Design developments have made hydrogenerators the most efficient form of sustainable auxiliary energy for yachts. Sam Fortescue reviews the latest innovations

The potential of hydrogeneration has been well understood for decades, but it’s only over the past 15 years that the idea has started to become properly commercialised. In essence, it’s a very simple concept: the boat’s motion through the water turns an alternator which generates electricity to recharge the onboard batteries.

‘Every yachtsman owns a sophisticated wind energy machine in the form of the yacht itself,’ explains Peter Andersen of Eclectic Energy, which manufactures the Sail-Gen and Duogen hydrogeneration systems. ‘Hydrogeneration provides a simple way of converting a small part of the power harnessed by the yacht’s sails directly into electricity.’

Because water is a much denser medium than air, a small hydrogenerator fitted to the transom of a yacht can potentially produce a great deal more energy than if a larger wind turbine was fitted to it.

‘Kinetic energy increases proportionally to the cube of the velocity,’ adds Andersen. ‘That means producing 300 amp-hours per day or more is a realistic prospect while cruising at 7-8 knots with a hydrogenerator fitted. And it’s the reason why more and more boat owners are now opting for water generation, although it’s still not as popular as it should be.’

By contrast, a tradewind run with a wind turbine might yield just 80 amp-hours per day, because the apparent wind on a run is lighter. Good performance might see 40W output from 10 knots apparent. Similarly, solar panels only produce a fraction of their rated output for most of the day, so you’d need a large array to match the return of a modern hydrogenerator.

A blend of all three will, of course, give the best outcome for different conditions at anchor and under way – but on passage, it’s hard to beat the benefits of a good hydrogenerator.

Watt&Sea’s hydrogenerator is a flexible option

Speed v drag

The speed versus drag issue has always dogged hydrogeneration technology, mainly because hydrogenerators only put out meaningful energy once a boat reaches speeds of five knots or more. However, the increasing use of hydrogenerators on the international offshore racing circuit has started putting paid to most of those concerns, with all manufacturers now agreeing that the extra drag of the impeller in the water is negligible. It also becomes non-existent once the boat reaches hull speed.

The main issue for cruising sailors is the boat speed required to obtain meaningful energy from a hydrogenerator, which is mostly a question of the diameter and pitch of the impeller. Just as you need to ensure that an auxiliary propeller is matched to both the power of the engine and the speed of the hull, so the impeller on a hydrogenerator must be correctly sized for the speed of your boat under sail and desired power output.

The pitch also needs to be set to offer the optimum angle of attack for the boat’s likely range of speed through the water. That’s why leading manufacturers offer a choice of impeller, or at least an adjustable one.

Hydrogeneration is the most efficient form of sustainable auxiliary energy for yachts

The Watt&Sea is the most flexible in this respect. Its two models (300W and 600W maximum output) can be equipped with a 240mm diameter impeller or a 280mm unit. The difference in output is striking: at five knots boat speed, the smaller one produces 100W, while the larger one can manage 120W. Over the course of a day, that amounts to an extra 40 amp-hours on a 12V system.

The smaller impeller has a maximum boat speed of 11 knots, which will be enough for most cruising monohulls. Only sporty catamarans would need to consider the smaller 200mm impeller with its 13-knot top speed. Faster than that and you’d need to look at a race model with a variable pitch.

Charging performance

Apart from price, the factors differentiating the various brands on the market are their charging performance and mounting system.

Eclectic Energy is the only manufacturer whose generators rectify the current on board to produce a DC output that can be wired directly to the batteries. This has the benefit of simplicity, but requires beefier cables running all the way from the generator to the batteries.

The Duo Gen hydrogenerator from Eclectic Energy

Watt&Sea’s hydrogenerator puts out a three-phase, low-amperage alternating current that runs through a DC converter. The converter monitors the battery voltage to determine whether it is in the higher power absorption phase or the lighter float phase of charging. After that, an electromagnetic brake stops the impeller if it’s spinning too fast and becoming dangerous.

Swi-Tec’s charge controller funnels all the power from the generator into the batteries until they reach a predetermined voltage. At this point, the MPPT (Maximum Power Point Tracking) algorithm reduces the charge to nearly nothing, a buzzer sounds to alert the skipper and the impeller brakes itself automatically. It can then be changed to ‘freewheel’, allowing the propeller to rotate freely.

The Remoran is perhaps the most ambitious of all, with a charge controller that can automatically detect the system voltage and then apply a ‘smart charging algorithm’. It also interacts with a smartphone app via Bluetooth, allowing you to keep a close eye on its performance and power output.

A Sail Gen hydrogenerator mounted on a yacht’s transom

Mounting systems

While Watt&Sea supplies a pod-based system that can be permanently fitted under the hull, a removable transom fitting is still the most popular choice, allowing the unit to be removed and stowed when not in use. Alignment, though, will have a significant impact on a hydrogenerator’s performance, with the best results coming from a clean, clear flow of water. Manufacturers therefore recommend an offset mounting outside a direct line with the rudder.

There is a balance to be struck, however, as heeling can make one tack more productive than the other. On twin-rudder boats, a central mount works well.

Swi-Tec and Watt&Sea both offer an optional pivoting mount for fitting to a raked stern, while the Remoran has in-built pivoting as standard, using a spring-loaded knob to tab between different settings and offering a rake of up to 40 degrees. Watt&Sea also offers different leg lengths to suit different installations, while the Remoran Wave 3 has a telescopic leg that can be adjusted by over 30cm.

Watt&Sea unit with an optional pivot mounting for attaching to a raked stern

The Seagen and Duo-Gen take a very different approach to mounting, with a flexible yoke providing a single point of attachment. The alternator is attached at this end, while the impeller is at the end of a 1.6m tube which can be fastened out of the water in a vertical position when not in use. ‘It means the units are not rigidly attached to the yacht but are free to pivot,’ explains Eclectic Energy’s Peter Andersen. ‘This de-couples the impeller from the hull movement, reducing stress on the mounting.’

Main propeller regeneration

While a standalone transom-mounted unit is the most common type of hydrogenerator technology, there is another approach – using your main propeller to ‘regenerate’ electricity whilst under sail.

Oceanvolt’s HighPower ServoProp

Fully electric propulsion from the likes of Oceanvolt and Bell Marine has long had this capability built in, as do systems from Torqeedo and ePropulsion. But parallel hybrid systems are also available, where an electric motor is installed alongside the engine to both generate electricity and drive the boat electrically. Lynch Motors in Devon has been supplying its permanent magnet DC motors to the Vendée Globe boats for years, purely as a re-generator, and now produces the flexible Red Snapper motor for cruising yachts.

The only problem with a regeneration system is that the pitch required for the propeller to drive the boat efficiently through the water may not always be the same as the pitch for optimum regeneration. ‘The lift surface on a propeller is on the wrong side for efficient extraction of energy from the water,’ explains Eclectic Energy’s Andersen.

Manufacturers have tackled this in different ways. Oceanvolt has developed its ServoProp for saildrives, which electronically adjusts its pitch depending on speed and function. The latest incarnation allows total 360-degree blade mobility and faces forwards, which increases efficiency, albeit at greater risk of collision damage. At six knots, it produces an astounding 1kW of power.

Bruntons has another solution with the cleverly engineered Autoprop, which automatically pitches up to match the boat speed. Its Ecostar version of the prop can generate 200W at five knots and up to 1kW at 10 knots when connected to an electric motor.

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