Select Page

Most problems with marine electrical systems arise from four possible sources, a lack of maintenance, a poor standard of initial installation, insufficient battery capacity, or ineffective charging systems.

Water ingress is a frequent issue – salt water can corrode contacts very quickly. If connections are not scrupulously clean – or are loose – resistance will be increased, resulting in progressively reduced power. Contacts should be cleaned with wet and dry paper until the surface is shiny. Investigate any evidence of water ingress and eliminate the source. Also make sure you don’t confuse a battery that’s almost at the end of its life with one that is simply flat. The old battery may give reasonable voltage readings after charging, but these will fall rapidly when even a small load is drawn and the battery will soon be flat again.

  

Fault finding

This is essentially a case of using logic to eliminate as many potential causes of failure as possible. Occasionally a large dose of perseverance is needed to identify an obscure problem, but equally it’s really easy to overlook an obvious problem, so always start with the basics.

circuit breaker boat electrics

In the case of a non-functioning navigation light, for example, the first action should be to check the fuse or circuit breaker. If not the fuse or breaker, the problem is likely to be a defective bulb, so examine the old one. If it’s blown – shown by a break in the thin filaments within the glass case – it can easily be replaced.

If the bulb appears to be intact, a voltmeter can be used to measure the voltage at the contacts in the lamp unit. If there’s power at the switch panel, but not at the unit, you’ll need to trace the wiring and attempt to locate the break in the circuit. How easy this is to find will depend on the individual boat – some boats may have a number of joins in the wire. In any case, a boat with separate red and green pulpit lights will have a junction box somewhere near the bow, where the single supply from the distribution panel divides to take power to the two separate lamps. There will similarly be a junction somewhere for the feed to the stern light.

A meter can also be used on its resistance (Ω) setting to check whether or not a component is damaged. At the most basic level, electrical current must flow through the component in order for it to work. The resistance function of the meter passes a small current through the device being tested. If no current flows, it records infinite resistance (often shown as a figure 1 on the left hand side of a digital meter display), telling us the component doesn’t work. Note that components must be isolated from the boat’s 12V supply before testing for resistance.

Alternators

Most marine engines are fitted with a belt driven alternator with an output normally ranging from between 40 to 60 amps. The same belt often drives the fresh water circulating pump. The condition of this belt and the alternator itself are both critical to keeping the batteries charged.

Alternators themselves need very little maintenance but correct belt tension is important in order for the batteries to be charged properly when the engine is running.

  • Adjusting belt tension – alternators normally have a support bolt and link adjust bolt. Belt tension is adjusted by loosening these two bolts. The alternator can then be swung outboard as it pivots on its support bolt. The link adjust bolt is tightened first and then the support bolt. The belt should be tightened enough so that it can be depressed by 12mm or so when pushed down by your thumb.
  • Worn belt – alternator belts need to be replaced when they can no longer be tightened sufficiently. Always keep a spare, if not two.
  • Charging test – to test whether the alternator is charging correctly, follow these steps using the instrument panel voltmeter or a digital voltmeter connected to the battery terminals:
  1. Check the battery voltage without the engine running, this should read in the region of 12.5 volts.
  2. Start the engine and check the voltage at idle. It should remain the same.
  3. Increase engine speed to 2000 rpm. The reading should now be somewhere between 13.8 and 14.4 volts.

Alternator problems

The electronic components of alternators can quickly burn out if the electricity they generate is not being channelled to a battery. This would happen if the battery isolator switch has been turned off, which should never be done when the engine is running.

Like all components, alternators can become worn and ineffective. This will need some troubleshooting to determine if the problem is a problem with the wiring, batteries, regulator or the alternator:

  • Batteries not charging – if the batteries are not charging this could be either a wiring problem, battery failure, a faulty regulator or a fault in the alternator.
  • Batteries not charging enough – this could also be a wiring problem, loose alternator belt or the alternator.
  • Batteries over charging – this could be a battery problem or a faulty regulator.

In my experience I have found having an alternator serviced and repaired professionally is the preferred option after identifying that it has a problem.

Jester Challenge 2022 – Sailing single handed from Plymouth UK to the Azores: Part 2 – Weather

Jester Challenge – A modern experiment in old-fashioned self-reliance, self sufficiency, and personal responsibility. This is the second of a 10-part post where solo sailor, Bernie Branfield, shares his first hand account of his single-handed, 2022 Jester Challenge, from Plymouth, UK to the Azores, in his 26′ Invicta Mk2, Louisa.

Capsize – understanding the risks

A skipper should know how their boat will cope with rough seas. By working within known limits and understanding the risks,...

Preparing for sailboat cruising

Preparing for a sailing trip entails a lot of planning. In this blog, we take a look at some of the many safety aspects that a skipper needs to consider before heading off on a cruise.

Rewiring a boat – overcoming the challenges involved

Skippers need to have a basic knowledge of boat electrics, to avoid potential problems and to be able to solve them when they happen.

Nautical paper charts – a reminder of the basics

The nautical chart is an indispensable tool for navigation. A chart is a graphic representation of an area of the sea which might also include coastlines, estuaries and islands. All cruising leisure boats should carry up-to-date paper charts.

Understanding marine sealants & adhesives

Sealants, adhesives and adhesive sealantsThere is a bewildering variety of sealants, adhesives and even adhesive sealants available for...

Seacock maintenance

If seacocks are always left open and neglected they can eventually seize which will prove a serious threat to boat safety should a connecting hose fail and the seacock refuses to close. There are three main types of seacock – ball valves, cone valves and gate valves.

Essential Knots: Sheet bend

Essential Knots: Sheet bend Use: Joining two ropes together. A sheet bend is particularly useful for joining two ropes of different...

Safety Briefings – leave nothing to chance

Before giving your crew a safety briefing, it is worth considering the specific circumstances of the planned trip, the...

How to tackle osmosis

Many owners of old GRP boats live in fear of osmosis, but what exactly is osmosis and what can be done about it? Osmosis comes about...

Boat gas system maintenance

There are correct types of hose for marine plumbing, sewerage, exhaust, cooling and gas and all hoses should be checked regularly for wear and deterioration.

Top five windvane self steering installation questions

Top five windvane self steering installation questions answered by Sarah Curry of Hydrovane International Marine, courtesy of Viki Moore from Island Cruising NZ

Sailing & Motoring in Fog

Sailing & Motoring in Fog You can only measure the visibility accurately if sailing & motoring in fog when you have...

Tools and spares for your boat

Tools and Spares to take to sea!

You won’t regret taking a look at our essential tips and checklist to make sure you’re prepared for routine maintenance and those unexpected jobs that come up whilst you’re afloat!

How diesel engines work

The basic principle of a diesel engine is less complex than that of a petrol engine. No spark plug or ignition system is needed, making the basic diesel engine a comparatively straightforward system that results in fewer faults and has lower maintenance costs than a petrol engine.

Wooden Hulls – Part 2

It is important to ensure the essential hull maintenance of a wooden boat is done, even if you are paying others to look after your boat for you. The priority is to prevent rot from taking hold. The protective layers of paint and varnish over wood are far more critical than on GRP boats, where the topsides are painted more for cosmetic reasons.

DIY boat upgrades: Budget-friendly projects to enhance your (older!) boat

DIY projects that can enhance both the functionality and aesthetics of your boat. In this article, we’ll explore a variety of DIY boat upgrades that won’t break the bank but will significantly enhance your boating experience.

Steel hull maintenance

A steel boat owner’s biggest enemy is corrosion. You don’t have to worry about osmosis or rotting timbers, instead rust is the number one issue that will keep you awake at night.

Essential Yachting + Power Boat Safety Briefing

Yachting Safety Briefing   Down below Lifejackets and harnesses - fitting, when to wear, clipping on Gas - risks,...

Boatyard Health and Safety

Boat storage facilities are potentially hazardous environments and it is the responsibility of both boat owners and boatyards to ensure that the...

Estimating and plotting your position at sea

This post covers how we go about estimating and plotting our position using traditional methods, when out of sight of land – covering Dead Reckoning, Estimated Position, Tidal Streams, Leeway and more…

Boat engine fuel system

If engines are installed and serviced correctly then most marine engines are very reliable, but one of the most important parts of the engine to check and service is the fuel system.

Marine engine oil system maintenance

The regular maintenance of a marine diesel is key to preventing engine failure at sea. This means doing regular checks of the fuel, cooling, electrical and oil systems.

How to ensure your boat is in proper working condition

In this article Eva Tucker from Volvo Penta presents a handy check list of all the things that you need to check regularly in order to make sure that your boat is in a seaworthy condition. Including maintenance, safety gear and electrical checks.

First aid at sea – four common emergencies

In this blog we look at what to do in the event of a crew member choking, drowning, or suffering from hypothermia or fatigue. Knowing how to cope with them could well save a life, while not knowing could result in an avoidable tragedy.