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Blue Water Energy: Planning Your Power System

Whether you’ve bought your bluewater catamaran or are still in the planning stages, one of the most important considerations for long-term cruising is your power system. Marine technical expert Duncan Kent looks into a successful onboard electrical system  
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A well-thought-out system will both maximize your freedom afloat and minimize your headaches 

Whether you’ve bought your bluewater catamaran or are still in the planning stages, one of the most important considerations for long-term cruising is your power system. Marine technical expert Duncan Kent looks into a successful onboard electrical system

Frombattery banks to reading lights–planning ahead for all eventualities will give you more freedom and less worry if you intend to linger awhile at anchor in far-flung destinations.


At the heart of any marine power system is your energy bank–the batteries. Before you buy what you think should be enough, you need to create a plan and decide what is essential, what would simply be nice to have and what is not worth the power cost.


Battery capacity is measured in cold cranking amps (CCA), marine cranking amps (MCA), or amp-hours (Ah). The CCA/MCA ratings refer to starting batteries only. They indicate the current, or amperage, that a battery can supply for 30 seconds at 0F (CCA) or 32F (MCA) without the voltage dropping below 7.2V.

The amp-hour rating is a measure of the amount of current a battery can supply over a known period, usually 20 hours (C20 rate), before the voltage drops below 10.5V. For example, a 100Ah battery can (in theory) supply 5A for 20 hours, after which it would be completely flat. In practice this is not strictly true, as the battery’s discharge properties change as the charge is reduced. However, it is close enough for approximate calculations.

Most deep-cycle batteries also have a more accurate “reserve” (R) rating, which is the number of minutes they will run a 25A load at 80F before dropping to 10.5V. Another important figure is the number of charge cycles they can endure, although these also vary with the depth of discharge. Ideally they would be quoted at a 50 percent discharge rate (the most you should ever discharge a deep-cycle lead-acid battery), but the figure can double if it is only ever discharged by 20 percent. 

With care, regular maintenance, minimal discharging and full recharging, a regular flooded deep-cycle battery should deliver at least 1,000 cycles before it packs up, while absorbed glass mat (AGM) and gel cells can deliver 1,500 to 2,000 cycles.

Calculating your requirements

Before calculating your required capacity first reduce any unnecessary loads. Changing all your filament bulbs (especially the masthead tri-color and anchor lights) to LEDs is a good start, as is improving the insulation on the fridge and installing manual water pumps at your sinks. 

To accurately calculate your total power requirements, simply multiply the amperage of all the equipment by the time it will be run over your chosen cycle period (usually 24 hours). Total the amp-hours, double the result (50-percent discharge max), and then add an extra 25 percent so you will have some juice to spare.


Flooded, AGM or gel?

Over the past couple of decades we have seen a proliferation of gel-cell and AGM batteries in marine applications. Known technically as valve-regulated lead-acid, or VRLA, batteries, their primary advantage is that they are sealed (though they do have emergency pressure-relief valves), so you need not top up cells with distilled water and can mount them at odd angles—sometimes even upside down!

Unlike traditional flooded batteries, the electrolyte in an AGM battery is trapped in a fiberglass mat, while in a gel cell the electrolyte is combined with silica particles, which make it semi-solid. Gases produced internally during the charging process (oxygen and hydrogen) are recombined inside the battery into water (H2O) to keep the moisture levels constant.

Also unlike flooded-cell batteries, VRLAs must be charged with a voltage regulator to prevent gassing, as they cannot vent except in an emergency. AGMs can tolerate a high-voltage charge (<14.8V), but gel-cells are more sensitive (14.2V max).

Most modern digital battery chargers have adjustable outputs or pre-sets for different battery types. However, if you have a gel-cell service bank and a flooded-cell cranking battery, you’ll need to charge the banks on two separate outlets. There’s nothing wrong with using traditional sealed lead-acid traction batteries for your house loads, though you should choose a quality brand such as Rolls or Trojan if you want to maximize their lifespan.

Lithium ion batteries are increasingly used in high-end boats; they are several times the price of gel or AGM batteries but will last much longer and they also weigh much less.


Size and shape

Once you have calculated capacity you must decide what type and size batteries to use. They should be the same type and capacity and should all be the same age.

Bigger batteries will survive more hammering, but are harder to carry and install securely. A single huge 500Ah battery, for example, weighs about 320lb. Better to buy four smaller batteries (12V/120Ah, say) and connect them in parallel. If you have the space it may be better to create banks of 6V batteries. They have a smaller footprint and are often compact enough to fit all around the boat or in a dedicated box inside an engine compartment.

If you’re fitting out a brand-new yacht, you might consider installing a 24V ring main for heavy loads such as winches, thrusters and windlasses to halve their current draw. This is becoming common practice on larger cruising multis.

Battery isolation

You must isolate the service and engine batteries so that they can be linked when charging and otherwise kept apart. A simple three-way switch is the easiest method, but you might flatten the engine battery if you accidentally leave the switch on “both” without the engine running. 

A diode isolator allows both to be charged simultaneously and automatically isolates them under normal use but also creates a voltage drop, which will reduce the charging voltage unless compensated for somehow.

The modern answer is to install a voltage-sensitive relay device that links the engine and service batteries together for charging only when the cranking battery has reached 13.8V, ensuring it is always prioritized for charging.



Once you’ve designed your battery banks you need to ensure you can easily recharge them. With two diesel engines on board already, it may not make much sense fitting a third dedicated generator just to create power. Better to use what you already have and add green-power options wherever possible.

A popular option, if you plan to fit big energy users like a washing machine, air conditioning or a dishwasher is to fit a clutch-driven AC alternator to one engine and a high-power DC alternator to the other. That way you can run the AC alternator a couple of hours each day, during which time you can bake, make water and launder your clothes while also charging the batteries. The other engine can be then used just to top them off as needed the rest of the time, allowing you to balance the use of your engines. With an inverter fitted, you can still run small AC loads when the AC alternator is not being run.

To simplify matters you can you leave the regular 12V alternators on both engines connected to their own cranking batteries. That way you will ensure they always receive a charge when their engine is running. A link switch between the two will allow either engine to charge either cranking battery; you should also install a link switch so they can be charged from other sources in an emergency.

If you do decide to install a diesel generator, realize that you will have to make sacrifices in other areas. Catamarans are weight sensitive and most builders will want to place the generator in the middle of the boat—typically in one of the bridgedeck lockers forward of the saloon. That will take up room that could be used to stow other cruising necessities.

Alternators, regulators and mains chargers

Deep-cycle marine batteries last longer and charge more easily if they are recharged in distinct sequential stages, commonly referred to as bulk, absorption and float, using a constant voltage throughout each phase. During the bulk stage the charger puts out its nominal current at the battery’s optimum voltage (14.8V for AGM) up to around 85 percent of full capacity. Voltage then drops to around 13.4V during the absorption stage, with the current tailing off until it reaches around 0.5A, when it switches to float mode.

This multi-stage process is used in most 110V shorepower chargers, but is far less common with alternators. Staged charging is not vital for cranking batteries, but is very desirable for deep-cycle house banks. Even high-powered aftermarket alternators can normally only achieve this with the help of an “intelligent” multi-stage regulator such as those from Balmar or Sterling.

The optimum current for the bulk-charging phase is 20 percent of the battery’s total C20 amp-hour capacity for flooded or gel-cell batteries, or 25 percent for AGMs. There is no harm in using a less powerful charge, but it will take more time. In our theoretical system, our 500Ah AGM house bank will be charged most efficiently by the AC alternator using a 125A charger, but in reality that’s one big beast of a charger. If you only run the AC alternator two hours a day, the charger will probably remain in bulk mode anyway, leaving the absorption and float stages for other charging devices, such as solar or wind. However, if you visit a marina with shorepower, you’ll have a 3-stage charger that can fully recharge your house bank overnight.

One word of advice—it’s best to buy a charger or smart regulator with temperature sensing circuitry. This works in two ways, both beneficial. First, the colder a battery is the greater the voltage required to bring it up to full charge, so the charger can adjust output accordingly. Second, the sensor will protect the battery from overheating and boiling dry by reducing or switching off the charge if necessary.


The combination inverter/charger is a very popular device for cruising yachts. This is a battery charger that is permanently wired to the shorepower input so that it automatically recharges your batteries whenever you’re at a dock with power. It also contains an AC-DC inverter so that it can provide constant AC power, if required, either from the batteries or the dock.

These devices reduce the amount of wiring and switching required with separate charger and inverter and are an efficient way of providing AC power via a ring main. Inverters do use a great deal of DC power, so you need to monitor this carefully, or just leave the ring main for small items, such as laptop and phone chargers, while running the engine-driven AC alternator for heavier loads.


A good quality charger/regulator should also include an equalization mode function, whereby a high-voltage charge (around 15.5V-17.0V) can be maintained to dissolve any lead sulfate crystals that may have formed on the plates. This vital but often overlooked, feature can add years to a battery’s lifespan and should be performed once every 20-30 charge cycles. It can be done using a good alternator regulator, but it is more common to use the shorepower charger after you have fully recharged the bank.

A word of caution, however–gel-cell batteries cannot be equalized! Also, as the voltage will be well above 14.5V during equalization, you should first disconnect any voltage-sensitive equipment.

Battery imbalance

Modern cruising boats are being fitted with much higher capacity house banks and more powerful charging devices, and in some cases, this has created a new problem. All boats with an inboard engine will have one or more thin-plate engine cranking batteries, which, unless they are of the dual-purpose AGM type, will have completely different charging requirements than the higher capacity deep-cycle house bank. Simply paralleling the engine and house banks for charging via a switch or VSR can cause the more easily charged engine battery to rapidly become overcharged.

More importantly, if you have gel-cell house batteries, but a simple flooded-cell starting battery, you will need a charger with two independent outputs that can be adjusted to suit the different batteries. This is why I suggested earlier that you retain your engines’ own simple alternators with in-built regulators for starting battery charging only. Where this is not possible, you can, in fact utilize a VSR to switch out the starting batteries from the charging circuit once fully recharged.



Photovoltaic (PV) solar panels are more efficient and less expensive these days, and cats usually have plenty of space to mount them. Although they may seem a little pricey, they are pretty much maintenance-free and will last 25 years or more. Most 40-50ft cruising cats have room for at least two 80W panels—probably twice that if there’s a rigid bimini. On average, 160W of PV capacity will feed some 80Ah to your batteries on a sunny day—possibly more in the tropics. That’s a third of your DC demand right there.


When you’re sailing a hydrogenerator, such as those made by Watt & Sea, makes a good deal of sense. Most can supply 8-10A at around 6 knots and some claim to put out 20A at over 8kn. On a fast downwind passage they are an excellent fuel-free charging system—even if they might take a quarter of a knot off your boatspeed. 

Choose a generator that can be wind or water driven, such as the Aquair or Duogen turbines, and you’ll have the best of both worlds—converting from hydro- to wind-power within minutes of arriving at your chosen anchorage.

Marine wind turbines are very effective in places like the Caribbean, where the wind blows almost constantly. Some older designs can be very noisy in high winds, driving the occupants of neighboring boats in an anchorage to distraction, particularly when left running overnight! Look for the types that stop rotating completely when your batteries are fully charged. Feathering or freewheeling blade types are the noisiest.

Unlike solar panels, wind turbines do need periodic maintenance, so you’ll need to carry parts if you’re venturing further afield. And oddly enough, some cat sailors have trouble finding somewhere out of the way to mount their wind turbines. They have a habit of falling victim to mainsail reef pennants flogging in a strong wind when the main is being raised. 

Boating writer Duncan Kent is always searching
for more efficient ways to generate
energy while cruising



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