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#### Missingyou

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I have Simrad NSS's. Until recently I wasn't sure how they determined True Wind. Were they using GPS SOG or the Boat Triducer Speed? Well it turns out after having a fouled paddle wheel on the Triducer that True Wind is derived using the Triducer. Is this the correct way? I would have assumed GPS SOG, since it's not affected by currents.

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The spinning cups at the tops of your mast spin with apparent wind. Your system solves for true wind by knowing your boat speed and direction.

#### svHyLyte

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I have Simrad NSS's. Until recently I wasn't sure how they determined True Wind. Were they using GPS SOG or the Boat Triducer Speed? Well it turns out after having a fouled paddle wheel on the Triducer that True Wind is derived using the Triducer. Is this the correct way? I would have assumed GPS SOG, since it's not affected by currents.
The classic definition of "True Wind", as used aboard, is relative to the yacht's speed through the water, if any. It is what the the sails "see/feel".

If the yacht's speed "through" the water is Zero; the wind from directly astern is two knots relative to the ground; and, the water is flowing at two knots in the same direction as the wind, the "True Wind" (so far as the yacht is concerned) is Zero. This is so as the "plane of reference" for the yacht is the water, not the ground below.

With the "True Wind" as calculated (as to speed and direction) one has the "apparent wind" relative to the ground and thereby, with the yachts COG and SOG can also compute the "Ground Wind" speed and direction in the same manner as the True Wind (relative to the water) is computed, a (relatively) simple vector calculation. While that might be useful for some, and particularly, perhaps, those racing, except under unusual circumstances the difference between the computed True Wind and Ground Wind will be relatively minor considering that most currents are generally quit modest except perhaps in smaller, relatively short, stretches.

FWIW...

#### Multihullgirl

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I have Raymarine, and it derives its true wind based on the paddlewheel readings. Thus, OP is correct, current does indeed skew the results.

Personally, I look round and make my own assessments, and only occasionally consult the electrical bits.

BTW...
True Wind: the wind relative to a fixed point the observation of which is not affected by the motion of the observer

Apparent Wind: The Apparent wind is the wind experienced by an observer in motion and is the relative velocity of the wind in relation to the observer. Apparent wind velocity is the vector sum of the true wind and the headwind an object would experience in still air.

#### tempest

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If the yacht's speed "through" the water is Zero; the wind from directly astern is two knots relative to the ground; and, the water is flowing at two knots in the same direction as the wind, the "True Wind" (so far as the yacht is concerned) is Zero. This is so as the "plane of reference" for the yacht is the water, not the ground below
SV In this example above, I have always considered the True wind to be 2 knots, and the Apparent wind to be zero.

I'm trying to understand the usefulness to racing sailors of the " ground wind" or what you seem to be calling the True wind now?

As I understand it...among other things, knowing the True wind relative to land is useful in determining the possible direction of an approaching weather system. ( relative to a position)

What am I missing?

#### Missingyou

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SV In this example above, I have always considered the True wind to be 2 knots, and the Apparent wind to be zero.
I agree. I did a little example featuring an extreme case of a 4.5kn current, say Hell Gate. There is quite a difference , especially with angle, more on that in a minute. I've always thought of true wind as that over the surface of the earth. .?? The one on the right seems more correct using SOG. However I agree with SV that in most cases it will be a minor difference.
[/URL]

I'm trying to understand the usefulness to racing sailors of the " ground wind" or what you seem to be calling the True wind now?

As I understand it...among other things, knowing the True wind relative to land is useful in determining the possible direction of an approaching weather system. ( relative to a position)

What am I missing?
With racing they are usually trying to maximize VMG (Velocity Made Good) either to the Mark or to the Wind, and not boat speed. This is usually done by referencing a Polar chart specific to the boat. Basically if you know True Wind Speed you can reference the chart to help you decide what True Wind Angle and hence the Point of Sail that will maximize VMG. I have it in my mind that SOG is the more correct way, but now that I start talking it through I think it's not that simple. Now I'm thinking the other method might yield a more accurate VMG. ??

I've been trying to develop a Polar Chart for my own boat so that's what got me thinking about how True Wind is calculated. I'd like to make a data logger that will allow me to go back and pull out the data needed to create a Polar Chart. I have too much time on my hands.

#### tempest

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Thanks MU,

Nice diagrams. I've done some reading on these distinctions, in this and other forums. Now I have to wrap my head around some of the math and it's relevance. The advent of instruments has added a level complexity and introduced new terminology. I guess I need to come out of the dark ages and get with it.

#### Gary M

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True wind is the natural wind and has nothing to do with the boat. Apparent wind is a vector addition of the true wind and the wind relative to the movement of the boat. i.e. the wind generated by the boat moving.

I race a lot and have fallen into the bad habit of having my instrument set for apparent wind. For instance I know which head sail to put up based on the apparent wind, at 16 knots I switch down to a #2 head sail. etc.

The true wind is much more useful to monitor for true wind shifts which of course are quite useful to a racer. You can watch it cycle a bit and see what it is doing and it is more obvious if it is making a permanent change.

If you watch apparent wind a I do, you will not see the shifts as clearly. For instance if beating and the true wind drops a few knots the "boat" wind will stay constant for a while until it starts to drop off. This will make the apparent wind go forward, a "velocity header" and it will seem that you are getting headed but in fact you are not.

Two wind speed displays would be very handy.

#### BarryL

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Hey,

Check your manual. My Garmin system can use GPS speed or paddlewheel speed to determine true wind. I have to tell it which one.

Barry

#### jackdale

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The GPS will give you your boat speed over ground, the knotmeter will give you speed through the water, the wind instruments will provide apparent wind angle and apparent winf speed. The fluxgate compass will show the vessel heading.

The CPU for your integrated system will use algorithms to calculate true wind speed and true wind angle using the dat that comes from all of the sources.

As matter preference, I set my wind instruments to apparent.

#### jackdale

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I sail with my instruments set to Apparent Wind. I dock with them set to True Wind.

#### svHyLyte

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I agree. I did a little example featuring an extreme case of a 4.5kn current, say Hell Gate. There is quite a difference , especially with angle, more on that in a minute. I've always thought of true wind as that over the surface of the earth. .?? The one on the right seems more correct using SOG. However I agree with SV that in most cases it will be a minor difference.
[/URL]

With racing they are usually trying to maximize VMG (Velocity Made Good) either to the Mark or to the Wind, and not boat speed. This is usually done by referencing a Polar chart specific to the boat. Basically if you know True Wind Speed you can reference the chart to help you decide what True Wind Angle and hence the Point of Sail that will maximize VMG. I have it in my mind that SOG is the more correct way, but now that I start talking it through I think it's not that simple. Now I'm thinking the other method might yield a more accurate VMG. ??

I've been trying to develop a Polar Chart for my own boat so that's what got me thinking about how True Wind is calculated. I'd like to make a data logger that will allow me to go back and pull out the data needed to create a Polar Chart. I have too much time on my hands.
FWIW "True Wind" on Polars is based up a water plane reference not the "bottom" under the water. Of what possible value would "Ground or Surface Wind" be to someone halfway between Newport and Bermuda or even Rodrigues Key and Cat or Gun Key when in the clutches of a 2-1/2 kn northeasterly flowing Gulf Stream when they're trying to optimize a course to steer which may but more often will not have much if anything to do with VMG per Polars? The "True WInd" reference is only relevant to the plane of reference upon which the yacht lives/depends, which is the water plane.

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