"Dave W" wrote in message
news

snip

Some further thoughts:

The display of Vo should stay fairly constant as the current is ramped up
and down.
If it doesn't, the assumed value for Rt can be adjusted until it does.

I suspect the motor current will be very noisy, making any digital display
very
erratic and difficult to read. Better to use a mechanical meter, with a big
capacitor across it to smooth out wobbles. Or perhaps use software to create
a
running average. A meter would be easy to read in bright sunlight.

The meter could have a big mark at 12.19V, with perhaps everything below
11.5V
marked in red, as that's where your discharge graph starts to drop like a
stone.

If you are using a simple rheostat to adjust the motor current, that's a big
waste of energy in the form of heat. Better to have an efficient
switched-mode
power converter to convert the battery voltage into motor voltage.

Ideally this would have current feedback to compensate for the resistance of
the motor, in effect raising the voltage as more current is taken. The
control
knob could then be calibrated directly in propeller speed.

The feedback could be further modified if you know the relationship between
boat
speed and propeller speed, and the knob could be calibrated in boat speed.
This
would be helpful for estimating journey time.

Regards,
Dave