Tobbera:
So what you are trying to say is that the PIDInput might be affected by the sail, even though is in front if it. Thats fine, I understand that. We are not talking about telltales and how they flow with the wind.
This isn't really what I am trying to say, and sorry if I have not been clear enough. What I have been trying to say is that your wind vane will experience significant short term variations in direction that will (a) be difficult to manage with a PID algorithm and (b) will be difficult to convert to effective rudder position data. The influence of the sail on the telltales makes them more effective guides to what is happening - but they don't tell you the free-air wind direction. Maybe another way to think of it is that you get useful data from the telltales that you would not get from a wind vane. Try installing the wind vane and disabling the tell tales and see how you get on just reacting to the wind vane.
I suspect it's not the average wind direction that is important so much as the average direction of the wind energy which could be very different in gusty conditions.
Another factor that occurred to me last night is that the force you feel on the tiller is also a vitally important element of steering a sailboat.
...R