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-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 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 volts AC, we have shore power, the generator, or, for making
AC out of the batteries’ DC, the inverter.
Section 4G: Electrical Systems, DC 4.12
