Help needed to contol speed of trolling motor to hold GPS position

Very new and inexperienced but hoping I can at least get pointed in the right direction. I am trying to create a DIY spot lock using an engine mounted trolling motor.

I already have steering sorted via a lowrance autopilot kit and can set tracks via my chart plotter, it steers my outboard which my trolling motor is attached to. I need ardupilot to be able to control the speed only of my Minn Kota RT80EM so that I can hold my gps position i.e. spot lock/anchor mode. Currently speed is controlled with a rotary switch. Max current at top speed is 58A.

Ideally I would l like to keep the rotary switch but have a single button that I press and the controller cuts in and out using PWM to keep the boat on the spot, the lowrance autopilot will make any steering corrections.

How possible is this for an absolute novice?

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I have no experience with boats but it looks like what you want is something close to Loiter mode: If you are not planning on using Ardupilot for directional control and only relying on the thrust output then you may run into trouble. I am sure that what you want is achievable with some coding…

That looks really promising. If only I knew the first thing about coding…

I have just learnt that the rotary knob that controls the speed uses PWM. There is an external box that the rotary switch plugs into with a lead to connect the power supply and then one to the motor. Minn Kota call it Digital Maximiser. From a hardware perspective this seems promising. Is there any chance that I could run ardupilot on a Raspberry pi and control the speed in loiter mode?

Is there anyone who can assist me with what hardware and software changes would be required to control PWM using the the Minn Kota Digital Maximiser and software changes required for Ardupilot to control thrust of a single motor only?

All flight controllers running ArduRover produce PWM signals as outputs to drive motors and servo’s. You don’t need a Rpi for anything, this is basic capability. What you will need to figure out is where to inject the PWM signal from the Flight Controller to the Motor Controller.

I’m not sure if you are still trying this project, but I imagine you will have little luck with a transom mounted trolling motor. All GPS trolling motors are bow mounted so the boat gets “pulled” along by the motor and swings and pivots behind it.