Multisource charging

Alternative energy for the offshore yacht
Courtesy Hydrovane
A voyaging boat equipped with a Watt & Sea water generator on the stern next to the Hydrovane wind vane autopilot.

If there is one thing we all seek when we slip the dock lines and set out for an offshore cruise, it is independence. For today’s cruiser, that means having all the modern comforts of life available at all times, wherever we go. Essential to our freedom is a robust, multisource charging network for our house and starter batteries operating while under sail or at anchor 24 hours a day.

Before running to your chandler and spending your cruising kitty on an ambitious list of charging devices, first determine what your battery charging needs will be, based on their rated capacity in amp-hours. A typical flooded 8D battery has a capacity ranging from about 200 to 450 amp-hours, with gel cell and AGM batteries lying somewhere in between. These large-capacity battery banks are charged one at a time to ensure adequate charging current, which is important to the health and lifespan of the battery.

The high-capacity 6-volt battery manufactured by Trojan — along with a number of lesser-known brands — is popular among offshore sailors, who connect two of the batteries in series to produce one 12-volt bank. A pair of Trojan T-125 6-volt batteries has a combined power rating of 480 amps while offering more installation flexibility than a single 8D bank.

Once you have established your amperage needs, you must design a charging system capable of keeping up with the demands of all your navigation and communications devices, along with lighting, entertainment, perhaps an electric windlass and — of course — the obligatory fridge. If you poke around a cruising marina these days, you will see that today’s fully equipped yacht draws power from solar panels, a wind generator, a hydro generator and an arrangement of controls to keep the battery banks topped off around the clock.

Solar panels
The most common dependable source of charging power is a bank of solar panels. Though they are the slowest charging input, once installed with a charge controller they do their job silently as long as the sun is up, even on a cloudy day. The Siemens 36-watt panel on the stern pulpit of our 1966 Cal 30 Saltaire has been charging our house bank faithfully for more than 20 years, having survived gales and a full knockdown during a world circumnavigation by way of the two canals.

Traditionally, sailors have depended on rigid monocrystalline solar panels to get the most charging power from the smallest amount of surface area. These panels are secured to a strong anodized aluminum frame that can be mounted on any fixed structure, which usually means either a deck or a stainless tube arch over the aft pulpit.

A solar panel mounted along the side of the cockpit.

Bill Morris

Nature Power offers two rigid solar panels, one rated at 90 watts and the other at 165 watts. At the peak power of 165 watts, this large panel produces a sizzling 9.43 amps, which necessitates installing a charge controller. Actually, any panel capable of more than 12 watts requires a controller to prevent frying the battery.

Amorphous thin-film silicon cell panels, or “flexible” panels, do not generate the same level of power per square inch as rigid panels, but they offer the cruiser a greater range of mounting options. Nature Power’s semi-flexible, 50-watt, monocrystalline solar panel, measuring 21 by 30 inches, can fit onto a variety of surfaces on a cruising yacht. Two of these panels should easily fit atop a rigid spray dodger on a medium to large sailing vessel.

When shopping for rigid panels, beware of cheap panels rated as “marine” quality but constructed of cheap glass with non-anodized aluminum framing and lacking proper waterproofing in the electrical connections. These may be adequate for home or RV use, but not for the grueling demands of an offshore yacht.

Installing solar panels on your vessel is open to many options. Besides the arch and dodger, hard panels can find room on the stanchions, where they may be hung vertically or propped up horizontally with a hinged set of stainless tubes, depending on weather and sailing conditions.

However you attach the panels to your vessel, always use marine-grade wire and connectors, such as those produced by Ancor or Marinco — not the cheap stuff you find at a home hardware store. Also make sure you are using at least the minimum wire size rated for the maximum amperage, along with the appropriate fuse or circuit breaker. For long runs, raise the wire size a notch to prevent overheating. I rarely use anything smaller than 16-gauge tinned Ancor wire. Another caveat: Never leave any wire connection — not even a heat-shrink butt connector — exposed outside the cabin or a sealed junction box, such as that found on the underside of a solar panel. As a rule, we try to expose as little wire as possible to sun and water.

Wind generators
Any sailing vessel with a fridge running 24/7 almost certainly uses a wind generator to keep up with the constant high demand for charging power. Boats with wind generators range between roughly 30 and 50 feet LOA, meaning just about every boat in every cruising anchorage on the planet.

An Air-X wind generator with streamlined styling and automatically feathering blades. These units can produce substantial output, even in light to medium winds.

Bill Morris

The typical cruising boat consumes anywhere from 40 to 100 amp-hours per day, a diet not always adequately fed by a set of solar panels. If it’s calm and sunny, a large array of solar panels keeps the batteries topped off. But when the clouds move in, bringing a breeze of at least 8 knots, a wind generator saves diesel fuel and adds to engine and alternator life.

If you have ever watched a wind turbine, even a toy pinwheel in a fair breeze, you have noticed the blades rarely spin at one speed for more than a few seconds. It will slow down to a near stop, and then as the wind freshens ever so slightly, the turbine becomes a blur. This is because the power produced by the wind theoretically increases as the cube of wind speed. For example, a 5-knot zephyr produces 125 units of power; at 10 knots, the power produced zooms to 1,000 units. However, according to Betz’s law, the actual power output is roughly 59 percent of the wind’s total power.

Modern wind turbines supply power to the batteries via an aerodynamic set of blades connected to a permanent magnet alternator, which is affixed to the interior of the casing. Inside the casing, a set of coils rotates on an armature while brushless contacts and a diode are employed to rectify the AC into DC power.

A number of manufacturers produce dependable marine wind generators, which range in size and power output, the choice of which depends on the charging needs of the vessel. At the humble end of the spectrum is the British-made Rutland 504 wind turbine, which produces up to 80 watts of power and features a safety shroud around its six-blade rotor. Priced at less than $500, you’d have a hard time beating this kind of value.

The ubiquitous Air-X, found in virtually every cruising anchorage on the planet, is in the middle of the pack in amperage, which is one of the reasons for its enduring popularity. After all, you need to power the fridge, not an arc welder. Air-X is built by Primus Windpower, a Lakewood, Co.-based producer of off-grid electrical charging systems.

The latest iteration of the Air-X features a brushless alternator in a sleek housing unit and a three-blade, 46-inch-diameter, carbon-fiber composite rotor controlled by a speed-dampening microprocessor for high wind conditions. A three-phase regulator allows a high bulk charge when the batteries are low, an absorption charge when the bank is close to maximum capacity, and a float charge at reduced voltage to prevent overcharging.

The complete Air-X package with the Deluxe Kit, including mounting pole and stabilizers, will run you about $1,800 plus shipping. However, the unit’s durability and self-tending capability make this central feature of your charging complex — and of your quality of life offshore — hard to pass up.

The setup for a Watt & Sea water generator. The unit can be rotated out of the water.


Hydro generators
The power we draw from the alternator, solar panels and wind generator may appear to be all we need to keep our battery banks topped off at all times. But when we are sailing downwind in a fair breeze offshore, the apparent wind is diminished considerably. If it’s blowing 18 knots and you are on a boat measuring 36 feet on the waterline (e.g. Sabre 426), you can expect to run at 9 knots, which leaves only 9 knots of wind to power the wind turbine. At 9 knots wind speed, the Air-X generates less than 2 amps of power.

Water’s greater density allows us to draw far more power at a given hull speed than with air power. Sailing downwind in that same 18-knot fresh breeze with a water generator deployed, we can expect a hull speed of about 8 knots if we subtract about 1 knot of drag from the water generator’s rotor. At precisely 7.5 knots hull speed, the Watt & Sea Hydrogenerator cranks out a blistering 24 amps, or 288 watts.

With this kind of amperage, you can charge 100 amps of battery power — more than most cruising boats use in a 24-hour period — in a little more than four hours. Considering all this power potential, it’s a wonder why every cruising boat doesn’t have a water generator.

While the Watt & Sea appears to be the market leader in water generators, it is the latest evolution of a long line of such devices manufactured or home built over the last half-century. And many of them are still in use.

The earliest version of the water generator was a deck-mounted alternator connected to a long length of three-strand towline, which was turned by a submerged rotor. This arrangement caused two problems. First, retrieving the spinning towline required the use of a cone through which the towline could spin freely while the propeller approached the stern. With some practice, a crewmember could do this without losing one or more body parts.

The second challenge posed by the towed propeller was its tendency to attract the attention of hungry, unsuspecting sharks. You can imagine the spectacle of a shark’s elaborately mangled dentition, the blood trail and the other sharks coming to eat the first one. With any luck, the skipper had remembered to pack an extra Cuisinart to feed the next customer.

The basics of an alternative energy charging system.


The DuoGen-3, manufactured by Eclectic Energy Limited in Nottingham, U.K., is a hybrid wind-water system requiring no towline and only minor reconfiguration when switching modes. The entire unit is mounted on the transom and stands upright with the air rotor installed, where it is capable of producing 6 amps of current in a moderate breeze of 15 knots.

For hydro mode, the operator removes the wind rotor, extends a carbon-fiber arm to accept a water rotor and then lowers the unit to the water. With the rotor submerged, the DuoGen-3 generates 8 amps at 6 knots hull speed and a respectable 16 amps at 8 knots.

Cruising guru Jimmy Cornell has installed a DuoGen-3 on his aluminum-hull Garcia Exploration 45 Aventura IV, and numerous participants in Cornell’s Atlantic Rally for Cruisers (ARC) have given highly favorable reports of the charger’s robust capacity in the often grueling conditions of the North Atlantic.

However, while the DuoGen-3 offers a choice between water and air power, the Watt & Sea Hydrogenerator, with its 24 amps at 7.5 knots hull speed and user-friendly design, is definitely the leader in power output and ease of use while underway. To deploy the unit, you simply push the entire assembly down to submerge the rotor and let the apparatus do its job of powering up the battery banks.

The choice between the DuoGen-3 and the Watt & Sea comes down to space accommodation, power needs and overall price, if we add a wind generator to the cost of the Watt & Sea. The Watt & Sea and a wind turbine require no reconfiguration to enable their use; on the other hand, the DuoGen-3 takes up less space and costs less than two separate charging components.

Charging controls
A multisource electrical generating array, such as the configuration you may install on your sailing vessel, must be backed up by a charge control system that is up to the task. Often, each charging unit comes with its own optional smart charge controller, while your vessel’s engine might still be controlled by a traditional mechanical voltage regulator. Overlapping all of these systems can cause confusion, with each one sensing what we assume is battery charge rather than charge current coming from an external source, which could be a diesel generator set, a solar panel, wind generator or hydro generator.

A solar panel arch above a bimini cover.

Bill Morris

Typically, no more than two charge inputs run simultaneously, so “confusion” may be a bit overstated here. Again, each of these can be configured to regulate its own output, although it does so not by arbitrarily setting a limit after X amount of time charging, but by constantly monitoring the charge level of the battery bank receiving the charge current. And with a multi-input charge controller, competing levels of current are managed by a microprocessor, alleviating our worries about over- or undercharging the battery banks.

In order to plan how we want to control and monitor our vessels’ overall charging plan, we must have a working understanding of the components involved and how they interact in order to keep our onboard electrics working consistently. These networks may include a traditional engine “black box” voltage regulator, one or more single-purpose “smart controllers,” a controller with multiple inputs, and one or more battery isolators. As you will see, the trend is toward centralizing the control and monitoring functions as much as possible to permit easier reading of the monitoring process, and to prevent the unnecessary and often detrimental overlapping of battery monitoring.

Many, not all, of today’s marine engine alternators are equipped with internal regulators to provide consistent current and voltage, bypassing the need for an external regulator. Internal regulators use a diode to rectify current from AC to DC and transistors to control voltage, taking the place of the old-fashioned spring and rheostat. Since modern components are internally mounted and thus difficult for the average skipper to repair, the cause of any malfunction in the alternator is likely to remain unknown to the operator. This leaves us two options: get the alternator repaired by a trained technician, or as is often the case, purchase a whole new unit.

An elevated level of current coming into the battery banks from a high-output source can fry the batteries if the current has no other place to go. A “shunt” regulator — which may be mechanical or transistorized — redirects excess current to some other target such as a water heater or a heat dissipater to protect the battery. Regulating current from charge sources can be done separately or combined in one multi-input regulator/controller. A number of manufacturers offer multisource shunt controllers for surprisingly affordable prices — an added boon when we consider the alternative of having to replace hundreds of dollars’ worth of battery banks, or potentially thousands of dollars in charging equipment from harmful reverse voltage.

SES Flexcharge of Charlevoix, Mich., produces the Flexcharge 7-amp PV7D shunt regulator, which is designed for small solar panel installations but is capable of charging two battery banks. The diminutive controller fits in the palm of your hand and can be used on flooded, gel and AGM batteries. The Flexcharge NC25A controller is capable of many times the amperage of the PV7D in a package only slightly larger. Roughly the size of a single-patty cheeseburger, the NC25A combines 0.1 amp to 25 amps of alternative energy charge current from one or two charge sources and is expandable to 35, 60 or 100 amps.

The Nature Power 8-amp charge controller is safe for use on a solar panel array of up to 130 amps.

Bill Morris

Flexcharge claims an incredible 99.9 percent efficiency rate for the NC25A. Rather than shifting through bulk, taper and float charge levels, the NC25A is a sliding controller, adjusting itself gradually by sensing the precise charge of the single- or dual-battery banks. The NC25A’s “Charge Divert” feature routes excess charging energy to other tasks (such as refrigeration or a water heater) or back to a permanent magnet charging source (such as a wind generator) in order to dissipate energy, and in this case to reduce turbine speed in high wind conditions. Charge Divert activates only after the battery banks have reached their full charge.

A less expensive alternative is Nature Power’s 8-amp charge controller, which is adequate for lighter loads such as a pair of 50-watt solar panels. The unit is actually rated for 130 watts, so you can throw in a small flexible panel to get full service from the charger. LED indicators show whether the battery is charging or fully charged, and a diode prevents discharge of the battery to the panel at night. At $20, this little marvel is a bargain that is hard to beat for small vessels with low-amperage alternative charging sources.

Taking the plunge
Deciding on the optimum combination of charging and control components is no easy task. A reasonable basic charging configuration for the average 40-foot cruising vessel should include a 100-amp solar panel array, and either separate wind and water generators or a combined system, such as the DuoGen-3. Charge current from these devices must lead to a single controller, which can sense the various amperages simultaneously without them incorrectly sensing each other as battery charge.

As all cruisers know, the level of creature comfort we enjoy offshore is commensurate with the amount of money we invest in both vessel and onboard equipment. My wife, Marilu, and I shared a memorable cruise across the South Pacific some years back, and our comforts were meager at best. If we attempt another ocean crossing in the future, our vessel will certainly be equipped with power from natural sources, which will not only save money on fuel but also add immeasurably to our comfort and happiness both underway and at anchor.

Circumnavigator-author Bill Morris is the author of The Captain’s Guide to Alternative Energy Afloat and is a frequent contributor to Ocean Navigator.