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Old 04-21-2003
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Charging with Wind Power


The mizzen-mounted wind generator here will help keep this crew's batteries topped off.
Sailors who are planning some extended cruising sooner or later must address the issue of how to keep their batteries full without endless hours of engine running time. For cruisers with moderate to hefty electrical loads, the answer may be blowin' in the wind. That's because wind-powered generators can be an efficient, effective charging source whenever your boat is away from the dock.

We all know there is energy in the wind; we see its power in a number of ways, including as a source of propulsion for our sailboats. Yet extracting that mechanical power and changing it into electrical power available via the batteries is not as straightforward as it sounds. On an average size sailboat, the cost, performance, size, weight, maintenance, and noise are all considerations that must be taken into account when considering the purchase of a wind generator. And despite roughly 25 years of wind-charger use in marine applications, there are still compromises to be made in order to take advantage of this renewable resource. Sailors who are able and willing to make these compromises are rewarded with a steady supply of free electricity.


Choosing an unfettered location for your wind generator is only the first step.
Let's first review wind generator capabilities. A wind-powered charger can be your main source of DC electricity, or one of several primary charging sources. Keep in mind that all charging sources complement each other and can feed simultaneously into a single bank of batteries without harm. Of course, there are many types of wind-powered generators on the market, each with a slightly different design approach. In general, smaller units weigh less and are easier to accommodate on board, while larger units can produce significantly more power. The type of wind unit you purchase relates mostly to your type of boat, your budget, and how much electricity you need. The amount of electricity you actually get out of a wind charger dependends upon these factors as well, but it also involves where and how you cruise. To understand why, we must delve slightly into the theory behind wind generators.

In mathematical terms, the maximum amount of electricity you can produce from a wind charger is proportional to the square of the blade diameter (i.e. the total swept area of the rotor) and to the cube of the wind speed. In simple terms, this means that although the blade size is an important factor in wind-charger output, wind speed is even more critical, and this relates to where and how you cruise. Some cruising grounds, such as southern Florida and on down through the Caribbean, have very good average wind speeds, while other regions such as the Chesapeake Bay have low average wind speeds during the cruising season.

When sailing, average apparent wind speeds are good when the wind is forward of the beam, but not so good when the apparent wind is well aft. In fact during downwind passages wind-charger performance, on average, is abysmal. And remember, the winds can be steady and strong in one anchorage (or in one part of the anchorage) that has good access to the wind, and variable or turbulent where the wind is blocked by trees, buildings, and headlands. Variable or turbulent winds don't have near the available power as winds with a clear fetch.


Wind generators generally don't produce well off the wind due to the decreased apparent wind.
You can see that simply having a good wind generator on board doesn't necessarily mean it will be producing anywhere close to its rated capacity; it takes good winds to have what I would term "acceptable" performance. To complicate matters further, most charging sources, including wind chargers, are rated according to their maximum power output in ideal conditions, which are typically much different from the average conditions encountered when cruising. For example, even though a popular wind unit on the market is rated at 400 watts, it takes almost 30 knots of wind to realize this output. Thus, on average, over the course of a cruise, this unit may produce only a small fraction of its rated power. The key to being satisfied is to know how much power you can expect from a specific wind generator on your boat and under given cruising conditions.

In addition to choosing a wind charger for its electrical output, it's also important that the unit satisfies your sense of aesthetics and preferred method of mounting. These may seem like quite different considerations, but aesthetics are greatly influenced by the way you mount your wind generator.

There are three main mounting variations from which to choose. The most common wind generator mount in use today is the stern-pole mount. In this mount a stainless steel or aluminum tube roughly two inches in diameter and long enough to keep the wind blades above head and hand height is mounted to the stern and held firmly in place by two or more rigid metal struts. It is critical that the struts keep the pole from moving in all conditions in order to minimize the noise and vibration transmitted below decks. The advantage of a pole mount is that it always keeps the wind generator in position and ready for use. The downside is that it causes additional windage and clutter on a passage.


The stern-pole mount for this wind generator allows it to work while remaining out of harm's way
The second type of wind generator mount is a mizzen-mast mount, where the wind unit is mounted on a short metal tube welded to a small platform attached to the mizzen mast, similar to a radar mount. The metal tube must be far enough away from the mast to allow the wind unit to seek the wind through a 360 degree rotation. The advantage of this rig is that the generator has better access to the wind and is out of harm's way. The downside to this mount is that the noise and vibration can be transmitted belowdecks if good rubber insulation is not used, and only smaller wind units are appropriate.

My personal favorite wind-generator mount is a rigging-suspended mount that hangs the unit in the foretriangle when you are in port, allowing the unit to be stowed below before you put to sea. The advantage is that, with the unit suspended in the rigging and held in place by lines instead of rigid stays, much of the noise and vibration is eliminated. This mount also frees up the cockpit area when in port and allows for clean decks on a passage. In addition, the rigging-suspended wind units on the market can typically be converted to water generators when underway.

Whichever mount you choose, make sure you follow the common sense rules of safety when using a wind generator on a sailboat. By taking simple precautions, and adjusting your cruising so you have good average windspeeds available, you'll make the most of having a marine wind generator.

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