A year or so back, we had a hybrid power list which Georgs wisely discontinued because of lack of interest. But as energy prices rise, the frugal boater finds it prudent to look for alternative ways of stretching fuel supplies. A couple of recent posts on wind powered electrical generators prompted me to dust off this almost forgotten memory. It should have been posted on the hybrid power list - but alas---- Wind power is a feasible auxiliary power source for passagemaking recreational trawlers. Wind power comes in many forms, sails, Flettner rotors, and windmills. Windmills have been very successfully coupled to generators to charge batteries and provide a source of electrical power for sailboats and even for trawlers at anchor. Windmills have also proven effective as a non-polluting method of augmenting electrical power ashore. While working on a USAF/NOAA weather reconnaissance program in the 60s, we experimented with an unanchored weather buoy which used wind power to maintain its offshore position. The buoy was an oval platform about 20 ft. long and 15 ft. wide fitted with weather monitoring equipment and a radio transmitter which submitted synoptic weather reports. Power was generated by a Darrieus rotor, a hoop shaped vertical windmill about 10 ft. in diameter and 15 ft. tall, coupled to a generator. The power was stored in a bank of batteries which also served as ballast. The Darrieus rotor is a windmill of moderate efficiency, about 35%, and has the advantage that it can receive wind from any direction. It also has a unique aerodynamic characteristic that permits the blades to stall and slow down, preventing overspeeding in very high winds. The weather buoy monitored its position by Loran. (GPS was not available at the time.) When the buoy got too far off station, an electric motor would cut in and the buoy would slowly move back to the proper position. The top speed was no more than a couple of knots so the buoy could not be placed in areas with significant currents. The buoy maintained position within a 10 mile circle for several months, transmitting weather information on schedule, until it was run down by a freighter. Several similar buoys were in development at the same time by other companies. As far as I know, none have been operationally deployed. I've had several e-mail letters from alternative power advocates suggesting that windmills could be directly coupled to the propeller of a trawler to provide propulsion power without the intermediate step of converting to electricity. Several have cited yacht research society articles from Britain and the USA. There is no doubt that a large enough windmill could provide sufficient electrical power to a trawler to handle most of its electrical needs, but would it have enough power to serve as its main propulsion method? The critical question is could the boat move on all courses regardless of the direction of the wind. The largest body of wind power data was gathered in the late 70s and early 80s when the search for alternative energy sources was at its peak. Calculations made at the time show that the maximum theoretical efficiency of a fan or windmill in extracting energy out of the windstream is 59.3%. The windstream in the swept area of the blades is slowed to 30% of its former velocity. In practice, real wind generators rarely extract more than 40% of the energy from the wind. The drag on a windmill working at high efficiency is nearly the same as the drag on a flat plate of the same size as the swept area. I'm not making these figures up. They come from a US Dept. of Energy report titled "Wind Power for Farms, Homes, and Small Industry" originally published in 1978 and revised several times since. Now lets put these figures in a boating context. Assume that we have a high efficiency windmill mounted on a suitable hull, coupled to a propeller. Further assume that we have used the highest state of the art design and the overall efficiency of the mechanical system for extracting energy from the wind and using it to propel the boat is 50%. This would require windmill and propeller efficiencies of greater than 90% and no loss whatever in the gearing. These efficiencies are virtually impossible to obtain in practice but I don't want to be accused of bias. Further assume that the blades sweep a circle 15 feet in diameter, resulting in a swept area of 176.7 sq. ft., and that the wind speed is 20 mph. Further assume that the hull is so well streamlined that it presents no air resistance at all. Now lets give the hull some dimensions. Assume a catamaran with LWL of 20 feet and a displacement of 2000 lbs. This is about the size of the Tornado, a popular racing class. According to the above mentioned DOE report, the power generated at sea level at a 60 degree air temperature by the windmill and transmitted to the water will be 3.67 kilowatts or 4.9 hp. The drag on the swept area is approximately the same as the drag on a circular flat disc of the same area. At a wind speed of 20 mph, the drag is approximately one pound per square foot. This gives a wind drag of about 177 lbs. pushing the boat backward. Marine engineers estimate the bollard pull of a highly efficient propeller at about 30 lbs. per applied HP. Assuming that the propeller is maximally efficient, the 4.9 hp would produce a thrust of 147 lbs or 30 lbs less than necessary to hold the boat in a fixed position against the wind. Thus the boat with windmill spinning and propeller churning at maximum efficiency would move slowly backward through the water. Note that I have assumed the best possible conditions, virtually unobtainable in reality. My back of envelope calculations show that the boat could make very slight headway at 34 degrees off the wind. This is in the same range as the most highly developed racing yachts. How fast would the boat move at right angles to the wind? Using Keith's formula from Skene, 4.9 HP would move a 2000 lb, 20 ft. LWL boat at about 7.1 Kts. This is faster than hull speed and is in the semi-planing region. This speed is comparable to that obtained with conventional sails. Now suppose that we double the windspeed to 40 mph. The power generated by a windmill rises as the cube of the increase in windspeed. The windmill will now produce 39.2 hp yielding a thrust of 1176 lbs. The drag on the circular swept disc of the windmill increases as the square of the change in windspeed so it will now be 708 lbs. Under these conditions our theoretical boat could make some progress directly into the wind. This assumes, of course, that the boat will not capsize under these storm conditions. But what would happen in practice? Windmills currently in use in power generating systems extract no more than 40% of the wind's energy instead of the 53% assumed in the previous theoretical model. Propeller efficiencies for boats are rarely greater than 60%, typically 50% or less. Mechanical losses in the gearing necessary to transfer windmill rotation to propeller rotation will further reduce the available power by 5 to 10%. Adding these inefficiencies together and we find that only about 20% of the energy in the windstream will be available to move the boat. The available thrust will be cut in half to 588 lbs, again less than the wind pressure pushing the boat backward. If we add in the wind drag of the hull and superstructure the picture looks even more discouraging. Even in a full gale the wind does not provide enough energy to move a boat against it. Autogyro boat zealots, for that is what windmill powered boat fans are called, find themselves in that gray area between theoretical possibility and practical impossibility. The conclusion is that the windmill scheme is not feasible for moving a trawler against the wind and is not significantly better than traditional methods of boat propulsion off the wind. We will have to look elsewhere for an energy saving means of mechanical propulsion. Perhaps wait until cold fusion comes on line. Larry Z