Sailboat autonomy score before extended cruising

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. This departure diagnostic rates four axes from 0 to 100 — energy, water, safety and boat-crew-route fit — and the weakest one sets the date. It complements the Nomad score, which combines energy, water, food and safety (weighted 25/30/25/20, from 5 to 95): route and rigging are not part of it, hence this grid.

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:

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.

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 — around 100 W from 5 knots, i.e. about 8 A at 12 V (the figure Watt&Sea gives for its cruising models) and close to 200 Ah/day under way: 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.

• • 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.

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

• • Liferaft — serviced at the interval the manufacturer sets: for an ISO 9650 liferaft, French rules (division 333-2) defer to its instructions. Hydrostatic release within its expiry date. Allow several weeks in season.
• • 406 MHz EPIRB — mandatory beyond 60 nm from a shelter under French rules (division 240). Registration is mandatory, with proof on board: in France with the ANFR when the beacon is coded with the boat's MMSI, otherwise with the FMCC (CNES register); elsewhere, with the national authority. Keep the shore contact current. Replace the battery before the date marked on the beacon; self-test as the manual says. 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. French rules (division 240) require a fixed DSC VHF programmed with the MMSI to supply the boat's position at all times.
• • 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, flares and hand-held lights within their expiry date, 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.

• • 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 — its age and condition matter, including to your insurer: tell them about the crossing and ask for their conditions. Rigger inspection 2 to 3 months ahead: a replacement takes weeks and is priced on a quote.
• • 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.

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

Departure score: (30 + 80 + 50 + 45) / 4 = 51 — targeted refit. After the actions: (85 + 90 + 90 + 85) / 4 = 88 — ready to sail. The budget for these actions comes from quotes, item by item, before setting the date: it depends too much on the boat and the yard to be priced here. 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.

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