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All boat owners should have a basic knowledge of electrics, both to avoid encountering electrical problems at sea and to stand a chance of solving them should they occur.

It goes without saying that the marine environment is bad news for electric systems and all their components, so maintaining them in good order is one boating essential that should not be overlooked.

Each and every boat has a unique combination of electronics, equipment, batteries and means of generating power. There is no one instruction manual that will talk you through your boat’s specific set up. So, in order to make sense of their specific boat’s electrical system an owner needs to have a reasonable grasp of how electrics work, especially with regards to boats. Understanding the basics means knowing about current, voltage, resistance and power.

While minor electrical repairs and updates can be carried out when a boat is afloat, major electrical installation work and re-wiring are best done when the boat is ashore. Undertaking a major job such as a complete re-wire is a time consuming operation and this does require more than a grasp of the basics. Unless you are confident you have the necessary skills it is best done by a qualified marine electrician or with the help of someone with advanced skills and experience of boat electrics, as has been my experience. It also calls for careful planning, problem solving skills, plenty of patience, dexterity and good eyesight. Being a contortionist can also help as well.

As with most boatyard jobs, time spent researching, planning, sourcing parts and having the right tools will pay dividends later on.

boat electrics

Basic electrics

Many first time boat owners have little knowledge and practical experience of electrics, sometimes little more than knowing how to wire a plug, change a fuse or reset a trip switch. This can be a little daunting at first, depending on the type and age of the boat, but the good news is that getting started is not that challenging.

The three basic concepts to get your head around are current (I), voltage (V) and resistance (R). Understanding what each means is key to understanding how electricity works –

Current – electric current is the flow of electrons. The number of electrons flowing through a wire per second is measured in amps (A). When used in a formula the abbreviation (I) is used.

Voltage – voltage is the force that pushes the current through a wire – it makes the current move. It is measured in volts (V).

Resistance – electrical resistance is the opposition or obstruction to the flow of an electric current. It is measured in ohms (Ω).

Voltage, current and resistance are closely related to one another and this relationship is known as Ohm’s Law where:

Current (I) through a wire can be calculated by dividing the voltage (V) by the resistance (Ω) or I=V/R.

Voltage (V) through a wire can be calculated by multiplying the current (I) by the resistance (Ω) or V=IxR.

Resistance (Ω) can be calculated by dividing the voltage (Ω) by the current (I) or R=V/R.

There is a fourth basic concept which needs introducing here and that is Power. Electrical power is measured in watts (W) and refers to the rate of energy consumed by a component or appliance. For example, in a domestic household a 2500 watt electric heater consumes electricity at a far higher rate than a 25 watt bulb – 100 times as much.

Power (W) can be calculated by multiplying the voltage (V) by the current (I) or W=VxI.

boat electrics

Essential boat electrics

Small yachts and power boats have 12-volt DC (direct current) systems, although larger vessels will have 24-volt electrics. In most cases the system is split into two parts, one for starting the engine, the other for running all the other electrical equipment on board. There are usually two batteries (or banks of batteries) to ensure there’s always a well-charged battery for starting the engine that’s never used for anything else. In some cases there will also be a third system, with another dedicated battery (or bank of batteries) for powering high current equipment such as electric windlasses, bow thrusters and electric winches.

AC systems

Larger boats often have an alternating current (AC) generator to power domestic electrics. The generator can also charge the DC batteries instead of using the main engine.

The more common type of AC system relies on connecting to shore power when the boat is docked. This can be used to run domestic appliances and also to charge the batteries. Note that AC and DC systems must be kept separate.

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