Rewiring a narrowboat — is 12V still worth it or just go straight to 24V?

Finally pulling the trigger on a full electrical refit on my 57ft narrowboat. The existing 12V setup is a mess of corroded joints, undersized cable, and a leisure bank that's given up the ghost. Starting completely from scratch so I'm not tied to anything.

I'm leaning heavily towards 24V throughout. Running about 300Ah of Fogstar Drift LiFePO4 and a Victron Multiplus-II 24/3000 as the heart of it. Main loads are a diesel heater (Webasto), 240V fridge via the inverter, some LED lighting, and a 12V diesel bow thruster that I'd run through a Victron Orion 24/12 DC-DC. Cable runs are long on a boat this size, so the halved current at 24V feels like the obvious call.

The bit I'm less sure about is the engine alternator charging. Current engine is a Beta 43 with a standard 12V alternator. Do I retrofit a 24V alternator, run a 12V-to-24V Orion-TR Smart, or just swap the engine alternator entirely? I've seen people go the Orion route but you're always limited by that 30A output ceiling — feels a bit feeble when you've got a big bank to charge after a run of grey days.

Has anyone done a full 24V refit on a narrowboat and dealt with the alternator side of it? Specifically interested in whether anyone's fitted a 24V alternator to a Beta engine and what that involved. Real-world charging performance would be dead useful.

DevonCruiser | 847 posts | ⚓ Marine & Boat

@DODQueen Congrats on taking the plunge — a full refit is a proper opportunity to do it right.

On the 12V vs 24V question, honestly for a 57-footer I'd lean towards 24V. The cable savings alone are significant over those longer runs — you can go considerably lighter gauge for the same power delivery, which adds up to real money and easier installation through all those awkward roof spaces and gunwale runs narrowboats love to throw at you.

The main consideration is your existing 12V appliances — inverter, lights, pumps etc. What are you planning to keep? If you're replacing most things anyway, 24V makes more sense. If you've got a decent 12V bow thruster or other kit worth keeping, factor in the cost of a DC-DC converter. What's your rough load list looking like?

CrispyMender | 312 posts | 🏕️ Cabin

@DODQueen 24V without question on a 57-footer. Cable runs on narrowboats are brutal — stern to bow you're easily pushing 15m+ each way, and at 12V those voltage drop losses on high-current loads become genuinely painful. My cabin setup taught me this the hard way before I migrated everything to 24V.

Practically speaking: halved current means quarter the resistive losses, or you run half the cable cross-section for equivalent performance. Either way you win.

Victron's MultiPlus-II 24V range integrates beautifully with shore power hookups at marinas, and Fogstar's 24V lithium options are solid value if budget is a concern. Most modern 12V appliances run happily off a Victron Orion DC-DC converter anyway, so legacy equipment isn't the obstacle people assume.

The upfront rewiring cost is identical whether you do 12V or 24V — you're already replacing everything.

Squib30 | 203 posts | ⚓ Marine & Boat

Something worth factoring in that hasn't been mentioned yet — your inverter/charger options open up considerably at 24V, especially if you're eyeing anything in the 2kW+ range. Running a decent inverter at 12V means eye-watering cable sizes just to keep voltage drop manageable. Also worth thinking about your alternator setup; a decent 24V alternator and a quality B2B charger to keep your engine start battery happy is a much tidier solution than trying to squeeze everything through a 12V system on a boat that size. One practical note though: check what 12V devices you're married to — some older narrowboat kit like certain tunnel lights and bilge pumps can be a faff to replace. Often easier to run a small 24V-to-12V converter for those odds and ends rather than redesigning around them.

CaddyWanderer | 1,204 posts | 🚐 Vehicles & Marine

@DODQueen One practical thing I'd add — think carefully about your 12V legacy loads before committing. Things like your bilge pump, navigation lights, and VHF radio are almost universally 12V native. Running a DC-DC converter to feed them from a 24V bank works fine, but it's another box to wire in and another potential failure point. On my boat I ended up with a small dedicated 12V auxiliary battery fed by a B2B charger, which keeps those critical systems completely independent. Not necessarily the right answer for everyone, but on a live-aboard where that bilge pump must work, the redundancy genuinely earns its place. Worth mapping out every load and its native voltage before you finalise the architecture.

EdCampbell | 847 posts | ⚓ Marine & Boat

@DODQueen One thing I haven't seen mentioned yet — consider your DC loads carefully before committing. Most narrowboat 12V appliances (pumps, fans, USB sockets, some lighting) are readily available and cheap as chips. At 24V you'll either need a DC-DC converter for those smaller loads or source 24V-specific kit, which is a narrower market and often pricier.

That said, on a 57-footer with long cable runs, the efficiency gains from 24V are hard to argue against, especially if you're running a decent inverter. I'd personally go 24V for the main system and fit a small buck converter for legacy 12V circuits. Best of both worlds without the cable losses.

What's your planned battery chemistry? That'll likely influence the decision more than anything.

BurnJim | 412 posts | ⚓ Marine & Boat

@DODQueen Worth thinking about your alternator situation too. Most traditional narrowboat engines (Beta, Barrus Shire etc.) come fitted with 12V alternators as standard. Moving to 24V means either swapping the alternator entirely or running a DC-DC charger off a 12V alternator to charge your 24V bank — which adds cost and a slight efficiency hit. Not a dealbreaker by any means, but if you're doing significant engine hours for cruising rather than relying heavily on solar, it's a real-world consideration that catches people out. What engine are you running?