Electricity 101 for Boaters

Ian Fortey by Ian Fortey Updated on May 8, 2021. In

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Since the majority of failures in boating have to do with something electrical, we thought it might be a good idea to post an article on electricity and trouble shooting.

Understanding the terminology of electricity is the first step to starting to understand electrical theory. I personally like to compare electricity to the more straight forward discipline of simple plumbing. I am assuming that each of you has, at some time, used a water hose. That is the simple plumbing to which I am referring.

When you turn on the spigot connected to a water hose, the amount of pressure that you have at the end of the hose depends on how much you open the spigot. This pressure can be measured in pounds per square inch. With the spigot opened fully you will have a constant pressure at the end of the hose. The pressure can vary if you reduce the length or diameter of the hose or add a nozzle which restricts flow at the end therefore increasing pressure.

Electricity works the same way. Think of voltage as equivalent to pressure in the water hose and the hose itself as the wire that carries the electricity. Water flowing through the hose has resistance from the inside hose surface itself as well as at the nozzle. This same resistance occurs as current flows through an electric wire. However, in electricity the resistance is measure by a unit called the Ohm . With greater pressure, water flow increases through the nozzle and resistance is greater. The same is true with electricity, with greater  voltage (pressure) the flow of electrical current increases though the wire and resistance (Ohms) also increase. On the other hand, if pressure drops, water flow decreases. The same happens if voltage drops, current slows down causing a “brownout”.

Resistance is an important part of the electrical equation. A smaller hole in the nozzle of a water hose increases resistance. In pipes, friction on the walls makes resistance. In wires there is resistance from the wire itself and, if you add a light bulb with a fine tungsten filament, even more resistance is produced. You might think of the light bulb as the nozzle. The larger the pipe the less resistance is created, the same holds with wire; the larger the wire, the less the resistance.

Water flow is measured in gallons per minute. Electrical current flow is measured by the Ampere or Amp.

So far we have explored in the plumbing analogy the following:



Pressure =


Resistance =


Water flow rate =

Ampere (Current)

How do these all interact? This brings in something called Ohm’s Law. Let’s say you have turned the spigot on only half way, producing some resistance, with a hose attached to fill a pot with water. As you fill the pot with a constant pressure from the spigot you have constant flow rate through the hose. The same is true with electricity; with a constant voltage you have a constant flow of Amps through the wire. If you open the spigot all the way and double the pressure you are also doubling the flow rate. Electricity works the same way, if we double the voltage we double the current in Amps.

Remember the resistance part? Water resistance flowing through a hose is the same as resistance through a wire. This resistance is measured in Ohms. The amount of this resistance can be measured by pressure and flow rate or voltage and amps. One Ohm of resistance lets one Amp of current flow when the pressure is one volt. Two volts would give you two amps of current, Six volts would give you 6 amps, etc. You can calculate the amount of Ohms, Volts or Amps with the following equation.


From this equation we can calculate any of the three variables if we know any two. For example, if we applied 12 volts and were able to measure that the 12 volts was producing 2 amps, what is our resistance in Ohms?

12 Volts/2 Amps = 6 Ohms

What’s a Watt?

Back to our plumbing analogy…the amount of power of the water flow can be controlled. Obviously, the power of a stream of water can be increase by adding a nozzle. (Ever try to wash leaves off the driveway without a nozzle or putting your thumb over the end of the hose?) Power comes from the volume and speed of the water jet. This is similar to power (Watts) of electricity. Volts times Amps give Watts.

As an example of why this is important, try to follow the following example. Remember Volts X Amps = Watts

Each of the lights below are 12 watts. As such they should give about the same amount of light. What we want to know is how many Amps are used under various voltage situations. To measure Amps we will use the formula but the illustration will show an Ampere Meter.

6 Volts

6 Volts X 2 Amps = 12 Watts

12 Watts

12 Volts

12 Volts X 1 Amp = 12 Watts

12 Watts

120 Volts

120 X .1 = 12 Watts

12 Watts

If you study the figure above more carefully you will notice that to produce the same amount of light at lower voltages more current (Amps) must flow to the bulbs tungsten element. In order to have more current flow through the 12 Volt or 6 Volt filament, the filament in the 12 Volt bulb must have a lower resistance than the filament in the 120 Volt light. Consequently the filament in the 6 Volt bulb must have a lower resistance than the 12 Volt bulb so that double the current will flow through its filament even though only half the pressure or voltage is there to make the current.

Remember that resistance, in OhmS, is also easy to calculate. It is the ratio of voltage to current:

Resistance (Ohm) =


Why are we concerned with the current flow rate or Amperes? Well…when connected to shore power or using a generator to generate 120 volts we might not be too concerned. However, when we are at anchor and operating off our batteries, we only have so many Amps we can use before your batteries die.

Using these formulas let’s try an example as shown below.

The diagram represents a 12 Volt battery powering a light bulb. We are measuring voltage with a volt meter, amperes with an amp meter and Ohms with an Ohm meter. (Actually, you can purchase a multimeter which has the ability to measure all of these) As shown, we have 12 Volts, 3 Amps and 4 Ohms. By using the formulas below we can find each of these measurements by only knowing two.

wpe18.jpg (10382 bytes)

12 Volt

= 3 Ampere (Amps)

4 Ohm

4 Ohm X 3 Ampere

= 12 Volt

12 Volt

= 4 Ohm

3 Amps

As the current flows through the resistor (light bulb) the resistor gets hot. How hot you ask, how about hot enough to generate 36 Watts, the heat of a small light bulb.

12 Volt X 3 Amps = 36 Watt

Now that you know the basics hopefully future articles will make more sense. We will, in the future, be covering a few more advanced items and then a series of articles on troubleshooting and solving electrical mysteries.

About Ian

My grandfather first took me fishing when I was too young to actually hold up a rod on my own. As an avid camper, hiker, and nature enthusiast I'm always looking for a new adventure.


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