Pascalou31: @Robin2 : You are right, I'm talking about racing boat and it should be better to find a solution which doesn't reduce boat speed in both racing and practicing mode.
I have no idea how you could measure speed, never mind mind measure it accurately, for such a small vessel.
Another significant issue is, if you do have a speed measuring device, how can you verify that it reads correctly.
It just occurred to me that a gyro/acclerometer might be able to let you know if the boat is speeding up or slowing down - in other words, if your control inputs are useful. However I suspect a sailboat would be subject to a huge amount of "noise" due to the effects of varying wind speed, angle of heel and water roughness (I don't know if there are currents in the water where you sail, but that would be an added complication). I suspect it would be very difficult (perhaps impossible) to filter out those effects to get usable data.
PS ... I'm assuming that timing the boat between two fixed buoys is not what you want.