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Battery charging Under Sail

Jan 1, 2003

For the past 20 years or so of voyaging aboard Fiona, my well-traveled Westsail 42, I have used a succession of generators coupled with the propellor shaft to pump a few amps into the 12-volt system. The shaft is free to rotate under sail, and at speeds over about 4 knots the current generated is usually sufficient to sustain a small refrigerator, the lights and low-power electronics like the GPS receiver. This technique is primarily of interest to the long-distance voyager. The average day sailor probably does not spend enough time under sail to recoup the drain while at anchor. I will not present detailed "how-to" instructions; all boats are different, so I will only spell out the problems and show how they were solved aboard Fiona.

The first two generators I used were made commercially by Lucas Marine in England and specifically intended for this application. They had a built-in regulator that energized the field when the generator output was sufficient to charge. The field was regulated to limit the output current to the thermal rating of the windings and to avoid over-charging. The field winding drew about 2 amps and this limited the balance of the output current available to charge the batteries to about 4 amps at a boat speed of 6 knots. Nevertheless, these generators worked quite well for many years; on a trip from Tahiti to Bermuda via the Horn in 1991-92 we rarely needed to run the engine to keep the batteries charged on a trip of more than four months. The first generator simply wore out and I replaced it with a similar unit that lasted many years with a little maintenance, such as changing the slip-ring brush.

When it finally died, an internal short drew such a heavy current that the brush holder literally melted. At that point I discovered Lucas Marine was no longer in business and a replacement was not available. I then turned to a DC generator intended for use with a wind-driven propellor. The manufacturer was quite happy just to sell me the generator. I made a support for it so that it could be turned by a rubber belt using pulleys to give a step-up speed ratio of about 6:1 from the shaft. The unit had a permanent magnet stator field and a wound rotor with a commutator that provided a DC output via two brushes. Because it required no field current, the unit charged at about 6 amps at a boat speed of 6 knots. I regulated the current by means of a transistor bank in series with the output.

Its weakness lay in the commutator brushes. Soon after I installed it I sailed around the world, but the output current dropped to zero after about 20,000 nm. I replaced the brushes but the unit only lasted another 5,000 nm before failing again. On disassembly I found the brushes had eaten away the commutator bars and the rotor was useless. The last generator installed on the boat was also originally intended for a wind-driven application. It features a permanent magnet rotor and a wound stator that provides a three-phase AC output rectified by built-in diodes. It has no brushes and, as it is slightly larger than the previous units, it easily provides 8 to 10 amps at a boat speed of 6 knots. This unit is still functioning perfectly after about 23,000 nm. Mechanical considerations

The generator is mounted on the underside of the cabin sole above a clear portion of the propellor shaft. The mounting frame is made by welding a 1 1/2 inch steel angle in a rectangular shape. The frame carries a rod from which the generator is suspended, and a slotted arm permits the belt tension to be adjusted (see photo and sketch). A pulley on the shaft drives the generator via a half-inch belt. The pulley ratio depends on the design of the generator; the idea is to start charging at a boat speed of about 4 knots. On my boat, 4 knots corresponds to a shaft speed of about 150 rpm using a 20 x 14 propellor. The earlier Lucas generators required a speed of about 850 rpm for the output to rise to 12 volts and start charging. The later model permanent-magnet AC generator requires only about 450 rpm to start charging, provided by a 7-inch pulley on the shaft and a 2 1/2-inch pulley on the generator. The propeller shaft diameter is 1 1/4 inches and the inside diameter of the pulley on this shaft has to be machined to match; it can be tedious getting the correct pulley for charging to start at the right speed.

Electrical issues

The generator output voltage is proportional to shaft speed and, hence, to boat speed. For example, if the generator output is 12 volts at 4 knots and charging just begins, the output voltage will be 21 volts at a boat speed of 7 knots, which could easily cause a current that exceeds the thermal rating of the generator. Thus, the current must be limited by an external regulator, and the generator must be protected by a fuse or circuit breaker. When the battery is fully charged the current from the generator must be reduced to avoid damaging the battery; another task for the regulator. When the DC generator was installed, a series diode was required to prevent a reverse current into the generator at low boat speed.

A new problem arises when the engine is put in gear; the shaft speed will increase by a factor of four or five, resulting in a generator output in the 60-volt range. On Fiona, the starter solenoid is powered via the starter key from an "engine on" circuit breaker. When this breaker is energized prior to starting the engine a relay switches the shaft generator output from the battery circuit to a fairly high-value resistor which draws a modest current and prevents voltage transients that may damage the rectifier diodes if the generator is left open circuit.

When I installed the AC generator I discovered the hard way that the manufacturer had incorporated a high-voltage limiting device in the rectifier circuit. The first time I ran under power the device overheated and damaged the generator itself. I removed the voltage limiter and depended on the resistor to protect the generator under "power on" conditions. This solution has worked flawlessly for the past year and a half. Over the years the voyaging benefit of charging under sail has been immeasurable. I have saved many thousands of diesel hours - not to mention the lack of noise. I believe an even bigger generator could provide virtually all amenities required on the average cruising yacht - except, perhaps, air conditioning! Even if 30 amps is produced by a shaft generator, this still corresponds to less than a horsepower at the shaft, depending on the regulator efficiency - far less than the power derived from the sails to drive the boat in the first place. The effect of charging on the boat speed is not noticeable.

Contibutiung editor Eric Forsyth has sailed hundreds of thousands of miles, including two circumnavigations.