On-board electricity from wind: When is a wind turbine a worthwhile investment?

Every sailor knows that wind energy can be harnessed effectively. On land, too, it is now one of the most important renewable energy sources: in Germany, around a third of electricity now comes from wind power – more than from solar power, coal or gas. It is sustainable and works even at night and when the sky is overcast, provided there is sufficient wind. On yachts, renewable energy sources promise self-sufficiency: you can remain independent for longer from shore power connections and fuels, which can only be kept on board in limited quantities. There are three main solutions to consider here: Solar panels, towing generators or wind turbines. However, direct comparisons of output are of little use, as these energy sources meet different needs and are therefore often used in combination.

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Solar panels are particularly worthwhile during the summer months, as they allow you to make the most of extended periods at anchor. This is because the output depends heavily on the intensity of sunlight, which varies depending on the season and geographical latitude. Towed generators, on the other hand, are primarily aimed at long-distance sailors, as they only generate electricity whilst the boat is under way. They are also only worthwhile whilst sailing, as the alternator fulfils this function when motoring.

Wind generators operate both whilst under way and at anchor – and do so even at night or when the sky is overcast. They are therefore the perfect complement to solar panels, particularly as the latter require a fairly large, largely shade-free surface area to deliver high output, which is a rare commodity on sailing yachts. A wind generator, on the other hand, takes up very little space and can therefore fill any gaps in power supply.

Numerous parallels with sailing

Wind turbines require nothing more than moving air. As with sailing, the power output corresponds to the wind speed – raised to the third power. This means that if the wind speed doubles, the power output theoretically increases eightfold. This sounds promising, but the same applies in the opposite direction: if the wind speed is halved, only one-eighth of the power can be expected.

Out on the water, as with sailing, the apparent wind speed is crucial. On a close-hauled course, the wind turbine benefits from this – on a downwind course, the usable wind power is reduced. Added to this is the operating height, as the wind is slowed down near the ground. The wind speed indicated by an anemometer mounted at the top of the mast therefore does not necessarily correspond to the force acting on the wind generator, which is fitted lower down. However, installing the latter at the masthead is not an option for various reasons: space requirements, masthead weight, difficulty in accessing it, and stress caused by heeling and yawing.

Another analogy with sailing arises in the relationship between surface area and power output: a larger rotor diameter means greater output. However, the number and shape of the blades also play a part. Some models therefore deliver higher power outputs than larger generators, despite having a slightly smaller diameter. On standard cruising yachts, however, there are strict limits on the diameter. After all, the rotor must operate at a safe height above the crew’s heads. Mounting masts commonly found on yachts are generally around three metres high. Added to this is the considerable weight of larger generators.

Performance and returns

For cruising yachts between approximately 35 and 50 feet, wind generators in the 350 to 450-watt power range currently dominate the market. Depending on the design, they usually weigh between 6 and 15 kilograms. The rotor diameter ranges from 1.10 to 1.50 metres. Smaller units are now rarely found on the market, which is likely due to increased battery capacities.

The 350 to 450 watts refer to the rated power output. This requires correspondingly high wind speeds. For example, the Superwind SW 350-II wind turbine only reaches its rated power output of 350 watts when wind speeds reach around 24 knots. The Silentwind 400+ requires a wind speed of around 28 knots to reach its rated power of 420 watts. In both cases, therefore, strong wind conditions are required. This rated wind speed is specified in the technical product data.

In a moderate breeze of 12 knots, both units deliver only around 40 to 50 watts. These figures can be found in the respective power output curve, which is available on the manufacturers’ websites for many wind generators. This provides a good basis for a reliable estimate of the yield. Some suppliers also provide graphs showing the expected monthly or annual yield as a function of the average wind speed in the area. However, the actual output depends not only on wind conditions but also on other device-specific characteristics, such as start-up behaviour in light winds, the shape of the power curve, and the control strategy in strong winds and storms.

Start-up speed and throttling

Just as any sailing yacht only gains speed once a certain wind pressure is reached, a wind generator also only begins to operate once the wind speed reaches a certain level. For most models in the specified power class, the cut-in speed is between approximately 2.00 and 3.50 metres per second. Depending on the model, power generation begins at around 4 to 7 knots of wind – this is the cut-in speed. However, the wind must not be too strong, otherwise there is a risk of what is known as overspeed. This refers to exceeding the safe rotational speed, which can lead to the turbine becoming overloaded and pose a danger. This is because a wind turbine contains a large number of moving components that are subjected to enormous centrifugal forces at high rotational speeds.

To prevent overspeed, the rotational speed must be throttled or the wind turbine shut down once a certain wind speed is reached. There are various ways to achieve this. The simplest is to manually turn the rotor out of the wind and secure it with a sling. However, given the risks to both the equipment and the operator, this is not recommended. A better option is electronic speed control. Here, the controller short-circuits the generator once a certain speed is reached, thereby electrically slowing the rotor. Advantage: Shutdown occurs automatically without the crew having to intervene. In most cases, the rotor continues to turn, but at a significantly slower rate – the altered airflow enhances the braking effect. Disadvantage: Energy output drops rapidly once a certain wind speed is reached. Furthermore, the system relies on the electronics functioning correctly.

In contrast, rotor blade pitch adjustment operates purely mechanically: some high-quality generators automatically adjust the angle of the blades relative to the wind at high wind speeds. This keeps the rotational speed within acceptable limits whilst the generator can continue to produce electricity. Here, the power curve does not drop during a storm but remains constant.

Regulated load

As another key component of any system, the charge controller therefore often performs several tasks simultaneously: it protects the batteries from overcharging, compensates for the sometimes significant fluctuations in power output caused by gusty winds, and helps to prevent the rotor from spinning out of control at very high wind speeds.

Furthermore, the charge controller should be able to divert excess energy to a load resistor should the wind turbine supply more electricity than the batteries can accommodate. This is also a prerequisite for safe electronic speed control. Caution: Such load resistors can become quite hot and should be housed in a well-ventilated area away from flammable materials!

The charge controller, generator and battery type should be compatible with one another. The cable cross-sections must also be able to safely withstand the expected currents. Wind generators for yachts are available for operating voltages of 12, 24 and 48 volts. In a 12-volt system, the rated current of a 400-watt wind generator can still exceed 30 amps.

Desirable qualities

In summary, an optimal wind turbine should therefore cover as wide a operating range as possible, have a power curve suited to the local conditions, start up reliably even in light winds, and still supply sufficient energy even at very high wind speeds (for example, through rotor blade pitch control). It is also important to have reliable overspeed protection that operates without manual intervention. In addition, the design must be robust and built to withstand harsh maritime conditions.

Prices in the aforementioned power class range between 1,500 and 3,000 euros. High-quality models impress with their durability, smooth operation and consistent power output. When combined with a solar panel system, they can help bridge gaps in energy demand, particularly in spring and autumn, when the sun is weaker but the wind tends to blow strongly. The same applies to overcast days during the peak season.

And this is true even with a moderate breeze: 45 watts of continuous power over 24 hours amounts to 1,080 watt-hours, or 90 ampere-hours in a 12-volt system.

Potential hazards

Standard yacht wind generators can reach speeds of up to 1,700 revolutions per minute during normal operation. With a rotor diameter of 1.10 metres, the tips of the blades then whizz through the air at more than 350 kilometres per hour (peripheral speed). At the same time, the blades become virtually invisible at high rotational speeds. Collisions – for example, with running rigging such as a flapping reefing line or loose backstays, or with a thrown mooring line – usually result in the destruction of the rotor blades. This poses a risk of injury to the crew or passers-by from flying debris. If moored alongside high quay walls, the crew could even find themselves directly within the rotor’s sphere of influence.

Against this background, it is advisable to fit the wind generator with a stop switch. This short-circuits the generator and enables it to be shut down in potentially dangerous situations – for example, before sailing manoeuvres or when mooring in port. Emergency stop switches must be able to withstand high currents and be compatible with the specific unit. Many generator manufacturers offer them as accessories.

All components of the wind generator should also be made from seawater-resistant materials. Otherwise, corrosion could cause parts to come loose or fail. When purchasing, it is advisable to look for products specifically designed for maritime use.

Noise prevention and installation tips

Occasionally, one reads in user reviews that a wind turbine robs the crew of their sleep. None of them operate completely silently, but many modern models have become significantly quieter – particularly thanks to aerodynamically optimised blades and improved bearing technology. As a result, the operating noise increasingly blends in with the general sound of the wind.

Whilst wind and blade noise is transmitted through the air, it is primarily structure-borne noise that is noticeable below deck. This is transmitted into the boat via the mounting bracket and can be prevented by thorough acoustic decoupling. Specialist retailers offer dedicated mounting kits with rubber or plastic buffers that effectively dampen the transmission of structure-borne noise. In some generators, such decoupling is already integrated into the mounting bracket as standard.

The generator should generally be operated at a height where, under normal circumstances, the crew cannot enter the area of the rotating rotor. Mounting on solid equipment brackets or special masts at the stern has proven effective. Moreover, this ensures a largely unobstructed airflow. The mount and suspension must be designed to withstand the constant load changes caused by sea state, heel and yaw over the long term. Mounting on a support pole usually requires bracing, particularly as stern baskets are generally not designed to withstand such stresses. Screw connections, dampers, bearings and rotor blades should be checked at regular intervals. Imbalances or unusual noises are often early warning signs that maintenance is required.

Book recommendations

Theory and Practice of Vehicle Electrical Systems explains the basics of a reliable on-board power supply – from batteries and charging technology through to DC power distribution and energy management. When it comes to installing a wind turbine, the book, above all, provides the necessary understanding to ensure that the generator, controller, loads and storage capacity are effectively coordinated as an integrated system.

Watts, volts and other hassles focuses on the technical principles behind on-board electrical systems: current, voltage, power, component selection and systematic troubleshooting. This title complements the overview of wind turbines by addressing how the energy generated can be utilised within the existing on-board system and how typical faults relating to installation, cabling or operation can be methodically narrowed down.

Are wind turbines a worthwhile addition to a solar power system, or do the noise, space requirements and safety risks outweigh the benefits for you? Please share your experiences and views in the comments.