Since the test with the new motor described in last week's blog ended abruptly with a tree collision, I went back the following day to try again, this time with a better "cold-temperature" magnetometer calibration. I also launched the boat from the kayak, just beyond the trees as a precaution. There was a light wind, a fair bit of current, and the sun was out providing a bit of warmth. It was almost possible to forget that it was the middle of December.
The new motor did great - going against the current I believe it was a lot faster than the old motor would have been:
I ordered a 6 foot kids surfboard:
a feel for the accuracy / precision without using the RTK base unit, I put the rover unit on AMOS and carried it around the perimeter of our fence, about 1 foot inside the fence: