A charter yacht lives on her batteries from the moment the shore cable comes off. At check-in you find out how the banks are split, what each breaker feeds and what the numbers mean; during the week you spend amp-hours like fuel.
1 · Batteries and switches
Two banks, kept apart
Why does a charter yacht have separate engine and house batteries?
So a house bank flattened overnight still leaves the engine able to start: each engine has its own starter battery, and the house bank runs everything else. Switch to BOTH only to start on a flat battery.
Each engine has its own starter battery; the house bank runs everything else. Kept separate, a house bank flattened overnight still leaves the engine able to start. On a catamaran ask which alternator charges the house bank: often only one does.
Battery switches
ENGINE, HOUSE: on at sea and at anchor
BOTH: only to start on a flat battery
Then BOTH off and an hour's charge
Engine running: never switch ENGINE off
Leaving the boat: all off
Switching the engine battery off while the engine runs can wreck the alternator. Terminals should be clean, tight and covered: a spanner dropped across them starts a fire, so tightening them is the base's job. Learn where the master switches are, for the dark.
Leave the battery vent open: batteries on charge give off hydrogen. A hot, bulging battery that smells of rotten eggs is being overcharged: charger off, hatches open, no sparks, call the base.
2 · Volts and amp-hours
Read the bank at rest
At what voltage do the boat's batteries need charging?
At about 12.2 V on a rested lead-acid bank: that is half charge, time to charge. 12.7 V is full and 12.0 V or less is flat; read it only after an hour with no charge and no load.
The voltmeter tells the state of a lead-acid bank only when the bank has rested: no charge and no load for an hour or so.
Rested volts
House bank
12.7
Full
12.4
About 75 %
12.2
Half: time to charge
12.0 or less
Flat
On shore power or with the engine running you read the charger, not the battery: up to 14.4 V in bulk, 13.8 V on FLOAT, when the bank is full. Lower volts with the amps negative mean nothing is charging.
A battery monitor counts amp-hours out and in since the last full charge: 76 Ah used from a 200 Ah bank is 62 %. If it says 100 % while the volts say 12.1, it has lost count: believe the volts. A lithium bank is the opposite: it holds about 13 V until nearly empty, so trust the percentage.
An old bank that falls to 11.9 V after a quiet night has lost its capacity. If the alternator fails mid-week: cut every load, make for shore power, call the base.
3 · Power budget
Amps times hours
How many amp-hours can I actually use from a 200 Ah battery bank?
About 100 Ah: only half of a lead-acid bank is usable between charges. Budget in amp-hours, current times time: a fridge drawing 5 A for 10 hours takes 50 Ah.
Current times time gives amp-hours. Only half of a lead-acid bank is usable, so a 200 Ah bank gives you about 100 Ah between charges.
One night at anchor
House bank 2 × 100 Ah: 100 Ah usable
Fridge 5 A × 10 h = 50 Ah
Anchor light and phones: a few Ah more
Morning: run the engine for an hour
With no solar and no generator, plan an hour of engine morning and evening and switch off what is not in use. A solar controller showing +7 A at 13.9 V is charging, but three fridges drawing 12 A round the clock take about 290 Ah a day, more than the sun gives back. Before a windy passage charge full: autopilot, plotter, VHF and bilge pump all run on that bank.
The 230 V sockets are live only on shore power or a generator. An inverter makes 230 V from the batteries, at a price: a 1,600 W hairdryer draws about 140 A (1,600 ÷ 12 = 133 A, plus the inverter's losses). A small one runs a laptop, not a kettle, and left on idle it still takes about 1 A: 24 Ah a day.
4 · Shore power
The cable goes live last
How do I connect shore power safely in a marina?
With the boat's 230 V main off, plug in at the boat end, then the pedestal, then switch the pedestal on; the 230 V main and the charger go on last. Never hold a live plug.
230 V on a wet pontoon can kill. Never hold a live plug, never accept a cable with a cut sheath, and never push bare wires into a pedestal: ask the marina office for an adapter.
Shore power, in order
Connect: boat's 230 V main off
Boat end, pedestal end, pedestal switch
Then the 230 V main and the charger
Leave: charger and main off, pedestal off
Unplug the pedestal first, then the boat
On the 230 V panel
What you do
Reverse-polarity light on
Unplug and tell the base
RCD trips with one appliance
Leave it off: it leaks to earth
Pedestal trips at breakfast
One big appliance at a time
Watts divided by 230 give amps: a 1,200 W water heater, a 2,000 W kettle and a 500 W charger together draw 16 A, too much for a 10 A pedestal. Keep the cable out of the water; nobody swims in a marina: stray current kills.
5 · Panel, lights and pumps
Every breaker shown, every light seen
Which breakers should stay on overnight at anchor?
Three: the anchor light, the bilge pump and the gas alarm. The all-round white anchor light burns every night at anchor (COLREGs Rule 30).
Have each breaker switched on in turn: you will be looking for them at night. Test the lights after dark, from the pontoon.
Navigation lightsunder power, from ahead
Anchor lightall-round white at the masthead
At anchor the all-round white light burns every night (COLREGs Rule 30). If it fails, hang a white lamp in the foretriangle; nobody climbs the mast in the dark. Overnight three breakers stay on: anchor light, bilge pump, gas alarm.
Bilge pump: switch to ON and hear it, lift the float to prove AUTO, find the hand-pump handle. Running in a dry bilge, its float is stuck: pump off, check the bilge by eye, call the base.
Gas and CO alarms: press TEST on each.
Windlass: engine running whenever you use it.
Electric winch: everyone knows where its isolator is.
Generator: its exhaust carries carbon monoxide; nobody swims at the stern while it runs.
6 · Fuses, breakers, fire
A fuse protects the wire
What do I do if a fuse keeps blowing on a charter boat?
Fit a spare of the same rating, never a bigger one; if it blows again, leave it out and call the base. A fuse is there to stop the wire overheating.
A breaker resets; a fuse needs a spare, so ask where the spares are kept. Both are there to stop a wire overheating.
What happens
What you do
A fuse blows
Fit a spare of the same rating, never a bigger one
It blows again
Leave it out and call the base
A breaker trips again at once
A short circuit: leave it off
Smoke behind the panel
Battery switches off, then the extinguisher; no water
Instruments dead by day? Steer by compass and paper chart, and ring the base.
Practice questions
Questions from this topic, as the app asks them. Open one to see the answer.
Catamaran 42 ft — batteries
Port engine: 1 × 80 Ah
Stbd engine: 1 × 80 Ah
House bank: 3 × 100 Ah
Fitted 2022 (base)
The technician opens the battery lockers of the catamaran and shows you this. What do you want to be sure of before he closes them?
That each engine has its own starter battery and the house bank is separate from both
That the house bank starts the engines when the starter batteries are low
That the port engine battery also runs the fridge, being the strongest one
That all five batteries are wired as one bank, for the most capacity
Show the answer
A. That each engine has its own starter battery and the house bank is separate from both
Separate banks: a flat house bank after a night at anchor still leaves each engine its own battery to start.
All 50 questions of this topic are in the app: practice by topic with the reason after every answer, your mistakes brought back, and a timed mock exam. Free, no sign-up, in the browser or in Telegram.
