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post #17 of Old 02-26-2019 Thread Starter
JoCoSailor
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Re: ambient wind vs true wind

Ok to confuse things more or maybe clarify (it's the offseason right) I found text that led to my question. The term there was not ambient wind it was ground wind.

It's from NauticEd Sailing School - Learn to Sail and Get Certified the course is, "learn electronic-navigation"

If anyone is interested here's the text:

"More about True Wind

By definition, the True Wind is the wind you feel when you are stationary with the water. Note that in this case the apparent wind equals the true wind.

Thus, we must define the difference between Ground Wind and True Wind. But wait, are they not the same? Well no! Think of this: In a current flowing at 5 knots, you stop the boat and drift perfectly with the current while measuring the wind speed. Since boat speed is zero, True Wind must equal Apparent Wind right? Right!

Now hop off the drifting boat and stand on a rock. The wind you feel will change. Since you are stationary now on the ground and with satellites, this is called the Ground Wind and you can see it is potentially significantly different from True Wind. But that’s not really a problem because since the sailboat operates in water albeit moving, the boat is not too concerned with Ground Wind. It’s almost like acknowledging that the Earth is spinning and moving through the solar system, the boat is not concerned with that at all.

The above discussion helped us understand that True Wind is the wind the boat feels when stationary with the water.

You probably had not considered that before, but it now makes sense. If we were operating in a high tidal current situation, by definition, when we tack the boat on moving water, the true wind must stay a consistent direction throughout all of the directions that the boat will head.

When sailing, you are more interested in the effect the wind has on the boat rather than measurements from a purely meteorological point of view, so True Wind is the preferred choice over Ground Wind.

However, racing navigators constantly convert between True Wind and ground wind to determine the accuracy of weather forecasts which use Ground Wind. This would be especially pertinent when navigating the Gulf stream off of the east coast of the USA or the EAC (East Australian Current)."

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