Humans have been harnessing wind power to perform useful work for hundreds of years. The same force that drives our sailing craft has been employed ashore in centuries past to mill grain into flour, and more recently it has been used both afloat and ashore to generate electricity. The wind spins a propeller attached to either an alternator or a generator that produces electricity by moving a coil of wire through a magnetic field.
In its simplest terms, the distinction between an alternator and a generator is the relative location of the coils and magnets. In an alternator, the magnets are on the rotor, and they spin inside the coils, which are mounted on the stationary component, the stator. These moving magnets induce an alternating current flow in the coils. This is then converted to direct current by electrically rectifying it through diodes that keep output current flowing in a single direction.
A generator uses magnets fixed in the stator to induce current in a group of coils that spin as part of the internal rotor. This also produces an alternating current, but it is converted to direct current by an electromechanical arrangement known as a commutator system. Carbon brushes are located around the perimeter of the rotor so the current they receive from the elements of the commutator always flows in a single direction.
Except where noted, the term "wind generator" will be used here to refer to a device that employs either an alternator or a generator to develop electrical power from the wind.
There are a couple of theoretical relationships that affect the ability of any wind-driven device to produce electrical power. All other things being equal (of course, they never are), doubling the diameter of the device’s propeller should cause output to increase by a factor of four. Also, doubling the wind speed should (other things being equal) cause electrical output to increase by a factor of eight. Thus, to obtain maximum output, it would appear logical for one to select a large propeller unit and operate it in high wind speeds.
There are practical limits to the foregoing considerations, however. At speeds of less than about five knots, the wind has insufficient strength to drive any generating device fast enough to produce useful output. When wind speed has built to about 10 knots, most generating devices can produce useful power, with actual output being roughly proportional to the diameter of the blades employed. As wind speed rises above an upper level of about 35 knots, other factors come into play to constrain electrical generation: output can rise high enough to damage the batteries that receive it, the wind generator’s internal parts can generate sufficient heat to damage mechanical and electrical elements, and high propeller tip speeds can cause mechanical failure.
Any assessment of a wind-driven electrical generating device cannot consider only output levels, but must also focus on a number of related issues, including the minimum wind speed at which it begins to operate effectively, the way in which it copes with winds above the normal (safe) operating range, and the way in which output is controlled to prevent battery damage. Other factors, such as noise, ease of maintenance, and mounting options also come into play to influence the selection of a wind generator.
Assessing minimum wind speed
With respect to the minimum wind speed a generator must experience to produce useful power, a crucial factor is an assessment of where and how the boat is to be operated. In regions such as Long Island Sound or Chesapeake Bay, where winds during sailing season tend to be light, a low wind speed threshold for power production may be crucial. In areas where stronger winds prevail, low wind speed output becomes a matter of less concern. Keep in mind, too, that wind speeds nearer the surface of the water, where the generator is likely to be mounted, will usually be less than those indicated at the masthead by wind instruments.
If a boat is used only intermittently for short cruises, a unit with low output at typical wind speeds may be quite sufficient. It may not keep up with the entire power load on the electrical system during the time the boat is in use, and it may need supplementary assistance from an engine-driven generator/alternator during longer cruises, but it can gradually recharge the battery bank in the extended intervals between uses. In contrast, if the boat makes frequent lengthy passages, it probably needs a generator with the highest possible output capacity, given the anticipated passage wind speeds. Otherwise it might not be able to keep the boat’s battery banks well charged in the face of the high daily electrical demand created by refrigeration, electronics, and autopilots.
Remember, too, that the boat’s use and location will also exert a significant influence on the total electrical load experienced. Boats occupied full-time in warmer cruising grounds may have far higher power demands for services such as refrigeration and ventilating fans and thus need a higher output power source to keep up with those loads.
In any comparison of output claims for wind generators, one fact is clear: there is no standard way to express output, so choices must be made with great care. Even a superficial review of manufacturers’ advertising claims reveals marked differences of opinion about rated outputs. Some companies may cite amperage output capability that is impressive until one realizes that those output levels only apply at 12.0 volts, a level far too low for useful battery charging. At the 14.0 volts needed to effectively charge the batteries, output amperage may be substantially lower than the advertised numbers.
The minimum wind speed at which the generator begins to produce power is also influenced by voltage, and output at less than 12.0 volts will be of no effective use. Some manufacturers may also cite theoretical output levels that cannot be attained in practice because the wind generator has an internal circuit breaker designed to protect components such as coils from overheating. This thermal protection keeps the unit’s output well below theoretical maximums.
At the upper end of the "useful wind speed" spectrum, at least five different techniques have been employed to keep the speed of wind generators (especially large ones) within safe limits:
1. The seemingly simple method of securing the blades with a tether or lashing. While it sounds easy enough, in practice this can be a daunting challenge. When the wind threatens to reach the upper limits for safe generator operation, the seas are usually getting rough, too. Before the blades can be secured, they must first be brought to a near standstill, usually by turning the boat or the generating device to a direction 90° from the wind. It doesn’t take much imagination to appreciate the hazards and difficulties involved in such a maneuverespecially when it must be done in the middle of the night on a violently pitching craft, with most of the illumination provided by lightning flashes.
2. An internal friction device or brake that is activated by the increasing centrifugal force that develops as propeller speed increases. These mechanisms may be adequate for coping with an occasional gust above the normal operating limits, but they will wear out or burn out rather quickly if sustained high winds are encountered. When such conditions are anticipated, the only safe alternative is to immobilize the propeller, with all the risks and difficulties already cited.
3. Use of an air brake, usually deployed by the increasing centrifugal force. These brakes create a drag that keeps propeller speed within safe limits. This technique is mechanically sound, but continued generator operation at high wind speeds may still produce dangerously large electrical outputs. This brings yet another considerationoutput regulation, discussed belowinto play.
4. Allowing the entire propeller/generatorunit to tilt backward as wind speed increases. This tilting reduces the effective blade area exposed to the wind, thus reducing the driving force and keeping rotation speeds within safe limits. A key requirement in using this method is sufficient clearance to allow the blades to spin in a near-horizontal orientation, rather than their usual vertical position.
5. Finally, designing and building the blades themselves so they flex, flutter, or stall at higher wind speeds. This reduces their efficiency to such an extent that their capacity to drive the propeller to higher speeds is impaired. While effective as a speed control, this approach can have the undesirable side effect of being extremely noisy.
Controlling electrical output
Aside from limiting the wind generator’s rotational speeds, controlling its electrical output is another crucialsystemcapability.Inanengine-driven alternator, the magnetic fields are produced electromagnetically by the field current, and the strength of the magnetism can be controlled by adjusting that current. Wind generators, however, use permanent magnets with fixed magnetic properties. Higher rotational speeds cause higher output levels, and in strong or sustained winds the output can reach potentially damaging levels. With winds in the 25- to 30-knot range, a high-output generator can produce as much as 400 amp hours of electricity daily, and, unless the boat is using a lot of power, this level of output can quickly overcharge and destroy even a large-capacity battery bank.
The most basic approach to controlling output is to have an operator monitor battery charge levels while the generator is running and shut the unit down manually (a simple matter in benign conditions) when the batteries reach full charge. While simple and effective, this method poses ob-viousproblemsin rough weather, and it also demands an attentive operator. Moreover, it cannot be used if the generator is aboard a boat that is to be left unattended. Just one squall with strong gusts while the crew is ashore can produce enough power to push a battery to the explosion point or to inflict internal damage to the generator through high current flows and resultant overheating of components.
Another control technique is to equip the generator with circuits that will electrically sense battery voltage and disconnect the generator from the battery when a maximum voltage level is achieved. The chief danger here is that suddenly reducing the electrical load on the generator can permit the propeller to speed up, perhaps to dangerous levels.
A widely used regulation system employs electronic circuitry to sense battery charge levels and divert excess generator output to some other load, such as a resistor or heat sink, where the unwanted power is dissipated as heat. This approach requires that adequate ventilation be available to protect other equipment from excessive heating and damage.
Regardless of the technique employed, every wind generator system must have an effective method for regulating output in a manner that prevents damage to batteries and circuitry.
Wind generator "noise" takes several forms. The first is the acoustic variety. Some of the units with two or three long blades can sound like a helicopter hovering over the boat. As abrasion from airborne dust, nicks from occasional impacts with foreign objects, and gradual weathering of the blade surfaces occurs, the audible noiselevelproducedbysomewindgen-
eratorscangetworseandworse.While the acoustic problem seems to be less noticeable with smaller-bladed models (especially those that use a larger number of blades), these units also tendtohaveloweroutputcapabilities, so a lower noise level has its price.
Generator noise can also take the form of vibration. If there is the slightest degree of imbalance among the components, all of which are rotating at relatively high speed, the result can be substantial vibration. This may occur over a wide range of wind speeds or it may focus on a few critical speeds, but the vibration can be transmitted through the generator mountings to the hull and be sensed throughout the boat. I have been aboard a boat with a large wind generator mounted at the masthead, and when wind speeds approached 10 knots the whole mast hummed like a guitar string, while at 15 knots the vibration could be felt in every solid object that touched the hull. This can wear on the crew as insidiously as other types of noise and induce subtle fatigue levels that impair alertness.
Wind generators that employ generators (as opposed to alternators) are also prone to creating another type of noise: radio frequency interference, the product of tiny sparks created as the brushes ride from one segment of the commutator to another. Alternator-type generators can also impair radio signals, but radio-frequency filters installed on the output circuits can usually prevent serious degradation of communications. Many generator systems also employ a system of slip-rings and brushes in the mountings. They do so to enable the power-generating unit to rotate freely in response to changing wind directions without risk of excessive twisting of the output wires. Salt, dust and corrosion prevalent in the marine environment can easily cause these slip-rings and brushes to generate radio frequency interference similar in nature to that produced by commutators.
Generator mounting is yet another important variable. Some units can be suspended from halyards on the foredeck, an arrangement suitable only for in-port use, while others offer options that include mounting on independent poles, on radar arches, or on mizzenmast brackets. Regardless of the location, the mounting must be strong and solid, capable of withstanding the significant stresses imposed as the boat pitches and rolls. The spinning propeller will function like a gyroscope to exert precessional stress on the mounts in addition to the stress caused by the weight and mass of the generator itself.
Any wind generator must be located where all of the crew and all of the rigging will be clear of its spinning propeller blades. Even at seemingly low speeds, propeller tips speeds are high and the end of a spinning blade can inflict a nasty cut. Units should be mounted high enough to prevent an upraised hand from being inadvertently extended into the propeller path. Obviously, the mounting site must also allow the unit to freely rotate through 360° (if so designed) without striking obstructions, and, if it is a model that adapts to high winds by tilting the propeller plane, adequate clearance in the tilted mode must also be assured. Protection against inadvertent entanglement with halyards, topping lifts, and other running rigging is also advisable, not only to prevent damage to the rigging but also to avoid fouling the propeller and damaging the aerodynamically sensitive blades.
While most generator units are relatively light in weight (20 to 40 pounds is common), when combined with their poles and mounting brackets they can adversely affect a boat’s stability. A higher mounting point will increase crew safety and put the generator up where wind strength is less degraded by surface friction. However, it will also increase the unit’s negative impact on stability.
A final consideration in the selection of a wind generator is its ease of maintenance. Many are advertised as "maintenance free," but experienced sailors agree that there is no such critter: any piece of gear you take to sea can and will malfunction. When it does, the availability of parts (brushes, bearings, blades) and service will become critical. Before choosing a generator know if the unit must be returned to a manufacturer or if it can be repaired by local service facilities. Also determine if the service/support network covers your cruising grounds. It is worth spending an hour or so to study the service/repair/maintenance documentation to see if it is clear and user-friendly or cryptic and obscure. A good unit is one that can be worked on by someone with less than an advanced degree in mechanics and electrical systems.
Directly related to the issue of maintenance is the weather-tightness of a wind generator. It will be enduring the worst nature can offer, with broiling sun, high internal operating temperatures, frequent baths in salt spray, occasional freshwater rain soaks, and perhaps a slug of solid sea water now and then. None of these conditions are favorable to parts that must spin freely with minimum friction, stay aerodynamically slick, and conduct electrical power through moving contacts. Most generators are remarkably well protected against nature’s onslaught, but eventually they will all require some degree of maintenance, if only a periodic cleaning of bearings and contacts. The manufacturer’s documentation should tell the owner how to do this work, not simply say the product is "maintenance free." It should also provide explicit guidance for assuring that the unit is restored to weather-tight condition after service.
Pricing of wind generators is quite competitive, with many units and systems carrying basic prices in the $1,200 neighborhood. When the necessary or recommended ancillary equipment (power monitors, regulators, isolators, linkages to the rest of the power system, etc.) are added, the bottom line gets closer to $1,800. Some units offer supplementary capabilities, such as the ability to connect the generator to a towed spinner. This will enable a boat to generate electrical power when running downwind, a point of sail where apparent wind is least, making wind generators less efficient.
Wind generators certainly provide an attractive and effective alternative to relying upon shore power sources or a propulsion engine for keeping the electrical system charged. They work best in conjunction with solar and other passive generation technologies that can kick in when wind alone is insufficient. In comparing units, the manufacturers’ performance data needs to be examined with the closest of scrutiny to assure a valid conclusion.
In making a selection, the opinions and experiences of those who have used wind generators in the area in which one plans to sail would certainly be a valuable addition to the equation. Regardless of the choice, the owner must understand both the capabilities and the limitations of the system selected, especially its minimum/maximum wind restrictions and its regulation system. Measures to keep noise of all forms to an acceptable minimum will make the generator much easier to live with. Finally, to ensure that "free" electrical power is always available, the owner must also be prepared to maintain and repair the unit, perhaps in a distant harbor using makeshift tools and without access to the manufacturer.
Hal Sutphen is a retired Navy officer, delivery skipper, navigation instructor, and freelance writer who lives in Kilmarnock, Va.