Why aren't these devices more prevalent? Answering this question requires a little historical perspective. Water power has been used in land-based applications for centuries. More recently, water in rivers and streams has been harnessed to produce electricity for a wide range of hydroelectric projects, from small-scale installations to enormous regional facilities. Yet it's been only in the last 25 years that water-powered electrical generators have been used for marine applications, and sailors are still just getting used to the idea that the relative motion between a moving sailboat and the surrounding water can also produce significant amounts of electrical power.
The most common and oldest style of water generator is the trailing-log type. On this type of unit the generator is mounted at the stern of a sailboat in some type of gimbal arrangement that allows it to pivot up and down and side to side. Attached to the generator shaft is a length of heavy line that trails in the water. At the aft end of the trailing line is a stainless steel or aluminum shaft with a rotor (similar to an outboard propeller turned backward) that makes the shaft and line spin rapidly when towed by the boat. The faster the boat speed, the faster the trailing line and generator shaft rotate. In turn, the higher the rpm the greater the generator output. Permanent magnet generators are typically used in this application since their power output characteristics are well matched to the rpms created at typical cruising speeds.
An alternative to the trailing-log type water generator is the outboard-leg model. On this type of unit the generator is mounted at the top or bottom of an outboard leg mounted off the sailboat's stern. If the generator is located at the top of the leg, similar to an outboard motor head, bevel gears are used to transmit the horizontal mechanical power of the rotor up to the generator. Conversely, water generators designed to be mounted at the bottom of an outboard leg are connected directly to the rotor, and they must be completely waterproof since they are under water when in use.
Usable power begins to be generated at about three knots of boat speed. While no one seems to have good supporting data, it is estimated that drag due to a water generator results in only about 1/4-knot loss of boat speed. With more than six or seven knots of boat speed, the spinner on the trailing log generators has a tendency to skip out of the water; certain sea conditions can even cause this to happen at lesser boat speeds, necessitating the retrieval of the spinner until conditions improve.
There is another type of water-powered generator that works well on any boat with an engine shaft and prop that free-wheels when the boat is under sail. A great deal of electrical power can be produced from this arrangement, where an electrical generator is mounted inside the boat to the engine shaft via a belt and a set of pulleys. A mechanical or electromechanical tensioner is used to disconnect the generator when the engine is used for propulsion, and a voltage regulator is needed to prevent battery overcharge.
Another method of using a freewheeling prop for generating electrical power is to install a complete electric propulsion system such as the Electric Wheel from Solomon Technologies Inc. In their system an electric motor powered by a bank of batteries serves as a silent means of propulsion. The same motor can also produce electrical power to recharge the battery bank whenever the boat is under sail and the prop is freewheeling.
No matter what type of sailing you do, water-powered generators can be an important, cost-effective source of charging power on board.
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