A rotating propeller creates a forward lift independent of the surrounding wind direction and wind speed (under typical conditions).
At the beginning the vehicle is pushed forward by the force of the wind where the propeller acts as simple sail (no blade's wing effect, no additional propeller lift, the max achievable vehicle speed is equal the tail wind speed).
The moving mass of the vehicle incl. inertia of the propeller and mechanically coupled wheels creates a storage of the energy compensating fluctuations of the wind speed/direction and by the propeller induced lift (ie. like a filter capacitor in our electronics).
At a specific vehicle/blades speed the propeller blades start to act as the wings (as they are designed that way) and start to create the lift.
The by the propeller created lift/speed adds up to the speed of the wind.
With already rotating propeller (the blades create lift):
with the wind coming from front: v_vehicle = j*v_vehicle + k*prop_lift - v_wind
with the wind coming from back : v_vehicle = i*v_vehicle + k*prop_lift + v_wind
where j and i are some params related to vehicle losses, k is related to propeller lift efficiency..
With a variable-pitch propeller you would be able to regulate the lift at specific configuration - thus to maximize the vehicle speed.
The max speed of the vehicle would be determined by the losses (friction in gear, wheels, aerodynamics, etc.) and the lift efficiency of the propeller.
For example with 50% "propeller's lift creating efficiency" and 40% mechanical vehicle losses the speed of the vehicle will be higher than the speed of the tail wind.
As soon as the vehicle starts to move the propeller will create thrust as propeller is connected to wheel so propeller will start to rotate as soon as vehicle moves.
So vehicle body and even the propeller is used as a sail and that is most of the power acting on the vehicle but then as propeller starts to spin faster it will be like increasing the size of the sail and at the same time propeller will push air back against the wind so that pressure behind the propeller increases compared to a simple sail where pressure decreases as vehicle speeds up.
So large part of the wind energy during initial phase (below wind speed) will be stored both as two types of kinetic energy one in the vehicle body and one in the rotating propeller but the important type of energy storage and what allows vehicle to exceed wind speed is this pressure differential created by the propeller.
You can test what I'm saying or just imagine having a propeller where blades have no pitch so they will not create any thrust in that case they will be just like a sail and only kinetic energy is stored (flywheel) but to be called a propeller they will need to have some pitch so it will create thrust using energy to do so but also storing that energy by compressing the air.
You can imagine that a vehicle pushed by wind will accelerate faster if propeller had no pitch as energy will not need to be stored by compressing the air and of course that vehicle can not exceed wind speed unless you are allowed to drive the wheels from the flywheel energy by changing gears not the case with blackbird tho blackbird can change the propeller blade pitch and that will also allow for kinetic energy from flywheel to be used if that was desired.
So wind power available during below wind speed will be used as follow
- small part of it will accelerate the vehicle increasing vehicle kinetic energy
- most will be taken trough the wheel and transferred to propeller that will store most of it as kinetic energy (flywheel) and also as pressure differential by creating a pressure differential between the back and front of the propeller. You may think that this sort of energy storage is not useful as it will quickly dissipate if propeller was to stop same way the magnetic field around an inductor will disappear fast if current is removed but anyone accepts inductor as being an energy storage device yes propeller pressure differential that is almost a perfect analog is not considered by most energy storage.
The propeller is a combination of capacitor (flywheel) and inductor (pressure differential when used in air or other compressible materials so this type of storage is not available in water).
If the propeller will not have been connected to wheel the this type of compressed air storage will not have been possible.
Imagine there is no wind and you push the vehicle to some low speed say 1m/s and say we ignore the pressure differential stored energy (maybe propeller used in water) and there is no wind or water flow.
Say wheel is connected to propeller and say total efficiency of transferring power from wheel to propeller and including propeller efficiency is 70% (realistic number).
Then at a 10W of breaking power your propeller will put the equivalent of just 7W as thrust thus vehicle will slow down.
Now imagine vehicle is at 11m/s with a wind of 10m/s or water flow of 10m/s so again we ignore the pressure differential energy storage.
In this situation breaking at 10W will result in the same 7W of thrust as before.
So there will be no difference between 1m/s with no wind and 11m/s with a 10m/s wind (or water flow so that we can ignore the pressure differential energy storage that you do not want to accept anyway).
Sorry for the long replay but without a form of energy storage the vehicle will have no change getting to wind speed let alone exceed that.
And as mentioned the most convincing experiment for you (all) will be to push the vehicle to designed speed and see how vehicle accelerates faster than pushed speed while there is no wind (this only works in air not in water).