Story by TMC Editor / May 15, 2017

By Kevin Jeffrey, 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.

Of the three renewable charging sources for marine use, water-powered generators are the most overlooked and least understood. The fact is that they can be an important part of your independent power system, quietly, reliably producing large amounts of electrical power whenever you are under sail.

Water-powered generators take advantage of the relative motion between a sailboat and the surrounding water, and so actually use the wind for mechanical power, unlike land-based water-powered generators that rely on falling water under the influence of gravity. Multihulls are well-suited for water generators, since they typically have high average speeds when cruising and have ample space on the stern for mounting water generator gear. With a charging rate of 8 to 10 amperes in cruising speeds of 5 to 6 knots, water generators can produce over 200 ampere-hours of electrical power per day.

There are several types of water-powered generators available. On Trailing Log units, the electrical generator is typically suspended on a gimbal mount at the stern, while an 8- to 10-inch diameter rotor mounted on a short stainless steel shaft trails behind the boat at the end of a 50- to 100-foot tightly wound braided line. The line spins rapidly, turning the shaft of the generator directly to create electricity for charging batteries. In use for many years, this type of unit is simple, moderately priced, and some versions such as the WaterPower 200 from Hamilton Ferris Co. can be converted to a wind-powered generator for use in port. But these water units are only intended for blue-water passage-making, and multihull sailors must ensure that the trailing rotor remains submerged—with optional pieces of gear—or is physically removed from the water at higher boat speeds or in certain sea conditions, otherwise the rotor line tends to kink and knot up unmercifully if the rotor skips clear of the water.

On Outboard Leg units such as the Aquair UW, the electrical generator is submersed on a pivoting bracket at the stern, similar to an outboard motor. These units are heavier, more expensive, and require more mounting space, but they remain operational at any boat speed or in any sea condition or cruising area.

Electrical generators can be coupled to a free-wheeling inboard engine prop shaft with good results—high power output and no additional gear mounted on the stern—and they can also be coupled to a small auxiliary rotor shaft placed through the hull for the sole purpose of battery charging—probably the cleanest, most efficient installation for those undaunted by yet another hole below water line.

Perhaps the most promising type of water-powered generator for multihulls is a combination electric motor/generator that serves as the boat’s auxiliary motor for boat propulsion as well as a generator when under sail. These types of systems are currently under development and should be available in the near future.


Recent News

We take pride in our happy customers

I had a very good experience with Will Miller at TMC. He is very knowledgeable about his business and took care of all aspects of my purchase, and where he couldn’t places me in touch with the people who could. The transaction went very smoothly as far as I was concerned.

— Jeff

“Thank you Conor, for your integrity, continuous input and guidance during this process. The Multihull Company is a class act that I have had the honor of working with twice. I look forward to referring Conor to all I know in need of a broker.”

— Jeff and Julie

Your team delivered on every mark. You’ve got an amazing broker in Mark Wattrus and I’m confident you will have continued success moving forward together. He was available 24/7 whenever I had questions. We had a great initial visit on the boat and his marketing plan and photos were superb

— Jimmy

I couldn’t be more grateful to have had TMC on my side throughout this process. Buying was a new experience for me and I know it wasn’t the easiest deal you’ve ever had, but you were competent and kind throughout.


— Chad

Thanks Jaryd, you made this process as smooth as we could have hoped for! We are most certainly going to have some memorable times aboard!

— Cordell and Jennifer

See All Testimonials

Trust & ExpertiseAbout Us

Successful relationships cannot exist without it. At The Multihull Company we base every relationship on a firm commitment to earning and retaining our client’s trust.

Advice of any kind is valuable only when grounded in hard-won expertise. It too, must be trustworthy. Trust and expertise define the heart and soul of The Multihull Company. We are a team of skilled professionals who thrive on providing expert, trustworthy advice and service to catamaran and trimaran sailors around the globe.

Read More

Follow us @themultihullcompany