No criticism aimed at anyone doing it themselves. Onboard wiring looks simple. The trick is in a few specific details where boats differ from land installations. Ashore, much works through fixed rules and built-in reserves; on board, every metre of cable matters, everything vibrates, salt and moisture eat at every joint, and a fault never comes at a convenient moment.
The five mistakes below are the ones I find with some regularity in my workshop and on board. They share a feature: they don't show up immediately. The installation works, seemingly. Until it doesn't.
1. Wrong cable cross-sections
The most common mistake: a new inverter, a new bow thruster or extra onboard lighting is wired up with the cable that "more or less fits". DIY stores stock 2.5mm² and 4mm². For most onboard connections that is woefully inadequate.
The reason: cables on a boat run longer than you think. A 12V installation where the onboard charger sits 5 metres from the battery means the current travels 10 metres (out and back). At a 50A load (a sizeable inverter) 4mm² over 10 metres delivers about 0.8V of voltage drop, while you start at 12.8V and your equipment already struggles below 12V. The cable also heats under load, which over years leads to insulation ageing and eventually shorts.
| Load (12V) | Length out+back | Min cross-section |
|---|---|---|
| 10 A (lighting, navigation) | 10 m | 2.5 mm² |
| 30 A (small inverter, fridge) | 10 m | 6 mm² |
| 50 A (typical inverter) | 10 m | 10 mm² |
| 100 A (bow thruster, large inverter) | 10 m | 25 mm² |
| 200 A (lithium installation) | 5 m | 50 mm² |
Indicative values for around 3% voltage drop. For 24V installations you can drop half a size. When in doubt: calculate or call.
2. No fuse on the positive, or one that is far too big
Ashore, the fuse usually sits in the consumer unit and matches both cable and appliance. On board I often see two kinds of mistakes:
- No fuse at all between battery and main load. "The inverter has an internal fuse": correct, but that protects the inverter, not the cable between them. On a short in the cable itself you burn out your whole positive bus and possibly the boat.
- A vastly oversized fuse. "I don't want it blowing unnecessarily, so I put in 200 amps." That protects nothing: your battery can briefly deliver 1000+ amps, so that 200A only blows on a serious short, long after the cable is already glowing.
Rule: fuse as close to the battery terminal as possible (within 18 cm per NEN-EN-IEC 60092 for marine installations), at a rating that matches both cable and load. Not at the inverter, but at the battery, otherwise the cable itself is unprotected.
A practical tip
A 100A MIDI fuse at the battery and a 200A MEGA fuse on the main positive terminal (for the bow thruster, say) are not a luxury. They are the difference between a short that is a nuisance and a short that needs the fire brigade.
3. Mixed grounding: DC, AC and shore power tangled
A boat often has three different grounding systems: 12/24V DC ground (the battery negative), 230V AC ground from shore power, and a galvanic protection system for underwater parts. These may connect at specific points, but certainly not just anywhere.
What I find: a DIY extension where the installer, looking for an earth point for 230V equipment, simply screws onto a DC bus, or worse: straight onto the engine or hull. That creates three problems at once:
- Galvanic corrosion accelerates: underwater parts, propeller and saildrive corrode three times as fast.
- Safety issues with shore power. Earth leakage current finds its way along unexpected routes and can be lethal on contact.
- Interference in electronics. Radar, plotter and autopilot suffer from stray currents they should never see.
On a boat that uses shore power regularly, there must be proper galvanic separation (galvanic isolator or isolation transformer) between shore-power ground and onboard ground. If you do your own electrical work, this is the area where you should bring in someone who knows it. The cost of a mistake here disappears below the waterline, where you don't notice it until the boat is on the hard.
4. Connections that don't survive vibration
A screw connection that holds fine on the bench can come loose under onboard vibration within a season. Especially at engines, behind the engine panel, or near pumps and generators. What I find:
- Open terminal-block connections that simply fall out of their clamps after 200 cruising hours.
- Wago clamps not rated for marine use. Work fine in a house, but the spring action does not hold up against vibration and salt-laden air.
- Uncrimped lugs where the cable just sits in the lug without a proper crimp: coming loose is a matter of time.
- Untinned copper strands in a salt environment, which oxidise within years into a green crust that no longer conducts.
For marine work: all connections mechanically secured and electrically protected. Heat-shrink with adhesive lining, lugs crimped with the proper tool (not squeezed flat with pliers), tinned copper, and preferably no solder joints on flexible cables (a soldered joint becomes stiff and breaks under vibration just outside the solder).
5. Non-marine components on a boat
The car radiator fan repurposed as engine-room ventilation. The DIY-store LED spot mounted at the helm. The household light switch installed in the cabin. All real-world examples.
The problem isn't always with how it works initially. It often works fine for the first months. The problem is in:
- Moisture resistance. IP rating of automotive or household components is typically IP20 to IP44. On board you want at least IP65 in exposed locations and IP67 near the bilge.
- Vibration and shock resistance. Components designed specifically for marine use are tested for marine vibration and brief shock loading.
- Material choice. Steel screws on a boat rust away within a season. Stainless and tinned brass are standard on board.
- Fire rating. Marine components are flame-retardant; many cheap alternatives are not. A short in a household switch in a closed space on board is not an attractive prospect.
When it makes sense to bring someone in
Plenty of work you can do yourself: replacing a light fitting, a small 12V extension, fitting a new USB outlet. The boundaries where I advise bringing in a professional:
- All 230V and shore power work.
- Connections to battery packs above 100Ah, and certainly to lithium.
- Bow thrusters and autopilots, because of high current peaks where sizing is critical.
- Changes to the grounding architecture or galvanic protection.
- A full refit, because the overview is what matters most.
For the work you do yourself: take your time, use marine-grade components, work calmly. An installation that looks like it has been thought through actually performs better. Not just because tidy work works better, but because the person who eventually has to maintain it (you or someone else) understands at a glance what they are looking at.
Unsure about your installation?
A brief onboard inspection flags the weak points before they become problems. Sometimes it's 30 minutes of work to fit a few fuses or upgrade a cable, and the worst worries are gone.
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