Hybrid Power Keeps Going
The diesel-electric hybrid as an auxiliary power source for sailboats has moved from the laboratory into the water. Though still in early development, it has advantages including fuel efficiency, ease of handling, responsive motor control, low sound levels, immediate-use capability, and, on some systems, power regeneration.
I have a Solomon Technologies motor and a Glacier Bay genset on my 43-foot Tom Wylie–designed, Schooner Creek-built sailboat, Prestissimo. I finished Prestissimo's sea trials in April 2005 and cruised about 3,000 miles in four months—sailing offshore, in coastal waters, and through the ICW.
Hybrid systems always have a fuel-powered generator and an electric motor. Some systems connect the generator directly to the motor while others use it to charge a bank of batteries, which then run the motor. Each approach has advantages. While the direct connection is simpler, it sacrifices some of the hybrid’s primary advantages.
Prestissimo has one 960-amp-hour (calculated at 12 volts) battery bank that serves her house and motoring needs. I chose this amount of storage based on my electrical requirements for cruising. If I had less equipment aboard, I could use a smaller, lighter, and less-expensive battery bank. This battery bank, though, is wired in series to achieve the 144 volts required to run the Solomon electric drive. To supply the boat's 12-volt demands, a DC-DC converter steps the 144 volts down to 12 volts. While a 960-amp-hour battery bank is large, it is not inordinately large, and it is not dedicated solely to the motor.
Now let me explain why a battery bank is so important and what advantages even a small-to-moderate-size battery bank will offer.
The most important reason for using a battery-powered motor is that it keeps running when the genset stops. Everyone accepts that when your engine quits, you loose power. We also know that engines seem to quit at the worst times. It doesn't have to be that way. For example, last summer we were motoring with the tide and against the wind out of the Cape Cod Canal. We had the genset on. Not surprisingly, waves built up where the outgoing tide met the opposing wind, and it was there that the genset cut out. (I suspect because of air in the raw-water intake.) The genset is so quiet and vibration-damped that we didn't hear or feel it shut down over the sound of the wind and waves. Eventually, I glanced at the amp meter and noticed that it was off. Thanks to the battery bank, we were able to motor at full power for another 15 minutes until we got safely out of the cut. Then I restarted the genset. With my 960-amp-hour bank, I could power for more than an hour before the batteries would run low.
The second reason to use battery power is safety related. There are times when I anticipate a tricky situation: sailing through a cut, going under a bridge, or picking up a mooring. These are situations when it is fun to sail, but prudent to have the engine ready, just in case. Other times I failed to anticipate an immediate need for power. In both cases, I'm ready for full power at moment’s notice with a battery bank and electric motor, and there is no need to warm up the engine.
Finally, when I'm sailing, the spinning propeller charges the batteries. On a good day I can run all the boat’s systems, including the refrigerator, freezer, stereo, computer, radar, autopilot, and instruments, and still finish the day with more power stored in the batteries than when I started. As long as the wind continues to blow, I can cruise without turning on the genset. In fact, when the wind is up during a passage, the batteries quickly recharge to full. Under these circumstances I can power up the motor just enough to use up the excess current and, as a bonus, reduce the drag from the propeller. If the wind isn't cooperating and I anticipate extended motoring, I turn the genset on when I start the motor.
Here are some technical details illustrating the advantages of using a small battery bank. The amp-hour rating on a battery assumes that it will discharge slowly and completely over a period of 20 hours. If it discharges more quickly, it won't reach the rated amp-hours; discharge it more slowly and you’ll get more amp-hours. In addition, a battery can charge and discharge only so many times before it no longer can accept a charge. If you run your batteries all the way down, you will get fewer cycles out of them. A good rule of thumb is to discharge the battery halfway before recharging.
There are two reasons why we can't use the rated-amp-hour capacity of the battery in our simple calculations: (1) With a powerful motor we will be discharging the battery much faster than at the 20-hour rate; (2) In order to preserve its life, we don't want to discharge the battery more than halfway. Peukert's Exponent is expressed in a formula to determine the useful capacity of batteries that are discharged at various rates.
This table takes into account the effect of rapid-discharge losses and battery-life conservation for AGM batteries.
As you can see from the Battery Discharge Rates Table, the modest bank of 108-amp-hours would give you 6 minutes at moderate power. That’s plenty of time to finish tacking or avoid an obstacle and hopefully enough time to deal with an emergency and get the genset started.
The ample bank of 336-amp-hours gives you 21 minutes at medium power or reasonable battery times for your house loads. With regenerative power during sailing, you might be able to avoid turning on the genset under good conditions.
My Solomon ST37 uses 37 amps at full power. I opted for about an hour at full power, two days of house loads, or some combination of the two before either a steady breeze or running the genset is required to charge the batteries.
There is one final reason to use hybrid power that may trump all techno incentives—the system is fun. I enjoy silently pulling into my slip without emitting exhaust, maneuvering without manipulating a transmission, and powering my boat's systems by sailing. Hybrid power is here to stay for cruising sailboats, and, as more people choose hybrid power, it will become easier to use and less expensive to install.
Schooner Creek Boat Works, www.SchoonerCreek.com
Solomon Technologies, www.SolomonTechnologies.com
Glacier Bay, www.GlacierBay.com
Powersonic Batteries, www.power-sonic.com
Prestissimo Sailboat, www.Prestissimo.org