Marine Electronics for Beginners – Terminology and Circuits

Whats a watt?

Up until a couple of weeks ago I had no idea either. So thankfully my friend Chris from Tweeds Marine agreed to do a beginners marine electrical course for the Little Ship Club to help me and other boat owners figure out what a watt is and all sorts of other interesting electrical stuff.

It turns out that a circuit is not something you sail around, a volt is not something you jump over, an amp is not something that makes music louder, a current is not something you eat, an ohm is not something you hum with your legs crossed, the common wire, does not mean a wire that is looked down upon by other more noble wires, resistance is never futile, ground is not something you have to be wary of when sailing, AC DC is not an Australian rock band, and an electrical connection is not something you feel when you are attracted to a person…

Thanks to Alan Wheeler, one of my very witty blog followers for clarifying that much for me!

So now we know what those things are NOT, lets find out what they are…

About two minutes in to our first lesson, I once again found myself wishing I had paid more attention in school. Thankfully Chris is very patient with my persistent questions and his partner Paula said that I was a ‘very engaged student’. I am hoping that is good…

So to start off with we have got four basic concepts:

A Volt (V) – which drives the current through the wire. It is kind of like water pressure. It can be measured with a voltmeter. You have probably got some big 12 volt batteries on your boat that you use to start the boat and to run your power systems – lights, GPS, stereo, fridge, etc.

An Amp (I) or Current – this is the flow of electricity – kind of like the water in the pipe. It is measured in amps using an ammeter.

An Ohm (Ω or R) – resistance, this is the measure of how difficult it is for electricity to pass through the wire. It is measured in Ohms using an ohmmeter. Thin wires have more resistance,  fatter wires have less resistance and can allow more current to flow through.

A Watt (W or P) – a unit of power – the amount of electricity being taken at any time. So a 100 watt lightbulb uses more power than a 60 watt lightbulb.

On land, power companies typically charge you for electrical energy by the kilowatt-hour (kWh), which is equal to 1,000 watt-hours. Watt-hours measure amounts of energy for a specific period of time, and watts measure rates of power at a moment in time. We are going to learn more about this in later lessons, including making a power budget.


So all these things relate to each other. If you know part of the equation – i.e. you’ve got a 100 watt lightbulb and a 12 volt battery, then you can work out how much current in amps would be required by using this handy table!

Ohms Law states V = I *R (or in English) Voltage = Current x resistance



Boats usually use DC power. DC stands for Direct Current – this means that the current flows in only one direction. DC comes mainly from batteries and is a low voltage. Most equipment on small boats is designed to run on DC power.

At home we use AC power. AC stands for Alternating Current – this means that the current flows backwards and forwards really fast and is usually a higher voltage 120 or 220 volts. Some boats that have shore power plugs have both AC and DC systems. AC can be easily transformed in to DC power by using special power supply devices.

Things that use a lot of power – for example TV’s, microwaves, air conditioning etc need to run on AC power.


AC power is very dangerous, and will kill you, but DC power is not so dangerous. Bear this in mind when working with electricity!

Never work on a live circuit, turn everything off at the main breaker and put a note on there saying “Do not turn on!” Unplug both ends of the shore power cord. Check electrical cords for cracks, melted areas, frays etc.

Wear rubber soled shoes to insulate you from the surface you are working on. Never work with wet hands or on wet decks. Wear eye protection.

When installing things consider that the electrical regulations state that the backing board and enclosure must be of a material that will not support combustion.


Series Circuit

Think of a series circuit like a circle. A break anywhere in the circuit will interrupt the current flow. The components are connected end-to-end in a line to form a single path for electrons to flow:


Parallel Circuit

A parallel circuit means that there are many paths for electrons to flow, but only one voltage across all components. (Basically lots of wires and switches going all over the place…)

IMG_1871.JPGSee that diagram drawn above? I did that! 


Wire for use in the marine environment on yachts should be:

  • Tinned copper (silver not copper coloured)
  • Multi stranded (not solid)
  • Should have oil resistant insulation
  • In engine spaces the insulation must be rated for over 70°C

You should use Red (hot) wires coming from the positive side of the battery, and black wires going back to the negative. If you have AC wires then the negative wire should be yellow.

If you use non-marine grade wire then it can rust/corrode quickly in the salty environment. Also if you use different types of wire then they can be prone to galvanic action – where one metal dissolves another.

Boats and motors vibrate, flex and move, so that is why we need to use multi stranded and flexible wire and avoid terminations that could crack.

When installing your wiring throughout the boat, label each connection as to what it goes to. Where a wire runs through a Bulkhead, label it each side of the bulkhead.

Wire Terminations

There will be times when you need to join bits of wire together – like for example at the base of a mast – so you can unclip all the wires when you take the mast out. Or for wires leading to bits of equipment that might need to be replaced from time to time.

At the bottom of my mast I’ve got a chocolate box – which is where you screw one wire down on one side and the other one you want to connect it to goes in the other side. Chris recommends you get a marine grade and one that has a pressure plate between the wire and locking screw. (I am pretty sure mine doesn’t)


If you haven’t got a pressure plate then you can use a ferrule (another new word for me) at the end of your wire and then screw that bit down. (I definitely haven’t got these…)


On the ends of the wires you should have crimped terminal connections. You should try and find terminal connections that are tinned copper (not aluminium) and nylon – translucent (not vinyl) insulation.

Terminals are colour coded for wire side.

  • Red = 0.5 – 1mm
  • Blue = 1.5 – 2.5mm
  • Yellow = 4-6mm

The joint should also be sealed and supported with heat shrink wrap (the adhesive lined stuff is best) remember to put the heat shrink wrap over your wire before you crimp on your terminal… (and then you do the heat-shrinking bit) otherwise you won’t be able to fit it over the terminal.

Place a little bit of heatshrink over the plastic terminal crimp part and the same length out over the wire. If you can, drip just a little hot melt glue inside the heatshrink before heating it to shrink it down. You can buy glue lined heatshrink, bit it is hard to find. This will ensure the wire does not pull out of the terminal and it provides a little strain relief and it helps seal the crimp joint from corrosion. In areas where moisture is higher, a little electrical grease in that crimp before crimping is a good idea as well. (Thanks Alan)


Well!, It’s fair to say it has been a steep learning curve for me. Next week we are going to do some soldering and look at different wire sizes to use. Click here to read my notes on wires and fuses and how to design a boat electrical diagram.


11 thoughts on “Marine Electronics for Beginners – Terminology and Circuits

  1. Like you Vicky I was, sorry, am, hopeless when it comes to all this eletrickery stuff. It just baffles me. Ask me to build a thermo nuclear bomb with an old cornflakes packet and I would have more chance of succeeding than getting a light bulb to work.
    I love the clear diagrams that together with your explanation help me to understand just a little bit more.
    I hope the next post is going to be how to build a 200 gigawatt generator to power our boat.

    Liked by 1 person

  2. Ah, home ground, I was a digital electronics engineer for seven years, but I still find going back to basics great fun, especially with older technology! A yacht is alike ‘going off the grid’ so the more you can learn how to preserve and create your energy the easier it will make your life, I hope he covers electrolysis, that ones a head scratchier!

    Liked by 1 person

  3. Viki, I’ve just been waiting for you to take a class on this and write it up! I’ll take my own someday (when I find my boat), but this tutorial will be added to the others I’ve saved. You are a great teacher! Thanks so much.

    Liked by 1 person

  4. Pingback: Marine Electronics – Charging your Batteries | Astrolabe Sailing

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