Is there always a clean distinction between turbine mode of energy capture and propeller function of thrust for a given physical device such as an air-rotor or sail?
For a rotor there is a clean general distinction:propeller
: rotates opposite to the torque from the fluidturbine
: rotates in accordance with the torque from the fluid
There are other critera based on energy transfer, but those unlike the above are frame dependent and fail in special cases.
How does that work in a conventional sail boat? Which is the turbine capturing the energy and what is the propeller equivalent providing thrust?
The energy question is frame dependent. But we can translate the above distincion for rotors to the a sail boat:
The rotation becomes linear motion perpendicular to the true wind vector. The tangenial componenets of the rotor blade become the true-wind-perpendicular -components (TWPPC). Considering only those components of the velocity & force vectors we can decide if the sail or the keel act as a propeller or turbine blade:propeller
: velocity-TWPPC opposite fluid-force-TWPPCturbine
: velocity-TWPPC in the same direction as the fluid-force-TWPPC
Applying this to a sail boat gives different answers depending on the situation:upwind VMG > 0
sail : turbine
keel : propellerdownwind VMG > windspeed
sail : propeller
keel : turbine0 <= downwind VMG <= windspeed
Any combination of: [propeller, keel, none] for [sail, keel] is possible. "None" means that some TWPPC component is zero, like when going DDW slower than wind.
For DDWFTTW is the air-rotor only a propeller (providing thrust) and the wheels only an energy capture device powering the load of the propeller? Or is the air-rotor also capturing energy and the wheels also a load at times? Otherwise how does it get going and later how does it slow down when the tail wind shuts off?
As I allready noted: Energy considerations are not usefull to describe how moving things powered by the relative movement of two other things work. Since kinetic energy is frame dependent different observers will disagree which thing provides energy. This can get very confusing:In the ground frame the DDWFTTW air-rotor always takes energy from the air. In the vechicle's frame it takes energy from the air below windspeed, and gives it to the air above windspeed.
This is much simpler:The air-rotor of the DDWFTTW is always a propeller because it always rotates opposite to the aerodynamic torque.