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AQUILA OPERATING MANUAL
NW Explorations
4G: Electrical Systems,
DC
4G1: DC Concepts
Each year it seems more folks are confused by the operation of electrical systems on yachts than by any other
subject! Don’t feel discouraged if something isn’t clear: you’ve got company in your confusion. So let’s try to
cover some theory here first.
Most of the equipment on any boat is run by 12- or 24-volt DC electricity from the boat’s batteries. This is true
because DC should always be available: we have batteries aboard even when there is no shore power! If the
batteries aren’t run down, everything should work, just like in the family car.
Since the batteries are used so much, we have to replenish, or charge them. The most important way we
do this is by alternators on the ship’s engine. In most cases one engine will provide enough electricity in
most every case to run everything, and still have some energy left over to add back to the battery, that is,
to charge it.
Ah, but what if the engine isn’t running? Then, the batteries are slowly depleted until they have “run down” and
there is no more electricity stored in them . . . a big problem, because then we not only can’t run all the neat
stuff on the boat, we can’t start an engine to get more electricity.
So a good skipper and crew has “electrical power management” in mind whenever they turn an electrical
gadget on or off!
It is with this concern that we can cite a reality: If we need more electricity than the batteries alone must
provide, and if the propulsion engine isn’t running, we will need to get our electrical power from an alternative
source! That’s the most important reason why we plug the boat in to shore power or use the generator: To keep
from running down the batteries. For by using battery chargers getting their power from shore power or the
generator, we can keep the batteries charged, or, at least, from getting too low.
In modern, luxury cruising boats, however, there is another important factor: Some of the “goodies” we like to
have on board such as hair dryers and
microwave ovens require ordinary household electricity. This is 120 or
240 volts AC. It is different from DC. So if we want to use these
things when we’re not at a dock, we must have another way to get 110 volts AC, and for this we use the
generator or an inverter, an amazing high tech gadget that takes 12 volts DC from the ship’s batteries and
makes it into 110 volts AC!
So here’s what we’ve got:
•
A lot of stuff running on 12 and 24 volts DC with that electricity from the batteries;
•
To keep the batteries from running down, we have alternators run by the engine, and battery
chargers that get their power from shore power or the generator;
•
For the stuff that runs on 120 or 240 volts AC, we have shore power, the generator, or, for
making AC out of the batteries’ DC, the inverter.