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The Ideal 12 Volt Power System for Catamarans

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These days the electrical power system on board a cruising multihull has to be rugged, reliable and, most importantly, easy for the crew to understand and operate. How hard is it to find a system that meets all these criteria? Easier than you might think, especially if you take the right approach to system planning. As a first step I recommend you assume the role of system manager of your own private utility company, for that adequately describes the situation you'll be in whenever you leave the dock. There are rewards and responsibilities that come with the territory. Your best chance of success is to take this position seriously.

Once you have the proper frame of mind and have roughly calculated your daily electrical load, you can set about the task of creating the ideal 12 volt electrical power system to satisfy that load. Whether you are outfitting a new boat or retrofitting an existing one for blue water passages, the considerations are the same. Your ultimate goal is to establish a balance between the energy you produce and the energy you consume, while making very real compromises between equipment size, weight and performance and the budget you have to work with. You'll probably have to make some tough decisions, but the choices become easier when you've adequately assessed your situation and know what gear to look for.

To help you determine the ideal 12 volt system for your needs I will illustrate three different electrical power systems: a simple system, one of moderate complexity, and one with all the bells and whistles. All three systems are geared for multihull cruising boats; the more complex systems simply have more capacity and sophistication. The system you choose may be some combination of these three examples. In Part I of the series we'll start with the simple system.

As you can see in System Schematic 1, I've assumed there is one inboard auxiliary engine with a high-output alternator and smart charging regulator. This arrangement is appropriate for trimarans as well as for catamarans that use a single inboard engine with an outdrive leg. The alternator in this type of system represents almost the total charging source for the boat, so it should be a true high-output version. Not only do high-output alternators charge batteries four or five times faster on average than standard alternators, they are much more reliable due to heavy duty construction and great cooling characteristics. Small case high-output alternators are direct replacements for the standard alternator that came with your auxiliary engine. I recommend a 125-amp small case alternator for most installations. Check the alternator's charging curve to find one that meets your needs. Sometimes lower amperage models produce more power at lower rpms (ie. when the engine is running just to charge batteries) than higher amperage models.

If you need more electrical power from your main engine than one small case alternator can provide, you can chose between installing a second small case alternator or a single large case alternator; both require adequate mounting space, so check the dimensions with your supplier before purchasing. Most sailors keep their standard alternator on board as a cruising spare. Don't forget to upgrade the output wiring to handle the additional current when installing a high-output alternator.

A good quality smart regulator is essential to battery protection. Some high-output alternators come with manually adjustable regulators that are perfectly suitable-assuming you know when to raise the voltage set point for increased performance (occasional or shorter periods of motoring) and when to lower the voltage set point for increased battery protection (frequent or longer periods of motoring). For worry free battery protection and good system performance, spend the $200 or so on a smart regulator.

A solar panel is included in electrical system Layout 1. I can't imagine a cruising sailboat putting to sea without at least one on board. I'd recommend a minimum 50 watt panel, which will give you about 16 amp-hours per day in reasonably sunny conditions. Mount the panel well away from the boom and other gear that can produce power-robbing shadows on the panel. On the stern rail, across a pair of davits or on a transom arch are ideal locations. Solar panels this size and larger must have a control to prevent overcharging the batteries, although on a cruising multihull the greatest danger of overcharging batteries is when you leave the boat. To keep the system simple and the decks clear, no other renewable charging sources are included in this layout.

I've shown a single house battery bank comprised of two deep cycle 12 volt batteries connected in parallel. I highly recommend a single house bank, since you don't have to worry about directing charging or load current, and system monitoring is less complicated. This makes the classic "1-2-Off-Both" battery switch obsolete. Appropriate battery capacity will depend on your electrical load, but assume at least 280 amp-hours of capacity for a cruising boat. You could also connect two large 6-volt batteries in series, reducing the total number of battery cells you have to worry about. Another great alternative is the Rolls 12HHG325M, a 12-volt/325 amp-hr battery comprised of individual 2-volt cells and with the footprint of a single 8D battery. Regardless of configuration, choose only high quality batteries for offshore sailing. Wet, gel and AGM batteries are all appropriate choices if you select brands with a proven track record in the marine market.

Having a separate, designated starting battery with no other electrical loads connected to it is recommended. A parallel charging device will keep the starting battery charged and ready for use. My favorite for this particular situation, where one battery bank is much larger than the other, is the echo-charge from Heart Interface. It simply uses the house bank to maintain a charge on the starting battery whenever the house bank voltage is adequate (when the alternator or solar panel is charging, for instance). Emergency parallel switches allow you to charge directly from the alternator or start the engine from the house bank.

Heavy duty circuit protection devices (fuses and circuit breakers) and distribution posts and busbars make system wiring simple and are essential for good system performance. The gear from Blue Sea is well suited to electrical power systems in offshore sailboats. Note that heavy duty On-Off switches are used liberally to allow for isolation of loads or batteries; you'll appreciate this when it comes to troubleshooting and maintaining your system.

Any DC house loads that can be accommodated by your main DC panel can be wired directly through that panel. Typically, heavy DC loads such as DC compressors for holding plate refrigeration and high-amperage windlasses are connected directly to distribution busbars that can handle the current. In general I favor DC refrigeration, but without strong support from renewable chargers (solar, wind, water generators) it makes as much sense to have a system with an engine-driven compressor. The windlass is often a long way from the batteries, so make certain to size the wires for the minimum voltage drop. The bilge pump is typically wired on the battery side of the disconnect switch to avoid accidental shut-off.

In the near future I will be offering a standardized, pre-wired marine High-Amp DC Panel that includes battery ON-OFF and parallel switching, circuit protection and switching for all charging sources and high-amp DC loads, positive and negative distribution busbars, and pre-wired system monitor shunt(s) and wiring. These panels will be offered on a semi-custom basis so customers can configure a panel to suit their needs.

The AC side of the system in System Schematic 1 includes a 30-amp/120-volt shorepower inlet, a simple AC panel to supply a few AC outlets on board, and an AC-to-DC battery charger for use only when dockside. The battery charger should be capable of recharging the batteries and supplying all DC house loads when in port; a 40 amp high-performance charger should be adequate. A small "pocket" inverter provides AC power for a laptop computer or other small-draw AC appliances when away from the dock.

Monitoring of this system has been kept simple using analog ammeters and voltmeters for both DC and AC circuits. Note that I've placed an ammeter in the output wire for all charging sources, and ammeters in the two electrical panels to track load current. A single digital system monitor could provide the same information in one device. Also, be forewarned that the DC voltmeter will give only an approximation of battery state of charge; more accurate information is provided by the amp-hour function of a system monitor.


Part I

In Part I of The Ideal 12 Volt Power System for Cruising Multihulls we discussed a simple system, one with a single auxiliary engine typically found on trimarans and cats with a central engine and pivoting catamaran drive leg. In Part II of this series we'll look at a more comprehensive electrical power system as shown in Layout 2.

In Layout 2 you can see that two outboard engines are used for auxiliary power and solar panels, a water generator, and an additional engine-driven charging device have been included in the mix. This makes for an interesting, flexible power system that can meet the demands of higher electrical power use.

The charging output from outboard engines is typically quite low. Some catamaran builders using twin outboards (Maine Cat comes to mind) make every effort to minimize electrical loads and maximize solar charging power. With the right approach you can have most of the conveniences of home, including efficient, small scale refrigeration, all powered by renewable charging sources along with the modest output from the outboard engine alternators.

Solar panels are the best option for renewable charging, since they are clean, quiet and have no moving parts to wear out or maintain. In Layout 2 you can see that two solar panels have been incorporated, yet more can easily be added. These panels can be standard panels with an aluminum frame, lower cost yet best suited for mounting well away from foot traffic, or low profile marine panels that can be walked on, great for placement on decks and hard tops where you might have to stand on occasion.

Solar panels and a water generator work well together and, depending on your electrical load and how often you are at a dock, may preclude the need for an engine-driven DC Charger. In fact on a typical passage a water generator can average up to 200 amp-hours per day. Water generators are great, but most of the units currently on the market aren't able to handle boat speeds of more than 7-10 knots, preventing their consistent use on fast mutihulls.

Solar panel and water generator output passes is regulated by charge controllers to prevent battery overcharge. Some multi-source charge controllers on the market can handle the output from both solar and wind or water generators.

A DC DieselCharger matches a small diesel engine with a high-output alternator, greatly increasing fuel efficiency and reducing the operating hours on the auxiliary engine(s). A 4 hp diesel engine can power a 100 amp alternator, while a 6 hp engine is well matched to a 150 amp alternator. Engine-driven DC DieselChargers are a good choice for boats with outboard engines and high average daily electrical loads, and that spend periods of time away from a dock with shorepower. These units are made for long-term use, and they put out a lot of power for their size and weight. If you do purchase one of these units, install it in a locker with good ventilation and a proper sound shield.

Note that one of the outboard engines and the DC DieselCharger in Layout 2 use the house bank for engine starting. These units could also be started from separate designated starting batteries, connected to the house bank through emergency parallel switches. Any combination of battery banks is possible; what you choose depends on your individual needs. Keep in mind that a separate parallel charging device (such as the "echo-charge") is needed to keep each individual starting battery topped by using the house bank as a charging source.

House battery capacity has been increased in Layout 2. I've shown a convenient set-up with three 12 Volt batteries wired in parallel, since this is a common choice among cruisers. Two large 6 volt batteries could easily give the same capacity and with only 6 battery cells total to worry about instead of 18. I'd personally chose the 6 volt layout using high quality Rolls wet batteries or premium gel or AGM batteries; this layout will be discussed in Part III of this article.

In Layout 2 independent stand alone inverter and charger are used. These units can be relatively small and lightweight, and offer a degree of independence over a combination inverter-charger. You'll need to size these units for your situation; a 20-40 amp charger is sufficient for most situations, and a 1500-watt inverter can run any normal household appliance, and provide 70 amps of battery charging at 12 volts whenever you plug into the dock. You can upgrade to a larger model if your situation warrants, as long as you keep in mind that your DC charging sources will have to replace the electricity you use on a regular basis.

There are electrical distribution panels for DC loads, AC ship loads supplied by the inverter, and a "Shore Only" AC load panel. The loads on the latter panel, loads such as the battery charger and appliances like water heaters and air conditioners, are only to be used when shorepower is available. A galvanic isolator is shown to protect the system from electrical leaks and subsequent galvanic corrosion when connected to shorepower.

At the heart of the system is a custom DC Power Center which provides convenient circuit protection and switching, high-amp main and parallel switching, monitor shunt, and main positive and negative connection points, all pre-wired in a neat package. My company Avalon House designs and builds these panels for each client.

A single bank digital system monitor is used in Layout 2. This device is invaluable for tracking system performance and troubleshooting, and is much more accurate and easier to use than a variety of analog meters. Notice that in Layout 2 I've included analog meters in the system. They are convenient to read "at a glance" and provide redundancy at a reasonable cost.

In Part III of The Ideal 12 Volt Power System For Cruising Multihulls we'll review a power system with two inboard auxiliaries typically found on a cruising catamaran.




Part III

In Part II of The Ideal 12 Volt Power System for Cruising Multihulls we discussed a more comprehensive electrical power system on a multihull with twin outboard auxiliary engines. The system included several solar panels, a water generator, and a small DC DieselCharger to handle a fairly large electrical load. In Part III of this series we'll look at an even more sophisticated electrical power system appropriate for boats with twin inboard diesels as shown in System Layout 3.

With dual inboard engines there is the opportunity for high-power charging by installing high-output alternators on one or both engines. These alternators can replace the standard units that came with the engine, keeping the standard units as spares, or the standard alternators can be left in place to keep the starter batteries topped up. When adding a high-output alternator I prefer to replace the standard alternator and use a parallel charging device such as the echo-charge to maintain the starting battery, as shown in System Layout 3. A smart regulator allows for a fast charge from the high-output alternator without fear of overcharging your batteries. If you choose to add a high-output alternator to only one engine, that becomes your "charging" engine (place your water heater and engine-driven refrigeration compressor if you have one on this engine, too), and you should balance the hours of use by using the other engine more frequently when underway.

In Layout 3 you can see that the renewable chargers have been upgraded in this system. Installing four solar panels reflects a determination to provide as much energy as possible from the sun. Larger solar arrays can be fitted on hard tops, reinforced biminis, and special transom arches. As always, a charge control able to handle the total current from the array should be selected.

A wind generator has been added to the system. Since wind generators are next to worthless on downwind passages, and since I like clear decks when at sea, my preference is a combination wind/water generator. They both essentially use wind power, but the water generator works regardless of point of sail. The wind generator can be placed in the rigging when in port, and in this mount it won't transmit sounds through the boat interior as easily. Of course, taking a wind generator up and down is not for everyone, so a modest size unit mounted on a sturdy pole or arch extension at the stern is a good second choice. With this type of unit you can still have a separate water generator for use on a passage.

The house battery bank consists of two large 6V batteries connected in series. Rolls Battery Company makes 6V batteries up to 920 amp-hours; two of these in series would give you 920 amp-hours at 12VDC, and with only 6 individual battery cells. In this system each auxiliary engine has its own designated start battery. The starting batteries are maintained with parallel charging devices that use the house bank as a charging source.

At the center of the system is one of my semi-custom High-Amp DC Panels. These prewired panels from Avalon House pull together all the high-amp components, and I design them to meet individual needs. Included in the panel are circuit protection and disconnects for all charging sources and high-amp DC loads, main house bank disconnect, crossover parallel switches for the starting batteries, convenient connection points for positive and negative cables, the parallel charging devices and the system monitor shunt.

The AC gen-set shown in System Layout 3 is more suitable than a DC DieselCharger when you have large, frequently used AC loads are on board. The gen-set can still charge the batteries through the inverter-charger; the inverter automatically becomes a charger whenever shore or gen-set AC power is present. An AC source selector switch allows you to choose which type of AC power feeds a "Shore/Gen Only" load panel. Loads on this panel include hot water heater, air conditioning, and other loads you don't want the inverter to handle, as well as the inverter-charger itself and the main AC electrical panel. Another AC source selector switch is used to select between inverter and Ship (shorepower or gen-set) to supply AC power to the main AC electrical panel.

In System Layout 3 an isolation transformer is used. These units provide the best protection against stray shorepower current and the subsequent safety hazard and galvanic corrosion problems. If you use an isolation transformer, match it to the size of the shorepower and Gen-set circuits. Since weight is always a concern on a multihull, try to keep your AC requirements modest so you can use a modest size isolation transformer and gen-set.

A single bank system monitor for the house bank is still adequate for this type of system. The starting batteries can be monitored with a simple 2-battery voltmeter. Another option is to install a 4-channel system monitor for ultimate system tracking and protection. Channel 1 of this type of monitor can be set up to track all current going into or out of the house bank, the same as a single bank monitor. It also displays house bank voltage, amp-hours remaining, and percent of charge. Channels 2, 3 and 4 can be set up to display current from the high-output alternators as well as the solar panels and wind or water generator. A host of other features are found in this type of monitor, including alarm functions and the ability to interface with a laptop computer.

After studying these sample systems, select the gear that makes the most sense for your situation, then work with a knowledgeable marine electrical system supplier that will be able to help you well after the sale. Ask yourself what it would be like to call this company from another continent with a technical question about your system; most general equipment suppliers wouldn't be willing or able to help. And finally, don't build in more complexity than you want or need, but make sure you have the ability to produce enough electricity to supply your loads and can easily monitor system performance.

Kevin Jeffrey is an electrical power designer/consultant specializing in renewable power systems for boats and land based applications. He is the author of the Independent Energy Guide and publisher of the Sailor's Multihull Guide (new Third Edition to be released October 2002).




Independent Energy GuideKevin Jeffrey is a long-time multihull sailor, independent energy consultant, author and book publisher.  He is the author of Independent Energy Guide, a valuable resource for cruising mutihull sailors, and is the publisher of Adventuring With Children by Nan Jeffrey and the first three editions of the Sailor's Multihull Guide. Sailors Multihull Guide


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