Up to now I had very poor range with my RC transmitter (which uses in band telemetry from the receiver for things like battery voltage and RSSI) giving me warnings about poor RSSI at the receiver onboard the boat, while being only a short distance away. Last session I swapped out the RC transmitter and have not had range issues since, so it appears it was the RC transmitter which was the cause of that problem.
I am finally starting to figure out how to analyse the logs produced by Ardupilot to diagnose why the autopilot sometimes refuses to change into "Auto" mode from "Manual" mode. (In auto mode the boat follows a pre-planned course consisting of a list of waypoints.)
The problem has been that sometimes Ardupilot would refuse to change from "Manual" to "Auto" with the error message "Flight mode change failed", which doesn't help much!
Luckily though Ardupilot sends large amounts of realtime data over the telemetry link and the data is received by the GCS ( Ground Control Station) running on a PC at the bank and saved in a log file. The logs can be viewed in "Apm Planner" , my GCS application, and am finally learning how to navigate the data.
Currently it looks like the issue may be "compass variance". The compass I am currently using (HMC5883L) is mounted on the Flight controller PCB quite close to the servo outputs and that isnt the best place. It may be necessary to disable the onboard compass and mount an external one, possibly in the bow of the boat, away from the power electronics. First though, I should probably retry calibrating the compass and see if that helps.