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.