Dougal, reading your blog re the Hundested prop controller & popping a breaker, I would say that whatever is using the power from that breaker is using too many amps for the breaker. I am guessing a motor drive or electric hydraulic pump is failing. Your schematics should trace to the part or you could use the half-split rule & disconnect (tape up & tie back live cables) along the path until the breaker stays in or disconnect at whatever drives the prop pitch & test.
In my experience as an electronics engineer, electric motors & pumps on boats don’t like salt air.
The other problem can be on the earth side (negative bus), all connections to the negative side (earth) need to be cleaned and retightened with marine anti-seize. This applies to all connections including the antenna outside thread on your radios & the metal case of all equipment.
When you have nothing to do? I would also check all the DC - & + brass Bus bars, I assume its 24V but check for tightness & any corrosion. A few cans of CRC Marine is great to have.
Checking all the connections from the Negative of the battery banks all the way to the zinc anodes connections helps a lot to fight corrosion.
If there is any “noise” in your radios, check that all motors are really well earthed all the way to the Zinc Anodes. I like to use copper zinc plated woven earthing straps about 1 inch wide held down with spring washers or Nylex nuts.