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Both electric anchor winches and bow/stern thrusters present similar problems related to very large current flows and the need to reverse the motor, so they will be dealt with as one item – after all they all use one large motor that needs reversing.

From a purely electrical point of view the best advice is to forget electrical units and go for engine driven hydraulic ones, that are much more suited to providing high powers, and probably have far lower long-term costs. However practically electrical units are often the only practical answer – especially as retrofits.

Although some fairly powerful 12v, permanent magnet, electric motors have been developed for car power steering I have yet to see them in marine use. That being the case the motors used require fairly complex switching to obtain reverse rotation. Half the motor needs the current to continue to flow in the same direction all the time, and the other half needs the current reversed to reverse the direction of rotation.

The problems can be split into the items below:

      1. Very heavy current draw – the smallest bowthruster is rated at 1500W, which equates to 125 amps at 12 volts. Probably two or three times as high as it starts or if it jams and stalls the motor!
      2. The need to reliably switch such large currents and fairly complicated reverse switching.
      3. The absolute need to minimise voltdrop between battery and unit – this requires very large, expensive cables.

ITEM 1 – Large current

First it is vital to look at a special power audit for the device. I have seen bowthrusters used for at least three, two-minute bursts when manoeuvring in windy conditions. This equates to 12.5 amp hours for the smallest unit, and as that should equate to 30% of battery capacity (see batteries & charging systems) this gives a battery capacity of 41 amp hours. This is a larger capacity than many cars’ starting batteries. The larger the unit, larger the battery capacity gets. This current is more like a starting load rather than a domestic load, so the battery should be a starting type battery. The need to protect engine starting from flat/damaged batteries indicates that it is sensible to use a separate battery for this duty, but the charging of the battery now becomes a further complication.


ITEM 2 – Complicated switching

The manufacturer solves this problem. Their unit will be equipped with two special solenoids (a bit like the starting solenoid) that are often called contactors. These are controlled by a low current circuit (again just like starter solenoids) from a two way switch that is placed in a position that allows easy access and operation.

The unit will be connected to its battery by two very large wires, one positive and one negative, and by a minimum of two thin wires that supply current to the forward or reverse contactor. This switch will also need a supply from the relevant battery. The supplier should supply full instructions and a wiring diagram.


ITEM 3 – Avoiding voltdrop.

The starting current is so high that a partially discharged battery or excess votdrop on the cables would prevent the motor starting, leaving it stalled, drawing a huge current and trying to burn itself (and your wiring) out. To prevent this ALL such motors should be fitted with either a thermal cutout or an over current cutout (a bit like a slow blow fuse) during manufacture. Check that any unit you are interested in has been so fitted. If you already have such a unit without thermal or over-current protection consult the technical department of a manufacturer of circuit breakers to find one that will protect the circuit by tripping on overload, but does not act so rapidly that it trips as the motor runs up to speed.

I would also advise that you see if you can accommodate a dedicated battery very close to the unit. This minimises cable length and thus voltdrop, however you must consider the dangers from the hydrogen and oxygen given off whilst charging/discharging and the danger of acid spills when seeking a location for such a battery. Also consider the ease of access for regular maintenance.

If you do decide to use a dedicated battery, wherever it is located, you must organise a means of charging it.


Charging the battery.

You will need to connect the battery to the charging system whilst the alternator is charging, and disconnect it when the alternator stops charging. Basically whatever method you chose to split the charge between engine and domestic batteries can be employed again for this duty. There may be problems with the length of charging cable running from the charge splitter to the remote battery, but these can be largely solved by using cable a bit thicker than calculations would suggest is adequate.

The lack of battery temperature sensing for advanced controllers may also cause a bit of a problem, but this is unlikely to demand more than more frequent topping up the battery with distilled water.

High current diagram

Once you have a 60 to 70 amp alternator split between engine and domestic batteries, the need to charge thruster or winch batteries plus a possible inverter supply, starts to justify a second alternator.


Check all connections for cleanliness, tightness and security on cables – especially the high current ones.

Check state of battery charge, top up, clean top and case, clean and re-dress terminals.

Check all wiring for chafing.

Top up any oil bottles or apply lubrication as recommended by manufacturer.

Visual inspection of contactor contacts and motor brushes (if you know how – otherwise call someone in when it stops working)

At long intervals clean brush dust from motor and renew brushes if required, but as part of the motor will probably need to go on a lathe it might be better to wait until failure and then take the whole motor to a specialist.


Fault finding

  1. Motor only runs one way – suspect faulty contactor, burnt contacts or solenoid coil burnt out, faulty control switch – try pulling the wires off the bottom and making a physical contact with the supply.
  2. Motor runs, but no drive/thrust – suspect broken drive pin, sheared drive bush, stripped drive splines.
  3. Motor slow or will not run in either direction.

Check circuit breaker or fuse.

Allow to cool in case thermal cut-out has operated (may require a manual reset – find out before it happens!!!

Check battery for condition and state of charge.

Check terminals for security.

With power turned OFF try to push armature round by hand – it may be jammed or seized.

Check motor brushes.


These are another set of devices that demand very heavy currents and suffer badly from voltdrop on their supply lines. They also tend to be used for far longer than bow thrusters and anchor winches etc.

Any inverter that is not isolated from its supply – turned off – will also draw a small current all the time. This will probably be quoted as about 0.5 amp, but using 1 amp in the audit (giving 24 amp hours per day) would be safer.

It is vital to do a power audit. A 70 amp alternator will only be able to provide total support for a 700 watt inverter. Using more will start to draw upon the batteries.

It is also vital to adhere to rules like only using certain loads when the engine is running, and also to turn the inverter off when not in use – that means NOT leaving it on standby. This may not apply to installations using something using a large generator (engine driven or self powered) together with a very large battery bank, but unfortunately when people start to calculate the cost of large battery banks corners start to be cut.

For reliable operation that minimises impact upon other services one should:

Consider a dedicated battery bank so the inverter cannot flatten the domestic batteries. The size will be dictated by the power audit (remember once you have 220 volts you usually find more and more to do with it), and it will require its own charging circuit. As stated in the previous chapter using a dedicated battery bank starts to make sensible use of a second engine alternator.

Mount the inverter as close to the battery bank as possible, whilst protecting it from damp and heat (the electronics need cooling air flow and I do not want 220 volts flying about over damp circuit boards etc.).

Supply by the thickest wire you can get hold of.

Fit the on-off switch in an easily accessible place.

Fit an easy to see warning lamp to indicate when the inverter is turned on.

The table below shows the consumption and the battery capacity needed to service that consumption with no supply from the alternator.


Time used


Battery capacity

600 watt drill

2 hrs during day

120 amp hours

400 Amp hours

150 watt sander

4 hours per day

60 amp hours

200 amp hours

1200 watt saw

Half hour

60 amp hours

200 amp hours

850 watt Hoover

Half hour

43 amp hours

144 amp hours

1800 watt hair dryer

15 minutes

45 amp hours

150 amp hours

27 watt battery charger

8 hours

22 amp hours

72 amp hours


The battery capacity calculation assumes no advanced charge controller, but even if you have one, you can only reduce the battery capacity required by 20%.

It also makes no allowance for the stand by current that the inverter will draw. This could be over 20 amp hours over a 24-hour period, requiring another 80 amp hours worth of battery capacity to supply it.

If you are supplying your inverter from your domestic battery bank you can see how important the power audit and increasing the bank size is.

Service and Fault Finding

We are dealing with fatal amounts of electricity. Do not fiddle with the mains side of things or inside the inverter unless you are sure you know what you are doing.

Just ensure the low voltage side of things have good, clean and tight terminals, well secured to the cable.

12 Volt Verse 24 Volt Systems

At first sight a 24 volt system looks very attractive when you start to consider high current devices like inverters and thrusters.

It is true that a 24 volt system only draws half the current that a 12 volt system uses for the same job.

It is true that a 24 volt system uses batteries of half the capacity of those on a 12 volt system.

It is true that a 24 volt system uses cables of half the size of those used on a 12 volt system.

However things are not as clear-cut as they seem:

    1. 12 volt equipment like radios, lights etc are easily available and relatively cheap because they are based on equipment designed and costed for the car and leisure vehicle industry.
    2. 24 Volt equipment is based on commercial vehicle technology and is therefore less easy to obtain and also be more expensive.
    3. You only buy the thicker 12 volt wiring once and there is not normally a problem accommodating it in a boat (unlike some luxury cars). Things like alternators and starters may well be replaced during the boat’s life and 24 volt equipment will cost more.
    4. Although you only need batteries of half the capacity on a 24-volt system, you need twice as many of them to get the 24 volts, so there are no cost or space savings.

12 volt system requires 220 amp hours – that is 2 X 12 volt, 110 amp hour batteries in parallel.

24 volt system requires 110 amp hours – that is 2 X 12v, 110 amp hour batteries wired in series.

Things may be different when the car industry goes to dual 48/12 volt systems, but at present they are still talking about keeping nearly everything apart from starting, power steering and charging at 12 volts. I can see no pressing need to go to a 24 volt system for most boats (as opposed to mini-ships).


The worst 12 or 24 volts can do is to set fire to your boat, you should have at least even odds on escaping, and more usually a fault just blows a fuse.

One fault or error at mains voltage and someone can easily be dead – TREAT WITH THE GREATEST RESPECT –TAKE QUALIFIED ADVICE, CHECK, RETAKE QUALIFIED ADVICE, CHECK AGAIN.


Never work on mains electricity unless you are accompanied by someone who knows what to do in case of an accident and can get help!

This section is intended to give you some guidance by which to assess the professional’s work in the UK – it is not intend to be an authoritative document.


The codes of practice can be found with such bodies as the Institute of Electrical Engineers (16th Edition), Maritime & Coastguard Agency, Lloyds etc.


Basic information

This section only covers single phase electricity like we have at home, if you want three phase, as you might have in a factory, you are on the wrong course or looking in the wrong place! Do not worry if you do not understand this paragraph, its purpose is to set a limit on the area covered.

The mains electricity keeps reversing itself at 50 times a second, so you cannot have a positive and negative part of a circuit.

One wire is called the LINE. This one can hurt you and should always be wired in a BROWN cable (kept well away from the low voltage circuits).

The other wire is called the NEUTRAL. This one should not (in a correctly wired boat) hurt you and is wired in BLUE cable (keep well away from the low voltage circuits).

To give you protection from faults and working in conjunction with the Residual Current Device (RCD) or mains fuse an extra wire called the EARTH is also used. This wire will also not hurt you and should be wired in a green wire with a yellow trace.



Mains circuits require three forms of protection:-

      1. Much thicker insulation on the cables to resist the much higher voltages.
      2. Ordinary circuit breakers or fuses to protect the wiring in individual circuits – just as in 12v dc work.
      3. A Residual Current Device (RCD) to protect living beings from mains faults.

So never be tempted to use an odd length of 12v cable to complete a mains circuit – if you must you can use mains cable on 12v circuits.


Circuit breakers

These are very similar to the ones used for the 12v circuits except they will be rated for ac mains.

They will monitor current flow by thermal and/or magnetic means.

They are totally "self contained" and only need inserting in a single wire to protect a circuit.

Residual Current Devices

RCDs react far more quickly and at much lower fault currents than fuses, so they are to be preferred for personal protection from mains faults.

One of these is inserted as close to the origin of the mains electricity as possible. It should be rated for the maximum mains load.

They work by magnetic means AND need a way for the electricity to bypass one of the wires in the circuit. This is provided by the earth wire.

A very simple diagram of the RCD is shown on the next page.

A Simple RCD Diagram


When the reset button (not shown) is pushed the contacts are closed and are latched together by something like a very light mechanical latch or magnet.

As the contacts are now closed current flows through both the line and neutral winding forming electro-magnets.

Because the two coils are wound in opposite directions they make opposite magnetic fields. As the same current is flowing in each coil they both create the same amount of magnetism, so they cancel each other out, thus there is, in effect, no magnet.

When a fault occurs most of the electricity flows back to where it came from via the earth wire instead of the neutral. This causes an imbalance in the magnetic fields.

Now the Line coil is stronger than the neutral, so it is not cancelled and can thus pull the contact apart; breaking the circuit and protecting you from the fault that would otherwise attempt to kill you.


Without a working earth system the RCD cannot work

EARTHING – Wood or GRP Hull – Onboard Generation

If the mains electricity is generated on the boat by generator or inverter the manufacturer should (in most cases) have bonded the neutral output wire to the earth cable outlet.

The non-conductive nature of the hull makes hull earth bonding unnecessary, but it is vital the earth cable joins all outlets and also is bonded to any metal cases on mains powered equipment.

Certain other metal parts of the boat’s fittings will also require bonding to the earth cable. These will be such things as sinks, taps, metal wastes etc. and any other metal (conductive) parts that can conceivably make accidental contact mains electricity.

If this is done any electricity "leaking" out of the line circuit onto a conductive material (where it can give a fatal shock) will return via the earth wire so the RCD trips.

EARTHING – Metal hull – Onboard Generation

With onboard generation and metal hulls not only should all the above be carried out, but it is possible for a line fault to make contact with the metal hull. To force this to trip the RCD before it electrocutes someone the hull should also be bonded to the earth cable.

If any electricity then leaks to a conductive surface or the hull the RCD will trip before any injury is caused.

Hull bonding

Many people recommend bonding the mains earth to the hull by utilising the battery negative connection.

This is a very dangerous practice for two reasons:

  1. There is no guarantee that a flexibly mounted engine will always have good electrical contact with the hull, and if it does not, a fault to the hull will not trip the RCD.
  2. If the connection fell off, whilst maintaining contact between battery negative and the mains earth bond any mains fault would, in all probability, put mains voltage onto the 12v circuits with possible very expensive and fatal results.

Make the mains hull bond to its own earth point. One fairly easy way of doing this would be to drill (and tap if you know what to do) one of the welded-in engine beds. Remember to use some form of locking device on the nut or screw!

Other methods would be by drilling a welded-in, metal bulkhead or by getting a setscrew welded to the hull to form a stud.


NOTE – this section only covers the UK.

Once you introduce a shoreline you begin to start adding problems:

      1. Submerged metal that is in contact with the mains earth system, either by design or fault, may be eaten away because the earth system connects your boat’s hull to all the other similarly equipped boats hulls via the earth wire. This means that there is a DC path between all the boats connected to the mains earth, and that will allow DC currents to flow between the boats. These currents are caused by differences in any connected but submerged metals. (galvanic corrosion)
      2. Under fault conditions it is perfectly possible to give swimmers and those stepping on or off your boat a fatal shock.

To understand the problems so it is possible to make informed decisions this aspect will be looked at in some detail, even if it is rather simplified to aid understanding.

The diagram below represents some kind of a shoreline installation that has no earth bonding to the hull or submerged metal parts.

At the substation the neutral connection on the transformer is also connected to an earth cable (usually the metal sheath on the underground mains supply) plus a direct connection to the ground – hence the name EARTH.

At your house all metal parts that might come into contact with the LINE under fault conditions are also connected to the earth cable. If a fault does connect the line to something that can give a shock the RCD becomes unbalanced by the fault current flowing through the earth line, so it trips out, cutting off the electrical supply.

Now the boat – for some reason the metal parts have not been bonded to earth (someone may have thought they were protecting their hull). If a fault connects the hull or metal parts to the LINE the whole boat/metal part becomes live.

With a bit of luck the resistance of the water would be low enough to provide some form of fault path back to the earth point at the substation (or somewhere within the marina’s system) so the RCD trips – if one was fitted at the entry into the boat, or if the Marina’s RCD is in good working order.

Unfortunately there are rather a lot of "Ifs" in the above description and it would be all too easy for the hull to become live with sufficient resistance through the water to prevent the RCD from tripping. If there was no working RCD, the hull would just remain live until: - - - -

If a swimmer makes a circuit between hull and ground, when someone steps off the boat, or if they step onto another boat that is correctly earthed they complete the fault current path with possible fatal results.

Now who fancies being sued for negligence?

The obvious answer is to bond the main’s earth to the boat’s hull, so in the event of a fault there is an easy path back to the substation to trip the boat’s RCD.

However this again poses a possible problem as shown on the next page.

Now if a fault does develop there is an easy fault path from the hull and back along the earth cable, so the RCD will trip. Depending on a number of other things a fuse may even blow, so you are protected.

Unfortunately there is now a path for the low voltage galvanic DC current produced by the immersed anodes that are trying to protect your boat from corrosion and any other submerged metal work (a boat shown) that is not protected or is protected by an anode made of different metal.

The result is that the small voltage causes DC current to flow through the earth cables, your hull, the water etc.

Regrettably this current flow will start to eat away at the hull and any underwater metal fittings that are earth bonded for safety.

Hopefully you will be alerted to this by very rapid anode erosion when compared with a totally 12/24 volt boat, but anode erosion is not certain, it could just as easily be your hull!

Isolation transformers

Probably the safest long-term way to make shoreline connections is with an earth-bonded hull and via an isolation transformer.

The isolation transformer breaks all physical connections between the shore mains and the boat’s mains, so it acts a bit like having a generator or inverter on board. All you need to do is to bond one of the isolation transformer outputs to the hull etc and also to connect an earth cable to the same point and you end up with a LINE (unbonded cable). Neutral (bonded cable, but running to all neutral connections), and EARTH (the other bonded cable that is joined to all the earth connections. This is illustrated below.

Now there is no earth connection between boat and the mains there can be no damaging current flow.

Because the boat is bonded to earth (see metal boat earthing above) WITH AN RCD FITTED CLOSE TO THE ISOLATION TRANSFORMER you are protected from faults.

However Isolation transformers are: -


Quite large and heavy

May well require different wiring for other countries.

Should be SOFT START to prevent tripping the marina circuit breakers as it starts up – they have a very high starting surge current!

Galvanic Isolators (Zinc Savers)

There is another option that involves introducing diodes into the earth bonding circuit, as shown below.

The mains’ earth now runs right into the boat’s system, but the GALVANIC IOSOLATOR prevents that small current flow in the earth circuit that eats away at the anodes or hull.

As a bit of a dc ludite I cannot readily understand how they work, unless they create a voltdrop that is approximately equal to the voltdrop through the ground. I have seen too many failed alternator diodes to personally trust my life to them – especially as its not very clear how they can be wired to either fail-safe or give a failure warning.

I have seen some correspondence that suggest that an American standards organisation is not very keen on them, however there are many people who appear to be happy with them.


It is important to make provision to ensure that only one source of mains supply is connected to your boat at any one time. If you manage to connect two, you may find some very odd and expensive things happening.

Ponder the reliability of automatic sensing and change over devices and match that against your ability not to forget to turn one supply off before turning another on.


Because of the dangers associated with mains work I will not go into wiring the various mains circuits inside the boat. They will be similar to the DC circuits with fuses to protect the cables, plus earth wires to all outlets and pieces of fixed metal equipment or equipment with an earth connection. They will require correctly insulated mains cable to be used.


Nowadays there are several pieces of "all singing and dancing" mains powered equipment available that combine several functions into one unit.

These should be fine when they work well, but do think about what happens if one function fails. A battery charger & inverter combination would leave you without mains whilst you await the repair of the battery charging part, and so on.


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