Marine electrical faults can feel mysterious, but most are caused by the same handful of things: corrosion, vibration, water ingress, and voltage drop on long cable runs. A methodical approach finds them faster than a guess.
Start at the battery, work outward
Almost every 12V fault is best diagnosed from the source. Check resting voltage, then voltage with loads applied. A bank that sits at 12.6V but drops to 11.8V when you turn on the nav lights has a problem at the battery or the cabling, not at the lights.
Corrosion is the usual suspect
- Green powder on a terminal means resistance and heat.
- Crimps that look fine often hide corroded copper inside the insulation.
- Crimp connectors used outside the cabin are a common source of intermittent faults.
- Anywhere bilge water reaches, suspect it first.
Voltage drop testing
A multimeter across the start of a cable run and the end, with load applied, tells you whether the cable or its connections are the problem. Anything over half a volt on a 12V circuit is worth investigating. Two volts or more and you have found the fault.
Earth faults and galvanic worries
A boat that suffers unusually fast anode loss often has an earth fault somewhere, leaking DC into the water. This is a job for a proper marine electrical survey, not a multimeter and hope. It will only get worse if ignored.
Get a proper diagnosis
Oli covers marine electrical diagnostics across the Thames. Most faults are resolved in a single visit at your mooring, with a written report so you have a record of what was wrong and what was done.
