of tricks.

But that doesn't mean that one should just settle for Ground Wind, which even with a very slight current will deviate a lot from True Wind.

If you intend to use the wind instrument at all (as opposed to just using the Windex), you should work to get at least somewhat decent STW data. I admit it's not easy.

I think the best low budget approach is to use a good paddlewheel sensor and clean it often, and calibrate it.

I had an ultrasonic CS4500 transducer for some years which was fairly decent, if no where near perfect. Advantage of this kind of transducer is that it reads the water some distance from the hull, so gets beyond some of the boundary layer effects. As a result it's much less sensitive to heeling. This was accurate enough to give reasonable True Wind even with only single point calibration.

After that died, I went to a UST850 which I found worse than the CS4500, although the electronics are supposedly the same. Don't know why.

Now I have a DX900+, which works on a different principle -- it's electromagnetic instead of ultrasonic. I'll report on it when I get it commissioned. I don't yet know whether it's any good or not. One cool thing about it is that it reads not only speed, but also direction, so you get directly sensed leeway, instead of calculated, which ought to be extremely useful.

For racing, whatever kind of transducer you use, you need multipoint calibration of the transducer data, including correction for heel. There are a few ways to do that; I opted for an H5000 Hercules racing computer. This calibration can also be done inside Expedition, and I bet our OpenCPN wizards could be persuaded to do a plug-in for O which would do it -- I don't think it's mathematically complicated.