I'm going to have to go ahead and "disagree". There are only two types of windspeed: "True" and "apparent". Whatever you call them. True windspeed is the speed over the surface of the earth and Apparent windspeed is the speed relative to the nose on your face as you go zooming along in your boat. Really, we should be saying wind velocity but we will keep it simple. Your wind gizmo has two settings True and Apparent. Both the windspeed and direction will be different as you toggle between the two settings unless you are stationary. If you are moving the computer will take into account your velocity over planet Earth to extrapolate the True windspeed. Your speed through the water has nothing to do with it as the computer cannot know the direction or speed of the current. That is it. There is no grey area here.Well, that argument has been made and rejected by sailors and I understand the reason why (not saying you're not a sailor! ). After reading the article that @DrJudyB provided, it's clear that the sailor definition of "True Wind" is in relation to the boat's movement through water and it is distinct from "Ground Wind", which is what you are defining as true wind. I accept the definitions as they are accepted by sailors, that there is a distinction between True Wind and Ground wind. Afterall, it's just semantics and I'll recognize the distinction.
Chartplotters can be set to use either GPS or speed through water as the input to calculate TWS. If using the speed through water the chart plotter can calculate the speed of the current by comparing STW and heading with SOG and COG. I understand you don’t want to call the wind speed over the water the True Wind Speed, but it absolutely is its own value and can be calculated with the right inputs.If you are moving the computer will take into account your velocity over planet Earth to extrapolate the True windspeed. Your speed through the water has nothing to do with it as the computer cannot know the direction or speed of the current. That is it. There is no grey area here.
How would you measure True Wind?There are only two types of windspeed: "True" and "apparent". Whatever you call them.
Will, you're way over thinking this. There is wind it is the same over water or over ground. It does change direction and speed as it passes over the ground or water and it is different at deck level and 50' up a mast, however, where ever you are if you are stationary with respect to the earth, it is the true wind. Apparent wind is what you feel as you move through the air. Doesn't matter if you are on a bicycle, walking, sailing, driving a car, the wind you feel as you move over the earth is the apparent wind.How would you measure True Wind?
You're standing in the cockpit of your boat, you have a radio, a compass, a wind vane, an anemometer, a knotmeter, a GPS and a chart with tide tables. How do you know what "True Wind" is, if it is velocity over ground? I'm not arguing for one one definition over another. Over ground is how I've always understood it. It's just that that is less useful than wind over water, to me.
To get "True Wind" (over ground), you could anchor and measure it, but that's just where you are at that moment. You could get it from a current weather report, but that's old data and an approximation from a distant location. The only other way, out there on the water, is to measure your progress over ground and compare it to your apparent wind. That would be the most accurate. Your progress over ground includes current drift and side slip as well as forward progress. A GPS will help with that, but what's the point? Your boat sails as she does and you, her owner, understands this. You know what conditions, wind speed and direction at what point of sail produce what effects. This, however is wind over water.
With a keel buried deep in the water, your boat is half in two mediums and neither one cares about the ground except where a stationary destination exists. For sail performance, wind speed and direction over ground are meaningless. For a thought exercise and trip planning, it is a worth while consideration. Still, there is no such thing as a set-and-forget scenario in sailing.
-Will (Dragonfly)
The movement of the air doesn’t change, but its movement is different relative to three things: (1) a moving boat; (2) the ground; (3) water that’s moving because of a current. The question of which one to measure is at the heart of the issues above with the definition of “true wind”. As Will says, the boat sails through the water, so the movement of the air relative to that water is what matters in measuring the performance of the boat. Whether that movement is “true wind” or some other term is something I’ll leave to linguists.There is wind it is the same over water or over ground. It does change direction and speed as it passes over the ground or water
That is the definition of apparent wind.s Will says, the boat sails through the water, so the movement of the air relative to that water is what matters in measuring the performance of the boat.
No, apparent wind is the wind relative to the boat. (And yes, I agree that matters a lot for sail trim and reefing). I’m talking about wind relative to the water. That’s what I’d use to measure performance with things like Polar charts. And that may be different from wind relative to ground, if there is current.That is the definition of apparent wind.
Oh!?!? I thought that was the point of this thread, to over think this issue.Will, you're way over thinking this.
Simple. the boat has an anemometer that measures the apparent windspeed and direction and it has a GPS that measures the boat's speed and direction over ground. Through the miracle of linear algebra your computer is able to subtract the boat's speed and direction to give you the windspeed as if you were standing still. If you have a really fancy pants computer you can add in the boat's speed through the water measured at the paddle wheel and heading (GPS) and it will tell you the current's speed and direction as well. Again, this is just math.How would you measure True Wind?
You're standing in the cockpit of your boat, you have a radio, a compass, a wind vane, an anemometer, a knotmeter, a GPS and a chart with tide tables. How do you know what "True Wind" is, if it is velocity over ground? I'm not arguing for one one definition over another. Over ground is how I've always understood it. It's just that that is less useful than wind over water, to me.
-Will (Dragonfly)
Thanks, Mark. That and, just as importantly, the definitions of the terms.Again, this is just math.
But @Stu Jackson if we just accepted that article on face value what we have done all day while sitting at home socially distancing? Huh?Thanks, Mark. That and, just as importantly, the definitions of the terms.
So, it's just math and English!
i.e., in Scott's OP, his link to Panbo, Figure 2 DEFINES ground and true wind in the vector diagram.
So couldn't you stop more often if knowing the true wind is that important and it is constantly changing? You're not racing, so you can come to a complete stop, take your readings, and then continue, whenever you like. Since apparent wind is what we sail in, why does knowing the true wind - which if you're moving is going to be dependent upon calculations from instruments that may or may not be too precise - make any difference?No, because the wind is constantly changing in direction and speed.
Basically, I opened this post because I was intrigued by the Panbo diagram that ties the 3 relative wind components together with vectors. My premise was that if I assume that the paddlewheel isn't accurate, how can you arrive at the True Wind component? I figured that the instruments should be able to calculate the true wind component with apparent wind, ground wind and current set and drift, instead of inputing the paddlewheel speed. I am interested in this because by adding a heading sensor to the network, the current set and drift is accessible. But I'm not sure how the chartplotter knows set and drift (it is displayed when the heading sensor is active). Does the chartplotter know set and drift from data input or does it measure it while you are moving across the water? I'm guessing that if it is measured while moving across the water, it must rely upon the accuracy of the paddlewheel speed somehow. So now I am going full circle and back to relying on an accurate paddlewheel. @thinwater you really know how to upset the apple cart! Now I'm supposed to distrust my wind sensor?The movement of the air doesn’t change, but its movement is different relative to three things: (1) a moving boat; (2) the ground; (3) water that’s moving because of a current. The question of which one to measure is at the heart of the issues above with the definition of “true wind”. As Will says, the boat sails through the water, so the movement of the air relative to that water is what matters in measuring the performance of the boat. Whether that movement is “true wind” or some other term is something I’ll leave to linguists.