Using it well · 7 minute read
Autopilot battery and solar sizing
Work out what your autopilot really uses, build an amp-hour budget for a passage, and size batteries, solar and charging for Australian conditions.
An autopilot is usually the largest single electrical load on a sailing boat on passage, and the one most likely to be underestimated because it draws current in bursts rather than continuously. Owners quote the catalogue figure, forget that it is an average under easy conditions, and then wonder why the batteries are flat off Gabo Island.
The fix is arithmetic done before departure. Work out the daily amp-hour consumption of everything, decide how you will replace it, and add a margin for the conditions that make the pilot work hardest. This guide walks through the numbers with Australian passages as the examples.
Key facts
- Tiller pilot
- Roughly 0.5 to 1 A average under way
- Wheel pilot
- Roughly 0.5 to 1.2 A average
- Linear drive
- Roughly 1 to 3 A average on a cruising yacht
- Hydraulic
- Roughly 1 to 2 A average on a motor cruiser
- Racing pilot
- Often 3 to 5 A average, much more downwind in a seaway
- Rule
- Budget the average, then add fifty per cent for bad days
Measure autopilot current draw for battery sizing
Published figures are averages measured in reasonable conditions. The instantaneous draw of a drive is much higher, often several times the average, but it only lasts for the fraction of a second the drive is running. What matters for the battery is the average over hours, and that average depends almost entirely on how hard the boat is to steer.
A well-balanced yacht reaching in fifteen knots might have its pilot working ten per cent of the time. The same boat running dead downwind under poled-out headsail in a following swell might have the drive running half the time, and the average current triples. That is why a single catalogue number is never enough for planning.
Use these indicative figures as a starting point and then measure your own boat. A clamp meter or a battery monitor with a decent shunt will tell you the truth in an hour of sailing, and the truth is worth far more than any table.
- Tiller pilot: about 0.5 to 1 A average under way
- Wheel pilot: about 0.5 to 1.2 A average
- Below-deck linear drive on a cruising yacht: about 1 to 3 A average
- Hydraulic pump on a motor cruiser: about 1 to 2 A average
- Racing pilot working hard: about 3 to 5 A average, more in a seaway
- All figures indicative; measure your own boat with a battery monitor
Worked autopilot battery and charging budgets
Take a 12 metre cruising yacht with a below-deck averaging 2 amps. Over 24 hours that is 48 amp hours. Add instruments and plotter at 1.5 amps, which is 36 amp hours, AIS and VHF at half an amp for 12 amp hours, navigation lights for twelve hours at one amp for 12 amp hours, and a fridge at 3 amps for a third of the time, another 24 amp hours. That is roughly 130 amp hours a day before anyone charges a phone.
Now the same boat running downwind in a Bass Strait swell. The pilot averages 3.5 amps instead of 2, adding 36 amp hours, the fridge works harder, and the total goes past 170 amp hours a day. If the battery bank is 400 amp hours of lead acid, of which perhaps 200 is usable, that is barely a day and a quarter of autonomy.
A small boat with a tiller pilot is a much easier sum: 0.7 amps average is 17 amp hours a day, plus instruments and lights, which a single 100 amp hour battery and a 100 watt panel will comfortably carry for coastal work.
Lithium against AGM
The practical difference is usable capacity and how the bank behaves under load. A lithium iron phosphate bank can be cycled deeply and holds its voltage far better as it discharges, which matters for an autopilot because drives lose power as voltage sags. An AGM bank of the same nominal capacity gives you noticeably less usable energy and a lower voltage at the drive by the end of the night.
Lithium also accepts charge much faster, so an hour of engine running puts far more back in. That is worth real fuel on a long passage. Against that, it costs more up front and needs a properly designed installation including a battery management system, appropriate charging sources and protection.
If you keep lead acid, size the bank so the autopilot never sees a badly depleted battery. The rule of thumb of using only half a lead acid bank exists for good reason, and pilots are among the first devices to complain when it is ignored.
Voltage at the drive is what counts
Solar, wind and Australian conditions
Solar is the backbone of most Australian cruising energy plans and it works very well north of about Brisbane. A well-sited 400 watt array on a cruising yacht can realistically return a useful fraction of its rating averaged over a day in the tropics, more in the dry season, and that covers a modest autopilot and instruments without the engine.
It is a different story in a Tasmanian winter, where short days, low sun angles and cloud can cut output dramatically. Boats cruising the D'Entrecasteaux Channel in July should plan on engine or shore charging rather than optimistic solar numbers.
Shading is the enemy on a sailing boat. A boom, a bimini frame, a pole or a backstay crossing a panel can cost far more output than the shaded area suggests, depending on the panel and controller. Mount panels where the rig does not sweep across them, use separate controllers for panels that shade differently, and check the array's actual output on a sunny day rather than assuming.
Engine hours, alternators and cable sizing
Most cruising boats still need the engine at some point on a long passage. What determines how useful that hour is comes down to the alternator, its regulation and the battery chemistry. A standard alternator with a basic regulator into a half-charged lead acid bank delivers disappointingly little after the first few minutes.
An external regulator, or a properly specified alternator and charging system, changes that considerably, and lithium banks can absorb a much higher rate for longer. If your passage plan involves running the engine to charge, it is worth having the charging system looked at by an electrician before the season rather than burning hours for little return.
Cable sizing to the autopilot drive deserves its own attention. Voltage drop is proportional to current and cable length, and drives pull high peak currents. Use the manufacturer's recommended gauge as a minimum, keep the run as short as practical, use proper crimped and sealed terminals, and give the drive its own fused feed from the battery rather than sharing a circuit.
What pilots do when the power runs low, and the zero-power backup
Most autopilots monitor supply voltage and will alarm and disengage rather than steer badly on a sagging battery. That is correct behaviour and it is also a nasty surprise in the middle of the night if you have never seen it. Find out what your system does and at what voltage, and set the low-voltage alarm so it warns you well before it acts.
A windvane is the only genuinely zero-power self-steering, which is why it remains popular with offshore cruisers regardless of how good electric pilots have become. It steers to the wind rather than a course, so it is no help under power, but on a long passage it can steer the boat for days on nothing.
For a Coral Sea passage from Bundaberg or a Bass Strait crossing, build the energy plan around the worst-case pilot consumption, not the average, and know how you steer if the plan fails. That is what makes the difference between a passage and an ordeal.
Questions
Frequently asked
How many amp hours a day does an autopilot use?
Roughly 12 to 24 amp hours for a tiller pilot, 24 to 70 for a below-deck linear drive on a cruising yacht, and considerably more for a racing pilot working hard downwind. The figure depends far more on how hard the boat is to steer than on the model. Measure your own boat with a battery monitor for a number you can trust.
Is lithium worth it for autopilot use?
It helps in two ways: more usable capacity from the same nominal size, and a steadier voltage at the drive as the bank discharges, which keeps the pilot working properly. It costs more and needs a properly designed installation with appropriate charging and protection. Many cruising boats get by well on a generously sized AGM bank and good cable runs.
How much solar do I need to run an autopilot?
As a rough starting point, a cruising yacht wanting to run a below-deck pilot plus instruments without the engine typically looks at several hundred watts of well-sited panel in Australian conditions. Output falls sharply in southern winters and with any shading from the rig. Build the budget from your own measured consumption rather than from panel ratings.
What happens if the battery voltage gets too low?
Most pilots raise a low-voltage alarm and then disengage rather than steer poorly, which is the right behaviour but startling at night. Set the alarm so it warns you in time to do something, and check what voltage your system acts at. A drive seeing low voltage also loses power well before it stops.
Is a windvane a realistic backup?
For offshore passages, yes, and it remains the only self-steering that needs no power at all. It steers to the apparent wind rather than to a compass course, so it does not help under power or in very light or fluky conditions. Many offshore cruisers carry both and use the electric pilot under power and the vane on long sailing legs.
Practise autopilot operation with an instructor
Habits are learned at the helm with someone watching. The training section explains what a good instructor covers and where to find one in your state.