Normally, a source of energy is required to make the propeller turn. Hydrogenerators turn this approach on its head. They are a true sailing speciality: as sailing yachts use the wind for propulsion, travelling through the water provides a renewable source of energy with remarkable efficiency, because the water flowing past generates enormous forces due to its high density.
In addition, with conventional displacement-type boats, there is the factor of hull speed: a cruising yacht reaches this at wind speeds of almost 15 knots. A hydrogenerator designed for this speed will then already be delivering a large proportion of its maximum output. If the wind speed doubles to 30 knots, the generator’s output changes only slightly – at least in theory – because the boat does not go significantly faster. A Wind turbine In contrast, its output increases with the wind and only reaches its maximum output in strong winds.
The output of a hydro generator therefore depends on the speed and the distance logged whilst under sail. On days spent moored in harbour or anchored in a bay, a hydro generator naturally produces no electricity at all. It therefore makes sense to monitor wind and Solar energy to be used as a supplement. Similarly, a towed generator can help to bridge gaps in the power supply if there is not enough space on deck for a self-sufficient power supply relying solely on wind and solar energy.
Once dismissed as a hindrance, now widely used
The idea is not new: some long-distance sailors used to employ wave generators, in which the propeller of the boat’s diesel engine was used to generate electricity whilst under sail. To achieve this, an alternator was driven by a belt connected to the drive shaft. However, the propeller caused significant drag and generated noise, whilst the energy yield was modest.
Another approach involved systems towed behind the yacht, such as the Aquair 100 and the LVM Aquagen. Here, the towed propeller unit is connected to the generator housing at the stern via a line. However, these towed generators require sufficient water depth and are not easy to handle.
In current models, the generator is usually much more compact and operates underwater directly behind the propeller. Only in the case of the Seawing the unit continues to be towed behind. On all other boats, the generator is usually attached to a fin or rod mounted on the transom. These units look more like an electric outboard motor or an additional rudder blade. They are highly efficient, whilst their hydrodynamically optimised design also reduces drag.
This makes them attractive even for the offshore racing yachts of the IMOCA scene. Yannick Bestaven, winner of the 2020/21 Vendée Globe, is even one of the co-founders of the manufacturer Watt & Sea, whose products are also used by Boris Herrmann on his “Malizia Sea Explorer” uses. Hydrogenerators are also found in the Class 40 scene: for example, Ari Känsäkoski had one on the 2024 Global Solo Challenge: Remoran product at the stern. His rival, Kevin Le Poidevin, used equipment from Ocean Power. In practice, cruising sailors can therefore safely disregard the braking effect of these generators.
A comparison of 10 models
Boat type and sailing characteristics are crucial
Many products are the result of sailors’ own developments, and some are even sold directly by their inventors. Which hydrogenerator is suitable for your own yacht depends on the type of boat and its sailing characteristics. This is because the units are always designed for a specific speed range. It is the speed through the water that matters, not the speed over ground as determined by GPS. You should pay attention to what is known as the ‘cut-in speed’. Power generation begins once the boat reaches this speed. There is also a maximum operating speed, known as the ‘cut-off speed’. If the rotational speed is too high, the propeller is at risk of cavitation and should be lifted out of the water.
When selecting a product, it is helpful to look at the generator’s performance curve. This shows the expected power output as a function of the boat’s speed and should match the specific characteristics of the yacht: For a fast boat, the generator must not reach its limits too early, even if this results in a higher cut-in speed. For slower yachts, on the other hand, an as early as possible rise in the power curve is desirable, even if this means the maximum output is reached sooner. The comparison table (pages 68–69) is limited to generators for a speed range of approximately 3 to 10 knots. These units are therefore of most interest to cruising sailors.
Some hydrogenerators can be adapted to different speed ranges. Eclectic Energy, Kitewinder, Ocean Power, SailnSea and Watt & Sea offer a choice of different propeller sizes for this purpose. The reasoning is that the faster the boat, the greater the force acting on the propeller. In such cases, a smaller propeller is sufficient to achieve the desired generator output, although this also increases the cut-in speed.
Conversely, a lower average speed requires a larger propeller to generate the same output. For example: Watt & Sea recommends the standard propeller for its cruising models at cruising speeds of 4 to 11 knots, the larger version for speeds of 3 to 9 knots, and the smallest version for speeds of 6 to 13 knots.
The shape of the propeller also plays a role, particularly the pitch. This determines the rotational speed achieved depending on the boat’s speed. Some manufacturers therefore offer adjustable propellers in their range, which also allow a wider speed range to be covered. At SWI-TEC, the propeller is adjusted according to the customer’s specifications before dispatch. Watt & Sea uses a controllable-pitch propeller for very fast racing boats. Its variable pitch extends the speed range up to 30 knots.
In terms of performance, the devices are very similar
Another selection criterion is the power rating of the hydrogenerator. Which one is required depends on electricity consumption and battery capacity, as well as on whether other energy sources, such as solar panels or a wind generator, are available. However, at an average cruising speed of 6 knots, the differences in the power curves of the listed units are not particularly significant.
It is also important to ensure the unit is securely mounted. Some mountings can be flexibly adapted to different transom shapes. Some manufacturers also offer additional mounts for specific installation scenarios. Height and shaft length are also important factors: a propeller that is too high could emerge above the water’s surface in rough seas, whilst one that is too low could unnecessarily increase drag. The manufacturer’s specifications regarding mounting height should be followed in this regard. It is also practical if the unit can be easily pulled out of the water or swivelled, and can be removed without hassle when not in use.
What is relevant to the purchasing decision
The purchase of a hydrogenerator should be carefully considered. First of all, it is important to make a realistic assessment of the yacht’s sailing performance: at what speed range does the boat mainly operate? At what speed does it become worthwhile to set sail, and what is the maximum speed your yacht is actually capable of achieving? You should then shortlist units whose power curve matches these factors or can be adjusted to suit them.
On top of that, a secure mounting option and ease of use are important. When making direct price comparisons, you should check whether a charge controller suitable for the existing batteries is included. This protects the batteries from overcharging and is essential.
Electric drive as a hydrogenerator
In principle, electric drives can also be used whilst sailing to generate electricity. This process, known as regenerative braking, is a feature of, for example, the electric outboard motors in ePropulsion’s EVO range. However, they can only charge their own battery, not external energy storage systems. The Spirit 1.0 EVO is said to deliver around 95 watts when travelling at 6 knots. This would mean its battery would be half-charged again after just under seven hours. The POD drives in the EVO range also feature energy recovery. Oceanvolt provides an example of the use of inboard motors as hydrogenerators. The Finnish manufacturer’s Saildrives are fitted with so-called servo-props – propellers with variable pitch that adapt to the respective operating mode. This is said to enable up to 1,000 watts to be generated at speeds of 7 to 8 knots. Some shipyards, such as Arcona, Garcia, Linjett and X-Yachts, already offer such Oceanvolt propulsion systems to their customers as an option.
The models in detail
Hybrid beings: Duo-Gen
The numerous similarities with wind turbines have inspired the British manufacturer Eclectic Energy to develop a combined unit: the Duo-Gen 3 operates as a hydro generator whilst underway and as a wind turbine whilst at anchor or in port. To do this, one simply needs to change the propeller and raise or lower the unit. The manufacturer describes it as an ‘excellent hydro generator with wind capability’. Eclectic Energy also offers a version known as the Sail-Gen, which is a pure hydro generator. In this model, too, the generator housing is positioned above the water. A wing plate keeps the propeller submerged.
Easygoing: Ocean Power
The Ocean Power was developed by Eckard Scholz. The engineer and sailor also markets it himself. When being raised or lowered, the device glides along a guide rail on roller bearings. The mounting system can be adapted to different installation scenarios. In addition, there are brackets for every conceivable stern shape. When not in use, the unit can be easily removed. The slim generator housing does not require a dynamic seal. Scholz refers to this as an ‘unsealed design’, which minimises maintenance requirements. The carbon blades of the propeller can be replaced individually. Alternative propeller designs are available on request.
Highly flexible: Remoran Wave
The Remoran Wave 3 from Finland is one of the most versatile products in terms of paddle blade design. This is thanks to a telescopic shaft and an angle of inclination that can be adjusted by 40 degrees. It also comes with three different rear mounts. For different speed ranges, there are two models to choose from, designated GD and GS, both of which fall within the 300-watt range. “95 per cent of customers opt for our Remoran Wave 3GS model, which offers better power generation at lower speeds,” reports Managing Director Samuli Aura. The unit is designed for an average speed of 3 to 10 knots.
Entry-level class: SailingGen
Armin Horn, a sailing enthusiast, qualified engineer and professor of mechanical engineering, sells his own invention, SailingGen, via his own online shop. Instead of being mounted on a streamlined shaft, the generator unit is attached to a simple pole. It can be swung up when not in use. Despite its robust construction from seawater-resistant stainless steel, the unit weighs just 5.5 kilograms, including the stern mount. The bracket can be adapted to different stern shapes. The charge controller (299 euros), which must be purchased separately, ensures continuous monitoring of the turbine load. An optional display and an Android app are also available.
All-purpose solution: Hydro Charger Universal
With the Hydro Charger Universal, SWI-TEC is launching a successor to its tried-and-tested standard model. “The major advantage of our new, lighter model is that we can adjust the pitch so that approximately 4 amps are generated even at a speed of just 3 knots,” explains founder Marco Bachmann. This is set to benefit smaller yachts. The adjustable propeller can be adapted to different speed ranges. In addition, various mounts are available for different installation scenarios – including a pendulum mount. The supplied controller can be operated via Bluetooth using an Android app.
Wide variety: Watt & Sea
Watt & Sea offers the widest range. Cruising sailors can choose between a 300-watt and a 600-watt model, both featuring a flow-optimised rudder blade design, each available in two shaft lengths. There is also a POD that is bolted securely beneath the hull. Propellers are available in three sizes for each generator, as well as one with an adjustable pitch. There are also racing models and special versions for specific boat classes such as the Mini 650 and Class 40. Bespoke designs are also possible. There should be a suitable solution to suit every requirement here, although the products are also among the most expensive on the market.
Diving equipment: Seawing
Just as exotic as it is new is the Seawing from France, which is towed behind a boat. The two propellers each drive a generator, producing a considerable amount of power. A buoyancy chamber and a hydrofoil keep the device at a depth of between 0.50 and 1.50 metres. The tow rope is 3.20 metres long and sheathed in a Dyneema braid, which absorbs the tensile force. “We are currently producing units 10 to 20 with improvements we have made based on customer feedback,” reports founder Oliver Normand. The connection box, featuring an integrated charge controller and Bluetooth, is included in the scope of delivery.
Book recommendations
Theory and Practice of Vehicle Electrical Systems explains the basics of a robust on-board electrical system – from ship’s batteries and charging technology to DC distribution and energy management. On the subject of hydrogenerators, the book provides the necessary technical framework to properly consider output, battery capacity and on-board consumption in conjunction with one another. Advice on materials, installation, maintenance and troubleshooting also helps when planning a seamless integration.
Watts, volts and other hassles It begins with the basics of electricity and guides the reader through the components of an on-board electrical system – from batteries and charging sources to electrical loads and bus systems. The focus is on explaining the relationships between current, voltage, power and resistance in an accessible way, as well as on systematic fault-finding.
Is a hydro generator essential on a long voyage, or are solar power and a wind generator still the more practical solution? Please share your experiences and views in the comments.
Sven M. Rutter
Freier Mitarbeiter