T200 high velocity operation

I’m trying to understand what the limits/parameters are for operating the T200 at high velocity, specifically to assist powering a racing kayak.

I did some experiments and at 200W power input to the ESC I’m hitting the wall somewhere between 7-8mph in that the thruster creates more drag than thrust. In the experiment, the racing kayak is surfing down a wave, so the wave (gravity) provides oodles of power and could accelerate the kayak beyond 7-8mph if it wasn’t for the T200 drag. So this is not an issue of “the T200 is not powerful enough to move you vessel faster”. I know I could pump more power into the T200 but that goes into diminishing returns.

I’m trying to understand what happens so I can devise a solution. My understanding of the way things work:

  • I’m providing PWM input to the ESC using a PID loop that keeps constant power (e.g. 200W)
  • this determines the fraction of time that the ESC provides power to the motor
  • thus assuming constant voltage then a constant PWM input produces a more-or-less constant power consumption (this is what I observe experimentally)
  • the ESC commutes the motor poles at a frequency that maintains the programmed timing advance
  • this means that RPM are not directly controlled, they are whatever they happen to be given power input and operating conditions
  • using the stock propeller, in order for the T200 to move in 7-8mph water without providing thrust it has to turn at something like 3500-4000 RPM (there was some thread that calculated the RPM based on the prop pitch, etc)
  • 4000 RPM requires requires 28k eRPM (14 poles), which is well within the ESC’s capabilities

So I’m left with a couple of possibilities:

  • the mechanical friction inside the motor requires ~200W to overcome when attempting to rotate at around 4000RPM
  • the drag of the motor assembly (shroud, etc) requries ~200W to overcome at 7-8mph
  • something else I’m missing

In the end I’m trying to understand whether I need to remove/change the shroud, change the prop, or change the motor.