| 4. The regeneration mode with modern electronic controllers is very effective over a wide range of speed. The mode works fine with a boat that has a narrow operating speed like moderate displacement sailing boat. A HP will typically motor at 6 knots and sail up to 20kts. When sailing at 6knots you do not want to be slowing down to 3knots orleven lower to charge batteries. If you forget to lift the motor once the wind builds it will be spinning well above its design speed and at risk of throwing windings. Cage type induction motors are usually most tolerant of overspeed but they are the most expensive option.

Rick, I would like to revisit this point. I question the idea of it even being possible to over speed an electric thruster by towing it.

While the hull may see some low knots when under power, certainly the electric prop will see many times the speed the hull does in that condition, in order to create the thrust needed to move the boat. Certainly it will see more than double the speed of the hull. I would guess 4 to 5 times is more probable, but I am often wrong.

Does anyone know a good number?

So, I am guessing, dragging a thruster at 20kts seems not problematic at all, as it, probably, will be no where near over speed.

But, the potential charging amps seem quite attractive, as the mass available to extract energy is quite high. And it literally costs almost nothing in weight and complexity.

Having said this, there is a problem of the tender thruster tolerating the forces of dragging the thruster at 20kts. But that's another topic.