Diagnostic & Decision 14 min read August 4, 2026

Sailboat autonomy score: assess your boat before extended cruising

A sailboat that performs well on summer coastal hops is not, by that fact alone, ready for extended cruising. Abandoned voyages rarely come down to the hull: they come down to unquantified margins — a battery bank that can't last the night under autopilot, water counted too tight, a liferaft out of its service window. Here are the numbers that turn the diagnostic into a decision: how many amp-hours a day, how many litres, which deadlines, and a complete example on a 40-footer.

160–360 Ah/day

typical passage budget at 12 V, autopilot included

3 L/person/day

the vital water need — carry × 1.3, watermaker not counted

3 yrs / 5 yrs

liferaft service / EPIRB battery

D-21

minimum margin between diagnostic and departure

Purchase score and departure score: two different diagnostics

The pre-purchase score assesses a potential: what the boat could reach after a refit. The pre-departure score assesses a state: what is actually on board, serviced and tested, for the precise route of the coming months.

An 82/100 at purchase coexists without contradiction with a 45/100 at departure: the original bank has aged, the watermaker hasn't run since winter storage, the liferaft is out of its service window. The departure score rates each of the four axes from 0 to 100; the overall score is their average, and the weakest axis sets the date.

Energy: the Ah/day budget, load by load

The load that sinks budgets is the autopilot: on passage it steers 24/7, and a cross sea makes it work twice as hard. Orders of magnitude for a 40-foot monohull at 12 V in the trade winds:

Load	Average current	Ah / day
Autopilot (electric or hydraulic ram)	3 to 8 A continuous	70 to 190
12 V fridge in the tropics	2 to 3 A, 60–80 % duty	30 to 60
AIS + VHF watch, instruments, plotter	1 to 1.5 A	20 to 40
LED navigation lights (12 h)	0.5 to 0.8 A	5 to 10
Satellite communicator, tablets, phones	—	15 to 30
Lighting, pumps, sundries	—	10 to 20
12 V watermaker, 30 L/h, 1 h/day	8 to 10 A	8 to 10
Total		160 to 360 Ah/day (1.9 to 4.3 kWh)

Build this table with your own equipment: a clamp meter at the dock and one night of measurement beat every data sheet. One figure to remember: a budget that ignores the autopilot is off by a third to a half.

Bank and charging: sizing without fooling yourself

Sizing rule: the usable capacity must cover 1.5 days of consumption, to absorb an overcast day without the engine. Usable capacity is not nominal capacity:

• Lead-acid (AGM, gel) — 50 % depth of discharge at most to preserve cycle life. 200 Ah/day × 1.5 = 300 Ah usable → 600 Ah nominal, about 180 kg.
• LiFePO4 — 80 to 90 % usable, steady voltage under load. The same 300 Ah usable → 350 to 400 Ah nominal, about 45 kg. Three times lighter, two to three times the purchase price, but 3,000 cycles against 500 to 800.
• Capacity test before deciding — controlled discharge at 10–20 A down to 12.0 V (lead); real capacity = amps × hours. Below 70 % of nominal the bank is at end of life, and it will sail with you.

On the generation side, on passage and without the engine:

• Solar — 400 Wp on an arch yields 80 to 120 Ah/day in the trades: sails cast shade, clouds too. Count 0.25 Ah/day per installed Wp, no more.
• Hydrogenerator — 8 to 15 A between 5 and 7 knots, i.e. 200 to 300 Ah/day: the only source that covers the autopilot on its own, for a tenth of a knot.
• Wind generator — disappointing downwind: apparent wind drops to 8–10 knots, and output with it.
• Alternator — 1 h of engine a day returns 40 to 60 Ah into a lead bank (acceptance current falls quickly), more into lithium. One hour of diesel a day for 20 days is also 40 to 60 litres you must carry.

The energy-axis criterion: daily generation without the engine ≥ budget, and usable bank ≥ 1.5 × budget. Both at once.

Fresh water: the vital need is calculated, the watermaker is a bonus

• Vital need — 3 L per person per day to drink and cook. Real comfort with a freshwater rinse: 6 to 10 L. Dishes and washing use seawater (foot pump at the galley sink).
• The rule — tanks + jerrycans ≥ the vital need of the whole passage × 1.3 (calms, breakdown). A 2,850-mile Canaries–Caribbean crossing takes 18 to 22 days at 6 knots: for three crew, 3 × 3 L × 20 days × 1.3 = 234 L minimum, 360 L for normal comfort.
• The watermaker — a 12 V unit making 30 L/h draws 8 to 10 A: 30 minutes a day yields 15 L and costs 5 Ah. Test it at least three weeks before departure (membrane to re-activate, filters and biocide to stock). It never counts towards the vital need: a membrane can fail, a jerrycan can't.
• Jerrycans — 2 to 4 × 20 L, lashed, in different places aboard: one contaminated tank must not take the whole reserve with it. Keep 1.5 L per person in the grab bag.

Safety & communication: the deadlines that disqualify

This axis is not scored by feel: every item has a date, and an expired date scores zero on its line.

• Liferaft — manufacturer service every 3 years (ISO 9650), hydrostatic release every 2 years. Allow 3 to 5 weeks in season, €300 to €600 depending on the model.
• 406 MHz EPIRB — registration is mandatory (with the national authority; the record must match the boat and the shore contact), battery replaced every 5 years, monthly self-test. One PLB per night-watch crew member, on the lifejacket.
• DSC VHF — MMSI programmed and registered, GPS connected (the position must show on screen), DSC test call passed. A VHF without MMSI or position is an alarm that doesn't say where you are.
• AIS transceiver — class B (SOTDMA preferred, 5 W): being seen by ships beats seeing them.
• Lifejackets and jacklines — cartridges and firing heads in date, light, personal AIS beacon; jacklines inspected.
• Grab bag — waterproof handheld VHF, handheld GPS, in-date flares and hand-held lights (3 years), 1.5 L of water per person, rations, medical kit, survival blankets, knife, mirror, torch, copies of papers, satellite communicator.
• Communication — a satellite communicator (Iridium GO or inReach) for GRIB files and a daily check-in ashore; subscription active and tested before casting off.

Boat-crew-route fit: windows and rigging

• Seasonal windows — Mediterranean → Canaries: August to October. Canaries → Caribbean: mid-November to February, after hurricane season (1 June – 30 November). Caribbean → Azores → Europe: May and June. Leaving outside the window means accepting what no score can compensate.
• Standing rigging — replace at 10–15 years or 30,000 to 40,000 miles; many insurers require under 10 years for an ocean crossing. Rigger inspection 2 to 3 months ahead: a replacement takes 3 to 6 weeks and €3,500 to €5,500 on a 40-footer.
• Crew — at least two people able to stand a night watch alone, take a reef and run a man-overboard recovery. A skipper alone with two passengers is not a crossing crew.
• Bail-out ports — Madeira, Canaries, Cape Verde on the trade-wind route: written on the chart, with course and distance from each third of the passage.

Worked example: a 40-footer, three crew, Canaries → Caribbean

A 2012 40-foot monohull, three crew, Las Palmas → Le Marin, 2,850 miles, 20 days planned for late November.

Axis	Finding	Score	Action → score after
Energy	Measured budget 190 Ah/day. Bank 2 × 120 Ah AGM from 2019: capacity test at 68 %, i.e. 80 Ah usable — less than half a night under autopilot.	30	300 Ah LiFePO4 bank (240 Ah usable), hydrogenerator (≈ 200 Ah/day at 6 knots), 400 Wp solar (≈ 100 Ah/day) → positive budget without the engine → 85
Water	2 × 180 L + 3 × 20 L jerrycans = 420 L. Vital need 3 × 3 × 20 × 1.3 = 234 L; comfort 360 L.	80	30 L/h watermaker tested at D-21, consumables aboard → 90
Safety	Liferaft serviced 03/2025 (valid to 03/2028), EPIRB battery 2029 and registration current. But DSC VHF without MMSI or GPS link, no AIS transmitter, expired flares.	50	MMSI + NMEA link + test call, class B AIS transceiver, new flares, one PLB per crew → 90
Fit	Original standing rigging (13 years; insurer requires under 10). Window respected, two crew self-sufficient at night, Cape Verde bail-out plotted.	45	Standing rigging replaced (4 weeks) → 85

Departure score: (30 + 80 + 50 + 45) / 4 = 51 — targeted refit. After the actions: (85 + 90 + 90 + 85) / 4 = 88 — ready to sail. Budget: €11,000 to €15,000 (lithium bank €2,500–3,500, hydrogenerator €3,000–4,500, AIS €600–900, three PLBs ≈ €900, rigging €3,500–5,500). Without the diagnostic this boat would have left with a bank that dies on the first night and a VHF that cannot say where it is.

The D-21 checklist

Ah/day budget written load by load, autopilot included, measured with a clamp meter.
Bank capacity test: real capacity ≥ 70 % of nominal, usable ≥ 1.5 × budget.
Generation without the engine (solar + hydro + wind) ≥ budget; diesel for the alternator as backup.
Water: tanks + jerrycans ≥ vital need × 1.3; watermaker tested, filters and biocide aboard.
Liferaft within its service period, hydrostatic release under 2 years old.
EPIRB registration current, battery valid beyond the return date, self-test done.
DSC VHF: MMSI programmed, GPS connected, test call passed; AIS transceiver active.
Grab bag complete, pyrotechnics in date, 1.5 L of water per person.
Rigging inspected by a rigger, insurer informed of the crossing.
Weather window and bail-out ports written down; at least two crew self-sufficient at night.

Reading the score: 3 decision bands

0–40 — Postpone departure

Too wide a gap between the intended route and the boat's actual state, or a safety axis at zero. Scale back the ambition (coastal) or push back a season.

40–70 — Targeted refit before casting off

Departure feasible after a costed, dated action list: bank, raft, watermaker, DSC, rigging. Re-score once the actions are done, not before.

70–95 — Ready to sail

Autonomy validated for the planned route. Re-score if the season slips, the route stretches or the crew changes.

Key takeaways

The departure score assesses the boat's actual state for a dated route; the weakest axis sets the date.
Typical passage budget: 160 to 360 Ah/day at 12 V, of which 70 to 190 for the autopilot alone.
Usable bank ≥ 1.5 × budget (lead: 50 % of nominal, LiFePO4: 80–90 %) and generation without the engine ≥ budget — the hydrogenerator is the only source that covers the autopilot on its own.
Water: tanks + jerrycans ≥ 3 L × crew × days × 1.3; the watermaker never counts towards the vital need.
Safety: liferaft serviced (3 years), release (2 years), EPIRB registered (5-year battery), DSC VHF with MMSI and GPS, AIS transmitter, in-date grab bag — an expired date scores zero.
Score three weeks ahead at minimum, two to three months if rigging or the bank are at stake.

FAQ

How does this differ from the pre-purchase score?

The purchase score assesses refit potential on a boat you don't own yet; the departure score assesses the actual state of the boat — equipment serviced, tested, consumables aboard — for a dated route. They complement each other at different moments of the project.

How long before departure should you score?

At least three weeks, and two to three months if the rigging or the bank are in question: a liferaft service takes 3 to 5 weeks in season, a rigging replacement 3 to 6 weeks, a lithium battery order 2 to 4 weeks. Scoring the day before departure leaves no room to correct anything.

Lead or lithium for a crossing?

Lithium (LiFePO4) gives twice the usable capacity for the same weight, accepts the fast charge of a hydrogenerator or alternator, and lasts 3,000 cycles. It costs two to three times more up front and needs a compatible BMS, charger and alternator regulator. A new, properly sized lead bank (600 Ah nominal for 200 Ah/day) also crosses an ocean — a six-year-old lead bank does not.

Is a watermaker essential?

No. It brings comfort (rinsing, showers) and a backup reserve, but the vital need must be covered by tanks and jerrycans: 3 L per person per day for the whole passage, plus 30 %. A membrane failing mid-ocean must change nothing about the arrival.

Does the score replace a marine survey?

No. It formalises autonomy preparation (energy, water, safety, route). The structural condition of hull, keel and rigging belongs to a marine surveyor or rigger — the score assumes that baseline is sound, and the rigging inspection is on the checklist.