Rover: boats may drift away in Auto if current is too high

[rmackay9]

During testing of PR https://github.com/ArduPilot/ardupilot/pull/19304 which adds support for "motorboat" to sim_vehicle I discovered that in at least one edge case the navigation controller will get stuck pointing the vehicle in the wrong direction with very low throttle if the current is too high and nearly perpendicular to one leg of the mission.

To recreate the issue in SITL do this:

• ../Tools/autotest/sim_vehicle.py --map --console -f motorboat
• param set SIM_TIDE_DIR 90 (current will flow from East to West)
• param set SIM_TIDE_SPEED 3 (3m/s current)
• create a mission that is a square box (linked here)
• Auto
• arm

You should see the vehicle gets stuck pointing in the wrong direction as it starts the North->South leg of the mission

[geofrancis]

is this related to this issue here https://discuss.ardupilot.org/t/improve-steering-if-motor-throttle-is-off/74572

his problem sounds very similar to yours, essentially lack of rudder authority at low speeds.