I think it depends how powered up you are at any particular time. The more powered, the deeper you can go.
But if you mean, ultimately it's probably as hardie says.
There is no unique number but 135 is a good place to start. The fastest sailing angle for a windsurfer is a function of the strength of the wind, the weight of the sailor, the size of the sail, the overall lift and drag coefficients and the sea state. In general, the higher the wind, the deeper you want to go.
Quick answer: The angle at which you go fastest in the prevailing conditions.
Another quick answer: Usually broader than you think! Watch your GPS as you bear away.
Yet another quick answer: Somewhere between 115 and 135 on a windsurfer, depending on a whole lot of things as noted in previous relies..
There is a function in GPS Results called Polar Speed Diagram that will be of great help if you are interested to know. It shows the angle of your maximum speed with respect to the wind angle. Look through your past sessions and you'll have a pretty good idea - you will see the pattern emerging. (just check if GPSResults worked out the wind angle correctly - the line representing wind should run through the void in between your runs - if not just adjust it manually to be smack in the middle of that void).
Below is an example of what I'm talking about.
This is a screen dump of GPSR showing the session max speed of 36.33knots in the direction of 330.2 degrees (see the bottom track highlighted in blue). The wind direction is 190 degrees (see in the popup window). So the sailing angle with respect to wind = 330.2 - 190 = 140.2 degrees.
In addition it tells you that to achieve this speed at that sort of angle the wind must have been at least 27.9 knots (see popup window).
Another conclusion - if next time I go out and it's only blowing 22knots then to match this speed I need to be more efficient and run at 127 degrees to wind, if I run at 140 I have no hope - how many people try to beat their times in weaker wind and sailing at exactly the same angle? - it just won't happen! (n.b. to get these numbers play with the wind angle in the popup window). Mind you at 127 there may be too much friction/resistance and I still won't match it, but at least I have a theoretical chance, otherwise I don't!
I agree with the Boss above, GPS results is great for analysing the speeds as you bear off with the polar diagram. Had my top speed at 125deg, but only started on the journey recently.
