True wind from GPS?

[FlyingScot]

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?

Thinking outside the box here. If the sufficient sea trials taught the sailor where the currents were, or maybe no currents at all. Couldn't you just steer the boat until the apparent wind became zero (direction and speed)? Look at GPS data and you have the answer, no?

[valhalla360]

I think this is the same debate as engineer vs physicist.
- A physicist will argue over the technicalities of how quarks interact with muons in a concrete made with iron rich aggregate.
- The engineer will tell you to build the bridge with certain specifications and it won't fall down.

No, you can't produce technically correct results without STW.
Yes, in most conditions, you can use GPS speed and course data and get a pretty good estimate (with standard wind instruments). Certainly good enough for your average cruiser.

Since most folks don't have the mythical ability to keep a paddle wheel operating accurately for any length of time and GPS does it by default, you are trading off two error sources For most, that's makes using GPS data the better option.

If you are racing and an extra 0.005kts makes a difference, it may be worth fiddling with the paddle wheel.

[El Pinguino]

Originally Posted by valhalla360

I think this is the same debate as engineer vs physicist.
- A physicist will argue over the technicalities of how quarks interact with muons in a concrete made with iron rich aggregate.
- The engineer will tell you to build the bridge with certain specifications and it won't fall down.

No, you can't produce technically correct results without STW.
Yes, in most conditions, you can use GPS speed and course data and get a pretty good estimate (with standard wind instruments). Certainly good enough for your average cruiser.

Since most folks don't have the mythical ability to keep a paddle wheel operating accurately for any length of time and GPS does it by default, you are trading off two error sources For most, that's makes using GPS data the better option.

If you are racing and an extra 0.005kts makes a difference, it may be worth fiddling with the paddle wheel.

Your accurate speed through the water is of no consequence if you are racing. All that matters is your speed relative to his speed. Is his bearing opening or closing?

[dady]

Originally Posted by Jim Cate

Umm... the GPS has zero knowledge of wind of any sort, so on its own (that is, without an input of apparent wind) it surely can not output true wind.

Jim

Actually, I think, the real wind can be deduced, knowing the apparent wind with the speed, that is to say that a GPS will not provide this value directly or the inverse one deduced, but it will give us the speed, then the question may be incomplete, even if it were not issue.
Regards

[valhalla360]

Originally Posted by El Pinguino

Your accurate speed through the water is of no consequence if you are racing. All that matters is your speed relative to his speed. Is his bearing opening or closing?

Similar could be said of auto racing. Who cares about compression ratios, turbos, downforce, tire types, etc....if you go faster than the other guy, that's all that matters.

Equally silly statements.

[s/v Jedi]

Racers have much different instrumentation and it starts with a computer that has a very accurate polar diagram of the boat. It can simply show a percentage of how you do compared to the computer model. If it’s less than 100% you curse at the crew to do better

[Dockhead]

Originally Posted by El Pinguino

Your accurate speed through the water is of no consequence if you are racing. All that matters is your speed relative to his speed. Is his bearing opening or closing?

Well, no. You need accurate True Wind for tactics. Races are won and lost based on the quality of tactical decisions as much as anything else -- when to tack; can you get past that mark on this tack; etc.

[Dockhead]

Originally Posted by Lodesman

Is that commonly used in the "marine world" or just the "sailing world"? I can't see how the difference between ground and true wind as described, matters to motorboaters?

I think you're right that motorboaters don't care, but the instrument makers -- who make instruments for sailboats, mobos, and ships -- do strictly distinguish between Ground Wind and True Wind.

[Dockhead]

Originally Posted by s/v Jedi

Racers have much different instrumentation and it starts with a computer that has a very accurate polar diagram of the boat. It can simply show a percentage of how you do compared to the computer model. If it’s less than 100% you curse at the crew to do better

Indeed.



I have an H5000 Hercules on my boat, and we put the same thing on my friend's D67 which we sailed across the Atlantic last year. But % of target speed is not the only useful function -- it also does very fine multipoint calibrations of STW and Apparent Wind data.


The only bad thing is that this device requires motion data from B&G's proprietary motion sensor, which costs a boat buck. The motion data from the satellite compass is about an order of magnitude better, but the computer won't take it

[Lodesman]

Originally Posted by Dockhead

I think you're right that motorboaters don't care, but the instrument makers -- who make instruments for sailboats, mobos, and ships -- do strictly distinguish between Ground Wind and True Wind.

I doubt many mobos have wind instruments, but that's not to say there aren't occasions that a moboer would like to have a "reasonably accurate" true wind - anchoring, spreading ashes, etc., etc. It really goes to the mindset of the test-writer, but with a handheld wind chummy and SOG/COG from GPS, I think the average moboer would figure they could get a reasonable "true" wind.

[Dockhead]

Originally Posted by Lodesman

I doubt many mobos have wind instruments, but that's not to say there aren't occasions that a moboer would like to have a "reasonably accurate" true wind - anchoring, spreading ashes, etc., etc. It really goes to the mindset of the test-writer, but with a handheld wind chummy and SOG/COG from GPS, I think the average moboer would figure they could get a reasonable "true" wind.

All the bigger mobos I've spent time on had wind instruments, usually el cheapo Raymarine ones which looked out of place to my eye, but I guess it's all they need. The way they get blown around with their huge windage and tiny rudders, it's hard for me to imagine doing without a wind instrument.

I agree that Ground Wind is all any moboer would ever care about. You need accurate True Wind for sailing.

Moboers DO care about STW, however, at least on bigger ones. I believe -- I guess, anyway -- that is because they need to understand what they are actually getting out of their engines. One bigger motor yacht I spent some time on actually had a doppler speed log like what ships use. Wow, I wish someone would figure out how to miniaturize those so that we could use them too.

[OneBoatman]

I believe it is the case for mariners that unlike how we use True when we are navigating and or determining our position where it is convention to use the "true" bearing from the North pole as opposed to using a magnetic bearing, when we say what is the True wind absent any qualification, we usually mean what is its velocity if measured from a point that is not moving. This I would say is its meaning whether on land or at sea. But at sea we say the wind is out of the SW at 12 Knots. Most would agree that we mean the wind is 12 Knots coming from a magnetic bearing of 270 Degrees because we steer our course by compass. Yet we all trim our sails relative to the wind and compensate for drift relative to our course. However, a GPS is displaying your "true" position as reflected at any moment which is the position of the receiving antenna which we have defined by convention, a specific place on Earth, using Latitude and Longitude.

[Lodesman]

Originally Posted by Dockhead

All the bigger mobos I've spent time on had wind instruments, usually el cheapo Raymarine ones which looked out of place to my eye, but I guess it's all they need. The way they get blown around with their huge windage and tiny rudders, it's hard for me to imagine doing without a wind instrument.

We clearly sail in different circles. Never seen them on small mobo's. We had anemometers on the big ships, as needed to know "true" wind for flight ops, calculation in the firing of some weapons, which way to turn for a MOB, Wx-obs, etc. Funny thing, is that what we called "true wind" we considered to be geographically-referenced, but as we used gyro-heading and doppler log, it equated to the water-referenced "true wind" that you describe. Nowadays, an OOW could just as easily take the ship's data from the INS and give a ground wind as "true". 99% of the time it wouldn't make a difference for our purposes, as ocean currents are rarely a large vector in the equation.

In all smaller vessels, we used a handheld anemometer and our hand as a wind-vane (for direction).

Where an assessment of wind was required, there were always other indicators - ocean buoy reports, forecasts, flags on shore, etc, etc.

[Jim Cate]

Originally Posted by dady

Actually, I think, the real wind can be deduced, knowing the apparent wind with the speed, that is to say that a GPS will not provide this value directly or the inverse one deduced, but it will give us the speed, then the question may be incomplete, even if it were not issue.
Regards

Dady, with due respect a GPS knows nothing about the wind... nothing at all, for it is a position and speed reporting instrument with no anemometer function. Other instruments can be linked to many systems and they can introduce all the info needed to calculate the various species of wind speed and direction, but the GPS on its own (as in the test question the OP reported) can't do it.

An experienced sailor can use his own senses to add info to the GPS output and make reasonable estimations of the wind speed and direction, but that ain't what the question was about.

Jim

[s/v Jedi]

I wonder who in this thread knows their leeway and wind-set… using that and a tide table can plan a passage like across the English channel. If you don’t know that True Wind is water referenced then I have a hard time believing one can do that because everyone doing these courses knows that.