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Power Your Cruising Multihull Using Self-generated Energy

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Solar panels like those seen mounted below and on the opposite page have helped cruisers go green

Solar panels like those seen mounted below and on the opposite page have helped cruisers go green. Photo by Tor Johnson

Energy self-sufficiency been a dream of mine since I first started cruising in small sail boats. In fact, it was the sole reason I chose to become a sailor, rather than a powerboater—there is nothing quite as exhilarating as putting your life into the hands of nature and trusting her to work with you to ensure the wellbeing of your boat and crew.

A sailing boat is a perfect environment for boosting your self-reliance. It makes me laugh when I hear today’s “environmentalists” harping on about how modern technology is now enabling mankind to live entirely “off-grid”—sailors have been managing it for centuries in one way or another!

That said, fast-improving technology has indeed not only helped to make this dream more of a reality, it also ensures you don’t have to give up the many electrical gadgets to which most modern sailors are now addicted. Solar cells have become cheaper, smaller, thinner and more efficient. Both wind- and hydro-generators are a good deal more powerful now that brushless alternators are all the rage, and the once futuristic hydrogen fuel cell can now be bought over the counter—if you have a thick enough wallet.

Photos by Peter Nielsen (left); Duncan Kent (right)

Photos by Peter Nielsen (left); Duncan Kent (right)


The widespread adoption of photovoltaic cells combined with efficient manufacture has brought the cost of solar power down. However, there are other considerations that go some way to explaining the inflated price of the typical “marine” solar panel. Monohulls are rarely designed with photovoltaic (PV) arrays in mind, but almost all multihulls have an abundance of flat surfaces on which to mount the panels. One of the best areas is high up on a bimini or gantry, followed by the coachroof and then certain less-trafficked areas of the deck.

The most efficient panels are rigid, usually glass-covered and set in easy-mount aluminum frames that are raised slightly to allow all-round airflow. They are usually made from the most efficient silicon crystal structure and work within their optimum temperature range, making for an impressive cost/Watt ratio. However, this type of panel is often more susceptible to impact damage and can’t really be walked on. For this reason they need to be mounted out of harm’s way, above a bimini or on a stern gantry.

For areas likely to be frequently walked on, flexible panels are more suitable and these can be mounted on or even directly adhered onto the deck, wherever there is the space. Although these PV panels usually produce only 70 to 80 percent of the power output of a rigid panel of similar area, their ability to absorb rough treatment more than compensates for this.

Which type is best? A PV panel consists of a number of silicon cells, usually 36 or more, which are connected together in series to produce between 18 to 24V in open circuit condition (i.e., no load). Their current output will not only depend on the amount and the intensity of the sun’s rays hitting them, but also on the quality of the silicon cells, their design structure and the number of cells in a panel.

A Silentwind wind generator (left); An AeroGen wind generator and an Ampair battery monitor (inset)

A Silentwind wind generator (left); An AeroGen wind generator and an Ampair battery monitor (inset)

Panels come in capacities from 10W to 250W, although the maximum size for flexible panels is more like 150W. Like batteries, they can be connected in parallel to give higher currents or in series (must be the same type/power panels) for higher voltages.
The active ingredient in all commercial PV cells is silicon, which is derived from silica (sand). Various methods exist to extract the silicon from sand, but usually it’s heated to 3,092F in the presence of carbon, which forces the silicon to form crystals as it cools. The slower it cools, the larger the crystals.

A transom-mounted water turbine drags a spinning propeller behind the boat to generator power

A transom-mounted water turbine drags a spinning propeller behind the boat to generator power

Panels are made using either a monocrystalline or a polycrystalline construction. The former is produced from a single, thinly-sliced silicon crystal and is therefore as “pure” as possible, resulting in the highest possible efficiency. Polycrystalline PVs are constructed from numerous smaller crystals and will convert slightly less solar energy into useable power. Although the gap is reducing these days due to improved production methods, polycrystalline panels still need to be a little larger than their monocrystalline equivalent. The former usually provide around 120W to 135W per square meter, whereas with monocrystalline it’s more like 140W to 170W per square meter.

Cost: It’s important to be realistic about how much actual power you will get from a PV panel. Do not just believe the sales blurb. Yes, a 24W/12V panel could, in theory, produce 2A when set at right angles to strong direct sunlight (Peak Power) and kept cool, but this is rare unless you install a sophisticated sun-seeking mechanism that follows the path of the sun at all times. If, like most, your panels are fixed onto the deck, then the movement of the sun through the sky during the day will reduce the effectiveness of the panels to a considerable degree. Taking the seasons into consideration as well, and the possibility of something partly obscuring the panel from the sun, then it is plain to see that what you actually get over time is a whole lot less than their peak rating would suggest.

To expand the math would require more pages than I have, but suffice it to say the average output from a PV panel mounted horizontally on the deck is likely to be somewhere between 15 to 20 percent of the stated power in northern latitudes, and 20 to 25 percent in sunnier southern climes such as the Mediterranean, Caribbean or Pacific islands.
Thus, a 100W panel could supply a maximum of 20Ah/day during average summer conditions in the United States, meaning if you consume on average 100Ah/day you would need an impractical number of 5 100W panels (each measuring approximately 5ft by 2ft) if you were to rely on solar power alone.

The cost, however, of 100W of good quality monocrystalline PV is currently around $200, including a controller. So, if they lasted for, say, 20 years, every Ah produced from each panel would have cost around a tenth of a cent!


Marine wind generators have been refined to a high degree of efficiency over the past few decades. Better alternators, CAD-designed blades and smart charge controllers have all played their part in making the latest devices extremely powerful, yet as quiet as possible.

Alternator advance: The most recent advancement in electrical generators has been the introduction of permanent-magnets rather than the traditional heavy and inefficient wire-wound armature coils, which effectively gives you more amps per dollar, as well as enabling power generation to start at lower wind speeds. The use of magnets means none of the valuable generated power needs to be used to magnetize the coils and they don’t need slip rings, which makes them more reliable and efficient, while reducing regular maintenance.

How many blades? Since the introduction of more powerful, permanent magnet models, three-bladed turbines have become increasingly popular. Early versions were efficient, but were noisy in high winds. Since then, self-regulating circuitry has been introduced and this, combined with the latest CAD blade design technology, has helped considerably in reducing noise levels while enabling the turbines to retain their electrical efficiency and low start-up speeds.

Suitability for Catamarans

Sometimes it can be awkward to find a safe place to mount a wind generator on a cat, due to their long booms, but with care they can be mounted aft, well outboard on specially designed gantries. With two hulls and such wide sterns you can even fit a pair, doubling the capacity and giving you redundancy should one fail.

Dave Abbott, who with his wife, Donna, has sailed over 33,000 miles on their 39ft Privilege catamaran, Exit Only, says in his blog “I sailed halfway around the world without wind generators, and I survived without a problem. I was always rationing power, and I ran my engines every day to generate electricity. During the last half of the trip around the world I had two wind generators working around the clock to meet our electrical demands. I no longer had to ration power, and I didn’t need to run my engines to generate electrical power except on rare occasions. That meant less wear and tear on the engines, a quieter boat, and I didn’t have to endure the smell of diesel exhaust.”

Cost: If you’re considering cruising along a tradewind route then wind power is a must. While wind generators are not very efficient when sailing downwind, as soon as you anchor you’ll get the full benefit of their generous electrical output.

Like solar, wind power is relatively cheap. Although the generators are initially quite expensive to install and require a certain amount of regular maintenance, once they start pumping out the power you’ll save a fortune on diesel and engine wear.

Around $2,000 will get you one of the latest high-output turbines, complete with mounting kit and charge controller. In tradewind conditions you could get up to 200Ah of power every day, from each generator. At that rate if it lasts 10 years that’s a mere 0.3 cents per Ah (plus any maintenance costs).

A battery management system helps the skipper monitor power levels (left); a dual charge control for both wind and solar power (right)

A battery management system helps the skipper monitor power levels (left); a dual charge control for both wind and solar power (right)


For many long-term bluewater cruisers the passage is a means to an end. Statistics record that most long-distance sailors spend just 15 percent of their time at sea—the rest is at anchor, moored or docked in a marina. Obviously, more power is needed when travelling from A to B as the navigation lights and instruments will be on in addition to the normal domestic requirements.

Running an engine to recharge the batteries is neither popular with the off-watch crew nor sensible in energy terms. Furthermore, if you’re travelling downwind along one of the favored tradewind routes, a wind generator will be a lot less effective and at night clearly you’ll get nothing from the PV array either. In fact, even during a sunny day the sails can often partly shade deck-mounted solar panels.

A Watt & Sea water generator mounts onto the transom just above the waterline

A Watt & Sea water generator mounts onto the transom just above the waterline

This is where a fixed or towed water generator comes into its own. Because the density of water is much greater than air, a water turbine will produce much more power at low speeds than the equivalent wind generator. Their output will also be more consistent, provided there’s enough wind to drive you through the water at four knots or more. Outputs are usually in the region of 1 amp to 10 amps between 3 to 6 knots boatspeed, which equates roughly to a modern wind turbine in 16 to 20 knots of apparent wind.

Hybrid wind/water generators: While the dual wind/water types of generator are versatile, they are a compromise and don’t necessarily produce as much power as a turbine specifically dedicated to one job. There are two distinct types of towed generator—those mounted on a fixed leg (such as the DuoGen, SailGen, Save Marine and Watt & Sea), which are deployed and recovered almost like an outboard. Then there are those with a separate alternator and a towed line with a turbine at the end (Aquair, Aqua4gen). The former are easier to operate, the latter rather more difficult to retrieve due to their spinning lines, which must be stopped by slowing the boat right down before they can be hauled in by hand. A trailing turbine can also prove tempting to hungry sharks, and setting them up to hang in the rigging as wind generators can often be tricky.

What a drag! The downside of all towed generators is that they produce drag, which can slow the boat down by up to a quarter of a knot—not that this is likely to worry a cruising sailor.


Rapidly improving technology has helped make the dream of power self-sufficiency more of a reality today. For around $300 you can keep your engine battery fully charged on a mooring. Just $600 worth of solar panels can power you for two days at anchor without having to run your engine; $2,000 spent on the right combination of charging devices should suffice for a week’s holiday away from shore power, and twice that will enable you to kiss the grid goodbye forever.

Duncan Kent has tried every means of power generation on his own boats

MHS Summer 2016




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