Narrow boats use these engine powered mini generators for charging batteries and producing electricity to run appliances.

230v battery charger on a narrowboat
230v battery charger on a narrowboat

Alternators… Engine Mounted Generators

What is an Alternator?

Well basically an alternator is a generator that takes it’s name from the term Alternating Current. It is also sometimes referred to as a Travel Pack.

Why are Alternators Used?

In three words, for power generation. Narrow boats may have up to 3 alternators fitted. Many older craft will only have one. They will all operate in essentially the same way, by generating AC electricity.

Single Alternator Set-ups

In these simple systems the narrow boat engine alternator is used to generate electricity for running and starting the engine and to provide power for the leisure circuit. The way we store power is by using batteries, which can only store direct current or DC. In the case of a single alternator system, the units AC output is passed through diodes, which block and direct the current. The diodes act like a valve which only allow direct current to pass through to the canal boat batteries.

Narrow boat alternators come in a variety of different outputs. We must match the ability of the alternator to replace whatever we consume from the batteries.

A common mistake made, for example, in the self build camper market is that people think that a massive battery will run fridges, music, lighting and all other power requirements and then wonder why the battery only lasts a season. The most likely cause is that they are taking more out of the battery than can be replaced by the charging system and as a consequence the battery is never fully charged and subsequently dies from sulphation.

Back to our narrowboat market and now we can see why it is important to have a well designed and balanced electrical system, based on what we are physically able to put back into the storage batteries for DC circuits and how we supplement DC for energy hungry AC appliances.

Multiple Narrow Boat Alternator Systems

A second alternator may be added to specifically feed power to the leisure battery bank and/or the narrow boat bow thruster battery. This allows the engine alternator to focus all it’s output into running the engine and keeping the starter battery in good condition.

A third alternator may be added which outputs pure AC electricity and is thus part of the AC circuit and totally separate to the DC system.

The High Output Alternating Current Engine Driven Generator

Marine specification high output AC alternators are usually bracketed to the engine & driven by a system of pulleys and a V belt. The pulleys are matched to engine revolutions to ensure that AC output is optimised, usually at around 300volts. A sophisticated box of tricks then converts this AC to DC and this DC voltage is inverted to give a regulated 230VAC/50Hz sine wave power output. In other words, the electricity generated from these bolt-on engine powered units is the same quality as our common “mains” electricity and therefore will run appliances with no adverse effects.

An engine powered AC alternator is a great space saving alternative to an auxiliary petrol or diesel powered generator. Top-end, sophisticated systems can also use an additional inverter connected to a DC storage battery bank to provide up to 7Kw of peak power. Standard systems will produce around 3.5Kw.

For continuous cruisers an AC alternator can prove both valuable and cost effective. An often overlooked advantage is the facility of charging leisure batteries directly from a 230v intelligent charger / battery management system rather than a 12v engine alternator. In theory this can provide you with an extra 20% of battery capability, something any continuous cruiser will openly welcome!

Depending on what engine you have installed a 230v alternator can also prove a much quieter method of generating 230v power in comparison to a traditional portable generator. Generator noise out and about on the canal and river system is something many leisure boaters do not appreciate nor sympathise with. It may also be worth mentioning in practice you may need to run a 230v power source to a battery charger for around 8hrs to enter in to the fully charged aka ‘float state of charge’.

If you are buying a new canal boat consult your narrowboat builder to understand what specification is included in your purchase and if there is any upgrade options available. Likewise if you are purchasing a sailaway narrowboat, check the specification to see if this is sufficient for your needs. Although not impossible to retro fit an alternator, this is really a decision to be made at the build stage.

AC Engine Alternators

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crick boat show 2024

(Saturday 25 to Monday 27 May 2024)


Britain's biggest inland waterways show.

Advance tickets can be ordered by phone and online until 12midday on Friday 17 May 2024. If you are planning to purchase advance tickets, we recommend that you do so well before this deadline to allow time for postal delivery.
Children 16 and under – free entry Saturday, Sunday, Monday

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Get ahead and visit The Fit Out Pontoon before everyone else on Preview Day!
Preview Day tickets are strictly limited to a maximum of 1,000 pre-booked visitors and give entry to the Show from 10am – 6pm on Friday 24 May.