A passage is where a boat's electrical system gets its real exam. At anchor you can be lazy: the sun comes up, the panels fill the bank, and a quiet day fixes most sins. On a two-night beat across Biscay or a Channel crossing in poor visibility, the loads run flat out, the engine may stay off for sailing, and the grey weather that triggered your weather window is exactly the weather that starves your solar. This is the moment power management stops being a hobby and becomes seamanship.
Before any of this you should have picked a sensible departure, and choosing a Channel crossing weather window is the first half of the same problem: the grey window that is safe to cross is often the one that starves your solar, so the two decisions are linked. I learned this the hard way on a passage from L'Aberwrac'h to the Gironde. We sailed beautifully for thirty hours, autopilot steering, radar and AIS on for the fishing fleets, navigation lights all night, and somewhere off the Vendee coast the battery monitor started showing numbers I did not like. We had been generating nothing under sail and consuming continuously. We made port fine, but I spent that night rationing electrons instead of resting, and I resolved to do the sums properly before the next long leg.
The continuous loads are the killers
The difference between coastal hopping and a real passage is that on passage the heavy loads never switch off. Day-sailing you turn the plotter off for lunch. On a multi-day leg the autopilot steers around the clock, the instruments run, the radar sweeps, the AIS transmits and receives, and at night the navigation lights add to the total.
Put real numbers on it. An autopilot in active steering mode draws meaningfully more than its standby trickle, and on a lumpy point of sail where it is working hard it draws more still. Chartplotter and autopilot together typically pull 8 to 12 amps. A radar adds several amps when transmitting. Navigation lights, even LED ones, run all night. The fridge keeps cycling at 5 to 6 amps. Add it up over 24 hours and a passage can easily double your at-anchor consumption, pushing a boat that burns 100 amp-hours a day in harbour toward 200 amp-hours on a leg. That is a verified rule of thumb across cruising boats, not a scare figure.
The autopilot deserves singling out because it is both essential and greedy, and it gets greedier the harder the steering. A balanced, well-trimmed boat costs the pilot far less than a boat fighting weather helm, which means good sail trim is also good power management. Reef early, balance the rig, and the pilot sips instead of gulps.
Build the budget before you slip the lines
The whole of power management on passage comes down to one piece of homework: a written power budget for the leg. List every device that will run, estimate the amps it draws and the hours it will run over 24, and total the amp-hours. Compare that total to your usable battery capacity and your realistic charging input.
Usable capacity is the honest word. If you run lead-acid, only about half the bank is usable before you start damaging it, so 200 amp-hours of lead gives you 100 to spend. Lithium lets you use 80 to 90 per cent, which is one of the reasons it has taken over offshore, and if you are weighing that change I worked through the trade-offs in solar and lithium for a French summer cruise. The general guidance is to carry three to four times your daily consumption in capacity, so a boat burning 150 amp-hours on passage wants 450 to 600 amp-hours of bank to avoid deep, life-shortening discharges.
If the budget shows you consuming more than you can store and replace, you have a decision to make before you leave, not at three in the morning off Belle-Ile.
Charging on passage: where the amps come back
You have three sources and a passage usually leans on the first.
- The engine alternator. Motor-sailing, or a deliberate hour of engine charging, is the workhorse. A standard alternator puts back tens of amps quickly, and an hour's run can replace a big chunk of an overnight deficit. The catch is fuel and noise, and a tired alternator that overheats at sustained high output.
- Solar. Wonderful at anchor, unreliable on passage. The weather window you chose is often grey, the boat heels so panels point at the sea not the sun, and you are using power day and night while the panels only give by day. Treat solar as a welcome bonus on a long leg, not the plan.
- A hydrogenerator or towed turbine. The serious offshore answer. Sailing at six knots, a hydrogenerator can match or beat your consumption, which is why short-handed ocean racers and Biscay regulars fit them. Overkill for a Channel hop, transformative for a Biscay crossing or a passage to Spain.
My own tactic on a French passage is simple arithmetic. I know my hourly consumption from the budget. I watch the battery monitor's state of charge, and when it drops to around 50 per cent on a lead bank or lower on lithium, I run the engine for an hour, ideally while motoring through a wind hole anyway so the fuel does double duty. Planned, not panicked.
Night-passage tactics that save real amps
Small habits add up over a long night.
- Dim the instruments and plotter at night. They run far cheaper on a low backlight, and your night vision improves into the bargain.
- Once well offshore and clear of traffic and hazards, you do not need the plotter glaring continuously. Glance, then dim or sleep the screen. The AIS and a watch alarm keep you safe between looks.
- Run radar in a guard-zone or timed-transmit mode rather than continuous sweeping where the kit allows it, saving the high transmit draw for when you actually need a look around.
- Pre-chill the fridge hard before departure and open it as little as possible. Some cruisers switch the fridge off overnight on passage entirely; in a French summer I leave it on but turned down.
- Keep the AIS transmitting. This is the one load I never trim, because being seen in the ferry lanes and by the Biscay fishing fleets is worth every amp. If you want the detail on why, I made the full case in the piece on AIS and reading French coastal traffic.
The passage that taught me
Since that Gironde leg I have never started a long French passage without the budget written on the back of the pilot chart. It takes ten minutes and it turns power from a worry into a managed quantity. You know going in whether you can sail through the night without the engine, or whether you have built an hour of charging into the plan. You reef early because it steers the boat and feeds the pilot at the same time. You dim the screens and you keep the AIS lit.
The Channel and Biscay do not forgive much, but they forgive a flat battery least of all, because a dead autopilot at four in the morning in a seaway is the start of a long, tiring night. Do the sums in the marina with a cup of tea, and the boat will look after you when it matters.
