The only one that really matters when sailing is apparent, you trim the sails to it and you feel the pressure of it and have to reef accordingly.
Going downwind apparent wind is less than true and upwind apparent wind is more than true as if you were driving in a car and tucked your hand out of the window. On a reach the faster you go the more it moves forward of the beam.
Using your burgee or windex Apparent is what you see them respond to.
Yes .. and also applies the wind direction as well to derive the true wind angle from the apparent wind angle. If the log reads zero then the true and apparent wind on the ST instruments will be the same
There was quite a long discussion on this a year or so ago, in that my older GPS (AP Mk6)and my Navman Wind instrument worked out true and apparent wind speed and angle using the log speed.
The discussion centre'd on what would happen if you were standing still (0 knots SOG) in the Alderney Race (for example) while the log said you were doing 6 knots (example), with, say, a close reach wind. Apparent and true wind angle and speed would actually be the same, while the GPS or wind instrument would say they were different.
I would be of the opinion that the speed should be by GPS SOG to be more accurate, but neither instrument had the facililty to allow a SOG input from the GPS.
Yes thats got to be right AFAIK there are no computing gizmos or meaningful data inputs between wind sensing unit and reading head,so the out put of sensor must be observed (apparent) with corrections to true with input from compass and log which are displayed as such if you so wish.
For practical purposes apparent wind speed and direction is the most useful.
The wind instrument senses and displays the apparent wind and can, if connected to the log, calculate the true wind speed and direction. There's no way that an instrument on the boat can directly sense true wind speed and direction.