Some of us at Antares Yachts are old enough to have worked on the construction of cruising vessels back in the 1970’s. The most obvious change in the fit-out of current cruising vessels is the extensive use of electrical power on board with all of its attendant hardware. This has resulted in a level of complication that would have astounded a traditionalist in the 70’s.

This may be compared with Antares vessels that are equipped with a multitude of hydraulic/magnetic breakers, switches and battery banks to supply an extensive array of electrical equipment, standard and optional. These days the customer expects to be able to support all the appliances and electronics of home in addition to the ship’s mechanical and navigation requirements.

The relatively enormous power requirement for this equipment is not apparent to most users who are accustomed to living with electricity delivered by the continental power grid. The vessel now has to act as a multi-source power station and distribution grid, an especially onerous task when away from dockside shorepower. All this has to be accomplished with the minimum of weight, complication and operator confusion. Ironically, this “high tech” requirement benefits from the “lowest possible tech” installation to achieve reliability.

Power Sources

  • 120 Volt AC
    • Shore Power
    • Generator
    • Inverter
  • 12 Volt DC
    • Engine Alternators
    • Battery Charger
    • Solar Panels

We commonly use 120 volt alternating current, (ac) power on board to supply household type equipment and 12 volt direct current, (dc) for ship’s operational loads.

We use dc power because it can be easily stored in batteries, unlike ac power which has to be generated on demand. This allows us to start engines, for example, with dc power from the chemically stored energy in the batteries. The batteries should be considered as a storage system rather than a prime source. The tank of diesel fuel stores immensely more energy pound for pound, but its energy requires conversion through an engine or burner to be usable, and it requires periodic renewal from the refinery.

120 Volt AC Power

Shore Power

When tied to the shore at a service dock, we have available the same 120 volt ac electricity that is supplied to virtually every household on the continent. The average home these days is built with an installation sized to deliver 200 amps of power flow at twice the voltage (think of voltage as pressure available to a garden hose, and amperage as the water flow rate through the hose, limited only by the resistance due to the hose size).

The power available is proportional to the voltage times the amperage, (wattage). Our vessels are typically “plugged in” to a shore power outlet with 30 amp capability. A little math will demonstrate that at the dock, we have about 7% of the power available to most homes. Unfortunately, we frequently need to support the heaviest loads common to both applications such as air conditioning (the utilities always experience their worst load conditions on the hottest days of the year). Powering air conditioning is a particularly difficult on board problem as there is no alternative power source such as propane that can be used for this function. Cooking, for example can be accomplished using gas or low demand microwave in preference to the electrically hungry household electric range. We find it expedient to install two 30 amp shore power cords in order to help support air conditioning loads.

When not tied to the dock, if we wish to enjoy 120 volt ac power, we have to resort to an ac generating set or an “inverter”.


A GENERATING SET essentially converts diesel fuel into electricity via a diesel engine turning electrical windings at a specific speed to create power closely resembling what is available from shore. There is little intuitive appreciation of the energy content in hydrocarbon fuels until one realizes that it takes the power of two “horses” working hard to light ten 100 watt light bulbs (1kw), and we install six times that much power with a typical 6 kilowatt gen set powered by a 12 horsepower diesel engine. This installation provides approximately 10% of the household power described above and about 1.6 times the power of one 30 amp shore cord, enough to power the vessel under normal circumstances.


An INVERTER is an electronically controlled device that takes 12 volt dc power from the ships batteries and converts it with some losses into 120 volt ac power. Even small capacity inverters place large dc loads on the batteries. A 2000 watt ac output inverter needs to be connected to the batteries with cables exceeding the capacity of even the engine start wiring. The math here says we will need to supply dc current of approximately 10 times the ac output current, say 166 amps plus losses in the inverter on a continuous basis. This is not a load that can be sustained for very long without enormous battery banks.

It must be appreciated that you can’t get something for nothing so the drain in the batteries has to be made up sooner or later by battery charging capacity. We consider that the use of an inverter is confined to short duration heavy loads such as microwave ovens or longer term light loads such as entertainment systems, (not air conditioning). Successful operation of the inverter requires some appreciation of the controls to avoid the accidental discharge of the ship’s service battery bank and take advantage of the substantial integrated battery recharging function, common in the large inverter/chargers we use.

12 Volt DC Power

We use 12 VOLT DC POWER extensively on board for ship operating systems. This is because unlike ac power, we can store surplus dc power when it is available and have it for offshore and emergency use. Commonality with equipment designed for general mobile use allows us to use generating and engine starting systems readily available from mass production. There is little if any difference between the starter motor on the engine in your boat and the one used for the same engine in a small tractor for example. Because we can draw power from the batteries, it is easy to consider them as sources, but in fact they are only a temporary storage system and the energy they contain must be regularly topped up. Carrying more batteries does not provide more power; it just alters the charge/discharge cycle schedule.

Engine Alternators

The ENGINE ALTERNATORS, (generators of direct current with misleading names), use rotational power from the engine, (courtesy of the tank of diesel fuel), to produce 12 volt dc power. This is available for use whenever the engine is running and any surplus beyond what is used for the engine control and monitoring functions goes to recharge the batteries. The typical marine alternator produces about 600 watts in practice, (60 amps x 12 volts), although they are rated much higher. (Whatever they purport to put out, don’t forget that the little drive belt on the engine has to transmit the engine power to turn them).

These units are of standard manufacture, and are engineered in respect to the pulley loads and mount suitability, with parts and service support assumed by the engine manufacturer. When the engine manufacturer offers optional larger capacity alternators, Antares uses them as standard.

Battery Charger

Another source of 12 volt dc power is the BATTERY CHARGER. The battery charger converts 120 volt ac power from the shore or gen. set to 12 volt dc, with the surplus going to recharge the batteries. The capacity of the charger measured in amps, (electrical flow), will influence battery recharge times.

Solar Panels

SOLAR PANELS are capable of providing maintenance level charging adequate to sustain some low consumption equipment and keep the batteries charged. 295 watts per panel, on a bright day, produce about 4 amps each. An array of four may produce a maximum of 16 amps as long as the sun shines.

You will note that the voltmeters on vessels and vehicles alike usually display voltages in excess of 12. This is because it is necessary to recharge a battery at a higher voltage than it is able to put out. The full charging voltage for the lead acid batteries we commonly use is 13.8 to 14.2. This value has to be closely controlled to avoid damaging the chemistry of the battery, hence the need for voltage regulators integral with the alternators and battery charger.

When we talk about 12 volt systems, we presume that the equipment used will be tolerant of system fluctuations between full charging and effective battery discharge (11.5 v +/-).  The voltmeter tells us the general state of things, for example if we don’t see a voltage higher than 13.5 when charging is available, we are not effectively restoring the battery charge.


It is necessary to safely connect the power available to the various loads while providing device control and preventing overloads of the wiring system. This entails the consideration of normal and damaged operating conditions. We have to design the system to provide the maximum of versatility and safety yet will be comprehensible to the vessel operator and “fail-safe” to the extent possible.

120 Volt AC

With a voltage this high, there is a potential shock hazard and as is common to all electrical power, too much current can heat wiring to the point where it becomes a fire hazard (A toaster is an example of the intentional use of this characteristic). These hazards are avoided by following the rules set out in various recognized standards of good practice, in our case those issued by Coast Guard and the A.B.Y.C. They insist on the use of conductors of adequate size to suite the loads, in addition to a configuration consistent with minimizing shock potential.

When we use multiple sources, such as two shore cords, an inverter, and a gen. set, the supply side of the configuration becomes our greatest challenge. The electrical panel board must allow for all possible input configurations but prevent interference between sources. For example, we cannot allow the switches to be inadvertently set to allow the generator output to be connected to a shore cord. This would result in “hot” prongs on the cord (shock hazard) when unplugged and a war between our gen. set and the local power grid if plugged in.

The expedient way to avoid this is to “interlock” the power source selection switches on the panel so that the engagement of one switch position physically precludes the engagement of another switch. You will see this set-up applied to the ac supply circuit breakers on the panel which disconnect one source before engaging another.

The excellent quality of the aircraft application derived circuit breakers installed in the panel allows them to do double duty as switches without any degradation of function. The “circuit breaker” function automatically disconnects a portion of the distribution wiring if it is asked to pass more than its rated current. The rating value reflects the current capacity of the wiring connected to the breaker and thus prevents its heating or burning in the event of an overload.

After the power passes through the source selectors it is distributed to the vessel via branch circuits protected by individual circuit breakers on the panel. In this way, a distribution “tree” is created. Antares supplies a “one line diagram” of this tree. This drawing simplifies the system as far as possible to assist in tracing the path of power flow and identifying the correct switch positions.

The 120 volt ac outlets used on the vessels are electronically equipped to detect any differential between the current going out on one wire and what returns on the other. In the event of current leakage, (such as may be represented by an individual creating an alternate current path by standing in a puddle and poking a knife into the heater wires of a toaster), the current flow balance is upset between the two supply wires and the outlet immediately disconnects itself. These specialized outlets are called “G.F.I.’s”, (ground fault interrupters), and are equipped with reset and test buttons.

12 Volt DC

We may consider the battery banks to be the power supply for the 12 volt dc system. Antares catamarans are equipped with separate battery banks for engine starting and ship’s service. This is done to avoid the accidental discharge of the ship’s entire battery capacity, for example by accidentally leaving on the cabin lighting, to the point where engine starting is impossible and we have essentially a “dead ship” condition.

Engine Start Batteries

Our start batteries are “stand alone” and duplicate the reliable configuration found in other mobile equipment. As standard we do not provide any means of using the start batteries for auxiliary loads. These batteries are therefore sized and selected to be suited by design for short-term high discharge rates and a rapid recharge cycle. They are not normally expected to be “flat” and spend almost all of their life in the fully charged state.

Ship's Service Batteries

The ship’s service batteries on the other hand are selected to supply long term lower current loads and are expected to endure occasional “flattening”. This requires a more robust physical construction and a larger size and weight. Electric vehicle batteries are commonly used as they are designed for just this type of operation. They are commonly supplied as 6 volt and two must be connected together, (in series), to operate as a 12 volt set. Our vessels use a second set connected in parallel with the first set to create the large capacity ship’s service bank consisting of four 6v batteries .

The several sources available onboard for battery charging will ideally charge any battery that may need it. The two engine alternators are directly connected to their respective start batteries. In order to allow them to charge each other’s batteries in the event of a failure and charge the ship’s service batteries, we use a “smart” switch called a charge combiner to detect when a charge source is on line and connect all the batteries together to share the charge currents. The switch is voltage sensitive and as soon as it detects a discharge condition, it disconnects and preserves the battery bank isolation. This effectively automates the charge process while precluding the accidental discharge of the start batteries. The battery charger may be directly connected to the ship’s service bank and work through the combiner to charge the start batteries in the same fashion.

For ship’s service applications, power is supplied to a distribution panel with one main supply circuit breaker feeding branch circuit breakers. Overload protection is the main concern here as the voltage is too low to constitute a shock hazard. Some loads such as winches and windlasses are too great for the panel capacity and are supplied via large circuit breakers or fuses from a main common connection called a bus bar. The bus bar connects to the battery bank with a high capacity manual switch.

Once again, the one line diagram can be used to study the distribution “tree”. To use the diagram, just follow along a line from a source or load to find switches and paths that constitute a circuit. The current return path is not part of the one line diagram yet it is essential if we expect power to circulate. Antares supplies more complicated distribution diagrams that detail both the positive supply, (red), and negative return, (black) current paths. Actual 12 volt dc cabling in the boat is colour coded to reflect this polarity. Current Antares catamaran wiring is also numbered with a common application code. The code sheet is supplied with each vessel.