Water power Page 2
The amount of energy that can be extracted from the wind is a function of the diameter of the blades, which for obvious reasons is limited on boats. Most wind generators kick in at wind speeds of around 7 knots, with output rising disproportionately as the wind speed increases. However, the output in the varying wind conditions experienced on a boat will be well below the steady-state numbers in published literature. Once you put these things together, most wind generators need wind speeds of around 15 knots before real-world output is much greater than 2 to 3 amps at 12 volts.
Most cruisers favor downwind passage routes, which keeps the apparent wind speed below 15 knots for much of the time. We then seek out protected anchorages, which again keeps the wind speed below 15 knots. As a result, unless you are anchored behind a Caribbean reef with tradewinds whistling through your rig, a wind generator can rarely keep up with a boats electrical load. However, it is a useful power contributor, which is why I always install one.
Because of the highly variable output, its impossible to develop the same kind of generic payback numbers as with solar panels. However, pretty much whatever way you slice or dice it, wind generators come out looking good, but will not eliminate reliance on a diesel engine for power generation.
Conservation is key
No matter how effective the mechanism used to extract energy from the wind or water flowing past a boat, there will be times when energy is in short supply. To optimize the use of renewable energy sources and minimize engine running time. Therefor, it is important to conserve energy on board. That is why it is important to replace incandescent and halogen lights with fluorescents and LEDs and to improve the insulation and door seals on refrigerators and freezers. Conserving fresh water minimizes watermaker use. If you have air conditioning, accepting a cabin temperature a few degrees higher than normal makes a big difference.
On ocean passages, using a windvane saves the constant drain of an autopilot, and in mid-ocean its not necessary to run electronic charts 24 hours a day. We also shut down our radar in fair weather, although it should be noted that insurance companies may try to invalidate an accident claim if the radar wasnt on at the time.
If you conserve energy, even the lowest-powered water generator will go a long way towards meeting a boats energy needs at sea and more powerful units can eliminate altogether the need to run an engine solely for power generation.
Nigel Calder has written many technical articles and books, among them Nigel Calders Cruising Handbook, published by International Marine.