True wind from GPS?

[Cwens]

Originally Posted by team karst

I will have to look up the difference between true and ground wind.

"Ground wind" is what anybody other than sailors would call "True wind," i.e. the movement of air relative to the surface of the Earth (the actual fixed solid Earth, not the water). Ground wind is the wind vector as measured by instruments sticking up into the air, minus the vector of the observer's movement relative to Earth's frame of reference (i.e. as measured by GPS)

"True wind" as used by sailors, is the movement of air relative to the surface of the water. True wind is the wind vector as measured by instruments sticking up into the air, minus the movement of the observer through the water, as measured by instruments sticking down into the water.

For anything related to boat performance (for example sail trim), you want "true wind." for weather prediction you want "ground wind"

[Dockhead]

Originally Posted by team karst

I will have to look up the difference between true and ground wind. Never heard of it, but, I'm new at this. Fundamentally, apparent wind is easy to measure, and for the sail trim exercise, that and a paddlewheel are good enough. I rather disagree with the GPS vs boat velocity unknown. That is essential for accurate True wind calculation. GPS describes it accurately; what IS missing is the component of the Water Current Vector. And, the paddlewheel gives only an approximate solution. And for sail trim: is the current flow important info? For that matter, what value is true wind for sail trim?

Try this: https://raymarine.custhelp.com/app/a...calculate-them


The three main uses of True Wind are (1) For finding your layline. (2) Sailing near dead downwind, where apparent wind flops around, and especially if you are sailing an autopilot in wind mode. And (3) Knowing what point of sail you will be on when you alter course by x degrees.


You don't need True Wind for sail trim except when near DDW. For that apparent wind is all you need.



GPS does not describe the boat's motion in relation to the water -- it has no idea what the water is doing.

[Dockhead]

Originally Posted by ChrisOwens

What is the mechanism that causes a foul bottom to cause incorrect speed readings? Is it turbulence that causes the flow over the paddlewheel to no longer be representative of boatspeed?

Exactly. Boundary layer effect is very complicated, and one of the inherent problems with paddlewheel transducers, although if you are as diligent and intelligent as WingSail you can eventually calibrate them.



One of the supposed advantages of ultrasonic transducers is they measure the water flow beyond the boundary layer.

[Uricanejack]

I just look at the water and use the Beaufort scale and guesstimate.
I’m probably more accurate. Than any wind instrument I’ve ever used.

[s/v Jedi]

Originally Posted by Uricanejack

I just look at the water and use the Beaufort scale and guesstimate.
I’m probably more accurate. Than any wind instrument I’ve ever used.

You’re not but you can be equally accurate. Of course that stops when you switch from Beaufort scale to knots, at which point the instruments will beat you.

[Happ]

Originally Posted by Welti

I got aquestion in a quiz, basically asking if one had proper calibration and sufficient sea trials, if one could expect to get a reasonably accurate true wind from a GPS? I thought it would be "no" since the GPS only knows SOG, but I got it wrong. (?) What am I missing?

The answer is quite simple. The GPS / Chartplotter is acting as only a display. The Anemometer at the top of the mast is doing all the work.

[Andrew Lippman]

In order to calculate true wind, you need to know the apparent wind and angle and the boat speed and heading. Normally one would use the basic log for boat speed, in which case you get true wind with respect to the water, not the earth. That is to say, if there is a current, then you are moving over the earth rather than resting stationary. If you want to know the true wind that you would if standing on land, then you have to eliminate the current. You can use GPS speed and course (COG) to do that. Then you are referencing the Earth.

Which one you want depends on what you are doing. And whether you can reliably know the current. For example, if you are motoring at five knows with a five knot current with a ten knot wind behind you, the apparent wind will be zero, and the calculated true wind will be five knots.

Then if you reverse direction, you will be stationary but the measured boat speed will be five knots as will the calculated true wind.

If you do the same thing with GPS speed rather than log speed. you’ll get ten knots. Which is the true wind over the earth.

I think I got that right…

[MAC-A-TAC]

Gee, if I really need to know true and apparent wind it’s a simple two step process.

1 stop boat. Wet finger. Get a wind direction

2 move boat in desired direction. Wet finger. Get wind direction.

Please feel free to use this technology anytime.

[OneBoatman]

Assuming the question is true wind direction and not true wind speed. Your GPS doesn't need to be calibrated per se as its accuracy was set when it was manufactured. It can give a true position on an e-chart display if your boat isn't moving. Then you would need at least a tell-tale to show you where the wind is coming from to render the true wind direction. One problem is that you rarely see a steady wind direction until you are experiencing the second or third day of a high pressure system. If you were at sea you could by trial similarly reasonably accurately estimate the true wind speed provided your boat was capable of rivalling the speed of the wind. Many sailors today equipment their boats with what's called a relative wind indicator which has a vane and paddle incorporating transducers feeding to a display either independent or using the same display as your GPS/Chartplotter or both. The instruments are by user selection able to display both relative and true wind speed and direction by including or cancelling the contribution of the boats movement.

[Happ]

Originally Posted by Andrew Lippman

In order to calculate true wind, you need to know the apparent wind and angle and the boat speed and heading. Normally one would use the basic log for boat speed, in which case you get true wind with respect to the water, not the earth. That is to say, if there is a current, then you are moving over the earth rather than resting stationary. If you want to know the true wind that you would if standing on land, then you have to eliminate the current. You can use GPS speed and course (COG) to do that. Then you are referencing the Earth.

Which one you want depends on what you are doing. And whether you can reliably know the current. For example, if you are motoring at five knows with a five knot current with a ten knot wind behind you, the apparent wind will be zero, and the calculated true wind will be five knots.

Then if you reverse direction, you will be stationary but the measured boat speed will be five knots as will the calculated true wind.

If you do the same thing with GPS speed rather than log speed. you’ll get ten knots. Which is the true wind over the earth.

I think I got that right…

I think you are wrong in using ground speed in your calculation. My proof is you can buy an anemometer and a display that is not hooked into a chartplotter. Tacktick is a perfect example.

[MJH]

Originally Posted by Welti

I got aquestion in a quiz, basically asking if one had proper calibration and sufficient sea trials, if one could expect to get a reasonably accurate true wind from a GPS? I thought it would be "no" since the GPS only knows SOG, but I got it wrong. (?) What am I missing?

Without the specific wording of the question it is impossible to give any kind of accurate answer or opinion.

[wingssail]

Originally Posted by ChrisOwens

What is the mechanism that causes a foul bottom to cause incorrect speed readings? Is it turbulence that causes the flow over the paddlewheel to no longer be representative of boatspeed?

Yes, sort of. It is not the turbulence but the boundary layer which affects the paddle wheel.

The paddle wheel sticks down into the flow about 1/4 of an inch. When scum or growth is present on the bottom of the boat the boundary layer of water not flowing past the paddle wheel becomes thicker and reduces the pressure on the paddle. It spins more slowly. This is when you need to change the calibration to agree with GPS measured speeds.

A second factor which is more serious is the presence of "bush-like" tufts of seaweed which may grow near the paddle wheel and restrict its movement. These can often be seen if you pull out the transducer and hold a drinking glass over the hole so that you can see out the hole. You may see bits of seaweed around the hole. These sometimes can be cleared with a bottle brush through the hole if you can control the inflow of water while doing it.

Generally, if you have reached this point you need to dive on the boat and clean the area around the paddle wheel.

And, as always, pull the transducer in put in a plug or a cap on when you are not going to be using the boat for more than a week or so. This does not clear the growth on the bottom but prevents growth and barnacles on the paddle itself.

[Sailor_Jeff]

Originally Posted by Welti

I got aquestion in a quiz, basically asking if one had proper calibration and sufficient sea trials, if one could expect to get a reasonably accurate true wind from a GPS? I thought it would be "no" since the GPS only knows SOG, but I got it wrong. (?) What am I missing?

Yes, uper end expensive GPS units can. Mine does. The GPS gives you speed and course over "ground" (or water) "IF" your gps has NMEA inputs for wind speed and direction sensors it's computer can calculate true wind speed by subtracting wind sensor data from speed data and displaying it. My ship does that. I have ultrasonic sensors for wind speed and direction. It's all math at that point.

but it's an expensive GPS and sensors plus integration costs are high, day or "trailer-sailors" can't usually afford it.

Jeff

[Cwens]

Originally Posted by Sailor_Jeff

Yes, uper end expensive GPS units can. Mine does. The GPS gives you speed and course over "ground" (or water) "IF" your gps has NMEA inputs for wind speed and direction sensors it's computer can calculate true wind speed by subtracting wind sensor data from speed data and displaying it. My ship does that. I have ultrasonic sensors for wind speed and direction. It's all math at that point.

but it's an expensive GPS and sensors plus integration costs are high, day or "trailer-sailors" can't usually afford it.

Jeff

What you say is true, but the quiz question doesn't ask, "Can you get true wind from a GPS plus other sensors?" it asks, "Can you get true wind from a GPS?" It's a sloppily worded question, but if you take it as written, the correct answer is "no". Not to mention that, as commonly used in the marine world, "True wind" refers to wind speed relative to the water, not relative to the ground, and the GPS has no idea what the water is doing.

[JimJohnston]

In answer to the initial question "if one could expect to get a reasonably accurate true wind from a GPS?", the answer is 'yes' if one is using a Raymarine Axiom multi function display with a connection to the internet and a subscription to Windy.com, both of which are supported by the Axiom.

Alegedly the Axiom MFD will allow a user to provide an overlay screen from a connected website, like Windy.com which provides real time wind conditions with a fairly reasonable resolution. So the 'GPS' display will provide 'accurate true wind'. I haven't tried it yet but it's in the plan.