On a long trip, there are various approaches to being energy self-sufficient on board a yacht or boat. For example, solar cells, and towed or wind generators can be used as regenerative energy suppliers. First of all: It is pointless to compare them since each system is intended for a different situation and has its justification.
Small wind turbines are predestined for the power supply on ships. Good wind conditions ensure high yields from the system. PV systems as a possible environmentally friendly alternative take up considerably more space than a small wind system, which is usually not available on sailing ships.
A wind turbine can also generate electricity at night. The infrastructure for a DC power supply including batteries is available on most ships, the wind turbine only has to be connected to it.
Here’s an example: Solar cells work primarily when the sun is shining, but also when there is no wind and regardless of whether the yacht is underway or not. The wind generator, on the other hand, only supplies energy when there is wind, but also when the sky is cloudy or at night and also regardless of whether the ship is underway. Drag generators, on the other hand, follow a completely different concept. They don’t care about the weather, but they only work when the ship is moving. This is disadvantageous for long periods at anchor. In short: one would compare apples, pears and bananas. Therefore, the following is only about wind generators without reference to other variants of energy generation.
Special requirements of small wind turbines on ships
The maritime use of a small wind turbine poses special challenges that differ from systems operated on land.
Noise development: On a ship, the turbine is usually located in the immediate vicinity of the crew’s whereabouts. Not only your own team can feel disturbed by the noise of a small wind turbine. Consideration must also be given to the neighbors at the jetty or anchorage.
Suitability for sea water: Salt water and the associated corrosion place special demands on the material of a wind turbine.
Stable yaw tracking: The heeling of a ship or rolling movements in front of the wind often result in turbulent wind conditions that have to be compensated for by stable yaw tracking.
Wind generators on yachts
The spread of wind generators on yachts has increased massively in recent decades. Whereas they used to only be found on yachts, they can now also be seen on many cruising yachts that sail from your doorstep. I think that’s because they don’t take up much space, are affordable and, if the workmanship is of the right quality, are almost maintenance-free. In other words, they easily generate electricity — at least when the wind is blowing. And that is definitely the case in most areas – apart from a trip in the doldrums.
If the purchase of a wind generator is planned, it is far from enough to just look at the expected performance. Rather, a wind generator is a relatively complex overall system made up of various components. The condition of the rotor blades plays just as important a role as the processing and the quality of the individual parts. Not to forget: charging regulations or dealing with storms.
Having a reliable system on board is particularly important for yachts that can find themselves in extreme conditions. I would therefore like to show below which components play which role in this overall system.
The rotor of the wind generator
The rotor is the most important unit on the wind generator since the energy yield depends significantly and directly on its design. Wind generators use the kinetic energy of the wind and convert it into electrical energy. The surface of the blades and the diameter of the rotor play a role here. A doubling of the diameter usually goes hand in hand with a quadrupling of the area. The more area I oppose the wind, the more energy can be generated. However, one should also bear in mind that cruising yachts in particular must have enough space to accommodate a large rotor.
An important rule of thumb applies to all wind generators:
WHEN WIND SPEED DOUBLES, THE ENERGY CONTENT OF THE WIND INCREASES EIGHT WARDS.
This means that the energy available not only depends on the area of the rotor but in particular on the wind speed – even to the third power.
In concrete terms, this means that if a wind generator has an output of 25 watts at 10 knots of wind, for example, it has an output of 200 watts at twice the wind speed of 20 knots. With the wind increasing to 40 knots, we would be at an enormous power of 1,600 watts. This is a major challenge for designers and manufacturers, because very high wind speeds supply a lot of energy, but also lead to extreme stress on all components of the wind generator.
If the load becomes too great because the rotor is turning faster and faster (also known as “Overspeed”) and because the bending load on the rotor blades is also increasing, you have to react because the wind generator itself, and possibly even the batteries, would be damaged. So I need a way to stop the wind generator. More on that in a moment.
The power of a wind generator
The fact that doubling the wind speed leads to an eightfold increase in power (also known as the cubic ratio) can also be viewed the other way around: For example, if a manufacturer states that the maximum power of its wind generator is 30 knots (wind force 7) wind and then 360 watts, based on the rule of thumb mentioned above if the wind is halved to 15 knots (a good 4 wind forces), only one-eighth of the maximum power remains. That’s 45 watts. If the wind drops again by half to 7.5 knots (weak 3 wind forces), only one sixty-fourth of the maximum power remains. In our example, that leaves 5.6 watts, around 0.5 amps for a 12-volt system.
It quickly becomes clear that promises made by some manufacturers that their generators would start up and generate electricity at particularly low wind speeds, even if they were true, would not result in any significant energy yield in practice. Physical laws cannot be outwitted.
Incidentally, the cubic ratio also applies to the fact that the wind sensor of a yacht is usually attached to the top of the mast, but the wind generator is often located at the stern at a height of around three meters above the deck. Turbulence caused by deck structures, sprayhood, or bimini can lead to a reduction in wind speed at the height of the wind generator.
The wind gradient also leads to a reduction. This is the effect that the wind speed decreases from height to ground due to the stratification of air molecules. For example, if the wind speed at the wind generator is only ten percent lower than at the top of the mast, the power contained in the wind is reduced by 27 percent – according to the above formula! (100 percent minus 10 percent = 90 percent. If you triple 90 percent, you get 73 percent [0.9³ = 0.729]). In other words: That is only 73% of what the wind generator would have achieved if the wind speed measured at the top had reached it fully below.
Understanding this cubic relationship between power and wind speed is important because we as humans are used to the fact that most things are in a linear relationship to each other. Example: If the water maker is supposed to process twice as much water, then it also needs twice as much energy.
Stopping the wind generator
There are many reasons – not just storms – to stop the wind generator. Depending on the yacht’s construction and the location of the wind generator, if it is operated while sailing, I recommend turning it off before maneuvers. When reefing, a line quickly snaps and gets caught in the rotor, or a lot of water comes over from the front and hits the rotor blades. Good seamanship here involves keeping safety in mind and acting responsibly.
Other times when you might want to turn off the wind generator are for inspection, in port, when the batteries are full, or when the wind gets too strong. Unfortunately, wind generators are still being offered, which causes unpleasant noises due to the lack of the required quality. The thought of switching off is also not far away.
Not all wind generators are storm-proof, and depending on the manufacturer, they have to be stopped above a specified wind force. For most models, the limit is between 25 and 30 knots. “Stopping” can be done in different ways:
Turning the wind generator out of the wind
In this variant, the rotor is turned out of the wind and tied down. This is a method that is not entirely harmless, as it requires a crew member to approach the device, which is running at full speed. Some owners use the boat hook and use it to turn the wind generator on the wing out of the wind and then tie it down. Personally, I think this is far too dangerous at sea, as the risk of injury or even falling overboard is disproportionately high. In addition, there is always the danger of touching the rotor blades with the boat hook and damaging them permanently.
The electronic shutdown of the wind generator
Some suppliers proceed in such a way that electronics monitor the wind generator and switch it off electronically when the speed and/or power reaches a certain threshold.
Here, the wind generator is electrically short-circuited. The short circuit represents the highest possible electrical load for the generator and causes the rotor speed to drop significantly. Due to the deceleration, the aerodynamic flow to the rotor blades is much worse, which means that their drive torque collapses. Even if the wind continues to increase, the rotor usually only runs slowly, does not accelerate significantly, and is therefore in a safe state.
However, the wind generator does not come to a complete standstill because the short-circuited generator needs rotation to build up the braking counter-torque. It is important to know in this context that the electrical braking torque of a generator is always limited. If the drive torque generated during generator operation exceeds the braking torque of the generator when there is a lot of wind, braking is no longer possible. The result would be Overspeed and that could be fatal.
In addition, electronics always entail a certain risk. The salty blue water environment is not ideal. In addition, it can fail in a thunderstorm due to overvoltage.
Stop switch for the wind generator
In my opinion, it is therefore advisable neither to turn the wind generator out of the wind nor to switch it off electronically. Instead, I would switch it off manually using a switch if the wind conditions still allow it.
In this variant, the rotor is stopped using a stop switch. If the switch is thrown, the wind generator is short-circuited. This creates a magnetic field in the generator, which makes turning difficult. The generator does not stop completely with this method, but it only rotates slowly. The price for this investment is extremely manageable and the impact is great.
The only catch: This variant only works for some manufacturers at very low wind speeds. From an energetic point of view, switching off too “early” is counterproductive: you have a wind generator on board, and it is switched off just when the wind is blowing hard and electricity could be generated very well.
Storm safety through rotor blade adjustment
Another way of preventing damage caused by overspeeding in a storm is not to let the excessive energy reach the electric generator at all. With small wind generators that are used on land, there are systems in which the entire rotor turns out of the wind in a storm or tilts up into the so-called helicopter position. These systems work tolerably well on land, but cannot be used on board because of the sea conditions and ship movements that are unavoidable in this weather. That is why this technology is not even offered for yachts.
On the other hand, the so-called rotor blade adjustment, which is standard in all large wind turbines today, works very well. This system is also available for small wind generators for use on board.
In wind generators with rotor blade adjustment, the angle of the rotor blades to the wind is adjusted by a special mechanism encapsulated in the rotor hub with the help of air and centrifugal forces when the wind increases. In this way, the impending Overspeed can be intercepted. The rotor then continues to rotate at a safe speed and continues to produce energy. The angles of the rotor blades are changed automatically and continuously in such a way that the performance does not exceed the rated power provided by the design.
Because the blades are turned mechanically, the system reacts in fractions of a second. This is of great benefit. Because every owner who has to deal with regular gusts on his trip or with an apparent wind of more than 25 knots benefits from it. Wind gusts, such as those that occur at a number of anchorages in the Caribbean, the Mediterranean, or on the Cape Verde Islands, should not be underestimated.
For safety reasons alone, Overspeed must not occur under any circumstances. This also includes uncontrolled idling at high wind speeds. It can occur if, for example, the wind generator is running without a load (no connection to the battery bank) due to a fault in the electrical connection. The resulting very high speeds mean that the rotor blades can no longer withstand the centrifugal forces, break and fly away. We’re talking about speeds of around 2,000 revolutions per minute and more! This would be an extremely dangerous situation as people could be hit.
Every reputable manufacturer must therefore be able to answer the question of how his wind generator is protected against such Overspeed. Automatic systems are always preferable to those that require manual intervention. Systems for automatically preventing Overspeed are the already described shutdown by the electronic controller of the wind generator. The automatic rotor blade adjustment is even better, as it always keeps the rotor within the permissible speed range purely mechanically and is not dependent on the functioning of electronics and electrics.
And anyway: In a storm, as a sailor, I actually have something else to do than take care of the wind generator because it turns at too high speed – it should work reliably and automatically in this regard.
Assembly of the wind generator
Ideally, the wind generator should be mounted on its own mast at the stern. The height should be chosen so that the wind generator is well above the heads of the crew. Mounting on the bow or in the mast is not recommended.
At the bow, the device can get tangled too easily with sheets blowing out, and too much weight is mounted at the top of the mast, which has an unfavorable effect on the stability of the ship. Here is the rule of thumb “One kilo of extra weight in the masthead must be balanced with five kilos of weight on the keel if the stability of the ship is to be maintained”.
If you own a ketch, however, it makes sense to mount the wind generator about halfway up to two-thirds of the way up the mizzen. However, the owner is only happy with a wind generator that already has structure-borne noise decoupling installed. This serves to prevent vibrations and magnetically induced running noises from being transmitted to the ship. It is constructed in such a way that the wind generator has no metallic connection to its connecting piece (mast mount) which is screwed firmly to the mast or to the bracket.
Ideally, this is achieved by mounting it in soft elastomer rings that are specially adapted to the frequency range of the generator. This design is very efficient, takes up little space, and allows the elastomer rings to be protected from harmful UV light. Compared to the widespread masts completely mounted in rubber, there is a further advantage in that the transmission of the vibrations to the mast itself is prevented and it cannot be excited as a resonance body.
In the case of generators without built-in structure-borne sound decoupling, it is important to note that the mast of the wind generator and any struts are acoustically decoupled from the ship with thick rubber plates. Otherwise, vibrations generated during rotation are transmitted to the interior of the ship and, depending on the installation location, can disturb the sleep of the crew.
By the way: On a boat, the forces acting on the wind generator are relatively unimportant for the dimensioning of the mast. Crucially, crew members will hold on to it and it must be designed to be stable enough to allow for this. The stability is then always sufficient for the wind generator.
Wind generator wiring
Depending on the distance to the battery bank, the cabling should be dimensioned accordingly in order to avoid losses due to insufficient cable cross-sections (otherwise part of the energy will be converted into heat). A cable cross-section of 10 mm² should be the minimum for a 12-volt system on a cruising yacht. 16 mm² cable cross-section is better.
Important: The larger the cable cross-section, the better the braking effect with the stop switch.
The energy management associated with the wind generator is also a very important issue that is often underestimated. Producing energy is one thing. Managing them is another. I think it is easy to understand that a wind generator supplies energy very erratically. If there is suddenly a strong gust that doubles the speed of the wind, the energy flow increases eightfold, as explained above. If the battery is then almost fully charged, I need a system that recognizes this and carries away the excess energy.
Again and again one reads in books about circumnavigators that they turn on all sorts of consumers to get rid of excess energy from the wind generator because the batteries are full. This is no longer up to date! There are enough wind generators on the market that can reduce excess energy. The rotor blade adjustment is only one step. Because even with the rotor blades turned away, energy is still being supplied.
I, therefore, need an energy management system that reduces the energy flow if necessary without damaging the batteries. This is just as true on the high seas as it is in port when you leave the ship for a few days with the wind generator switched on.
Excessively produced energy can be dissipated via load resistors. They convert the excess energy into heat, thereby preventing damage to the batteries. As the name suggests, load resistors are resistors. They have a metal body with a rib-like structure. This creates a large surface area through which heat is dissipated.
Attention: Load resistors can get very hot and therefore the installation location should be chosen carefully. Some owners stow them in the engine compartment – assuming a fireproof lining.
Wind generator charge controller
To ensure that everything works, a charge controller is connected between the wind generator and the battery, which can usually be obtained from the manufacturer and therefore also fits the system. The charge controller should have two separate outputs for the starter and consumer battery. Each output line is protected with a sufficiently dimensioned fuse.
What the individual manufacturers understand by a charge controller is very different. Some manufacturers solve the problem of protecting the battery from overcharging simply by having the controller measure the voltage across the battery during charging. If the voltage exceeds a preset limit value (end-of-charge voltage), the generator is stopped electronically. This feature is often present in the controller anyway to protect against Overspeed. It is true that the battery is not overcharged in this way. Unfortunately, it is not really fully charged either. At least not when the generator is delivering a high current in strong winds, which is precisely why the voltage at the battery terminals is increasing at that moment. Then the wind generator switches off loudly, only to start up again a few minutes later, also clearly audibly, in order to try it again. This process can be repeated frequently and can be very annoying when lying at anchor at night.
Other charge controllers are more complex and work in such a way that they actually deliver an electronically regulated voltage to the battery and can thus fully charge the battery. If the wind generator is currently supplying more energy than the battery can absorb without being charged with too high a voltage, the controller uses so-called pulse width modulation (PWM) to derive the excess energy from a load resistor.
The energy supplied by the wind generator in large variations depending on the wind is therefore constantly distributed between the battery and the load resistance in such a way that the battery is charged correctly, but not overcharged. The wind generator as a whole always remains electrically loaded to match the current wind. If the charge controller did not feed the excess energy to the resistor, this would result in a lower electrical load on the wind generator and an undesirable increase in speed.
Note: The output voltage on the charge controller must match the battery type and be adjustable according to the battery manufacturer’s specifications. When ordering, the battery type should be specified at the same time so that the charge controller can be configured appropriately. For a reputable manufacturer, this is a completely normal process. In addition, the charge controller should be designed in such a way that the voltage can be adjusted. For example when changing the batteries.
When using lithium-ion batteries on a yacht, there are special requirements for the charge controller. One of the most important is that the charge controller can autonomously control the wind generator. What does this mean? A correctly constructed lithium-ion system (LiFePO4) has a battery management system (BMS), which under certain conditions must be able to completely separate the battery from consumers and chargers to protect it. However, with one exception, charge controllers for wind generators are fed from the battery for their own consumption. If the BMS now disconnects the charge controller from the battery, it will not function and will no longer be able to control the wind generator. As a rule, the wind generator will then run up to high speeds up to overspeed without a load, generate a high no-load voltage and possibly destroy its charge controller because its components were not designed for the high voltage that has now occurred.
A charge controller that gets its supply directly from the wind generator is not only more economical in calm periods, but when the battery is disconnected it sends all the energy to the load resistor, thus limiting the voltage and speed of the wind generator. The setting of the end-of-charge voltage is also carried out with this controller according to the specifications of the battery manufacturer.
Another important point is that the wind generator can be safely on board even if the connection to the battery is lost. It could be a bad wire or a blown fuse. There are models that damage the turbine if the wind gets too strong. I would also like to pay attention to this.
With such a solution, the wind generator can work completely independently and the skipper does not have to intervene. Or to put it another way: The blade adjustment prevents overspeed and at the same time reduces the energy flow, while the charge controller distributes the remaining energy flow – including the peaks from gusts – to the battery or the load resistors. According to need.
Noise level at wind generators
The faster the wind generator rotates, the more audible it is. Many sailors are familiar with the typical hissing and whistling noise that occurs when the wind generator is running at high speed. In this regard, there is always criticism from neighboring residents who complain that the rotors are too loud. That no longer has to be the case. The technology has meanwhile developed further and some manufacturers offer so-called whispering blades (silent blades). Small, raised triangles can be found on the individual blades of these rotor blades, which ensure an acoustic break and thus make the blades quieter.
Influence: True and Apparent Wind
Another point of criticism we hear at trade fairs from time to time is that the wind generator is rather worthless on a circumnavigation along the barefoot route because it is said not to be very useful in the trade wind latitudes at sea. There, the true wind when sailing often comes from aft. On board itself, however, only the apparent wind blows, which is less by the headwind force. In the eyes of the critics, this is not enough to generate enough electricity. This statement is not entirely true.
Here is an example: Suppose the wind is blowing from aft at 20 knots and the ship is speeding at five knots. Then a net 15 knots of wind remain on board. The wind generator mentioned then still produces five amperes of electricity. That’s 120 Ah over 24 hours! On ships that sail under the wind vane and therefore do not need electricity for the electric autopilot, this is usually more than the daily requirement.
It should also be noted that the curve rises steeply as the wind continues to increase. If the apparent wind increases by just five knots, the current generated increases to 15 A. That’s triple the amount!
With this criticism, it should not be forgotten that most long-distance sailors spend a large part of the trip at anchor on a circumnavigation. The yacht is usually in the wind, and the apparent wind corresponds to the true wind. That means: At 15 knots of wind, the batteries are properly charged. The wind often blows stronger. I know enough sailors who have always had full batteries at various anchorages in the trade wind zone.
There are several ways to charge the batteries — although a sailor should not rely on one way alone. Instead, a sensible combination of the various options must be selected depending on the energy requirement and the shipping area. The wind generator is a valuable resource. Not least because it does not incur any further costs for years after purchase and installation. What’s more, it reliably charges the yacht’s batteries. At anchor or at sea. In the sun, in the rain, in a storm. And also in the absence of the crew.
Anyone who decides on a wind generator should not only look at the performance of the wind generator, as described. It is much more important to look at the system as a whole. This includes rotor technology, generator efficiency, charge regulation, safe ways to start and stop the wind generator, storm safety, and how to deal with excess energy. If the right components are chosen, I have a system on board that works reliably in the most extreme conditions – over a very long period of time and completely independently! Exactly that should be the claim of every cruising sailor.