Ive created a 24-volt system by connecting two 200-amp-hour 12-volt batteries in series to drive an electric outboard as auxiliary power for our 25-foot sailboat. The 24-volt bank will be charged using a 24-volt charger on shore power and by a 24-volt series of solar panels when mooring.
I would like to eliminate the 12-volt batteries. I bought a 24- to 12-volt converter to stand in place of the 12-volt batteries, but I learned that the converter is not compatible with driving any kind of motor due to the back-voltage created by the collapsing field when the motor stops. I have a freshwater pump and a motorized outboard-motor bracket, so this particular converter is out of the question.
Do you have any suggestions? Must I maintain a 12-volt battery for all the 12-volt equipment or is there a step-down technique?
Michael Hoffman
Concord, Mass.
Puffin, Com-Pac 25
Unfortunately, you are working with electric motor circuits in your electric outboard, its lifting bracket, and your potable water pump.
In electrical systems, motor circuits fall into a special category generally described as “inductive” loads. This load type has several characteristics that make the use of a typical DC-DC converter as a replacement for batteries essentially impossible. Besides the back EMF (electromotive force) issue that you mention-which is quite correct-there is also the matter of motor start-up current measured in amps, as well as the typically high amperage that motors need to run continuously. Start-up current for any motor, AC- or DC-powered, can often run as much as four to six times the continuous running current or amperage. So lets say the running current draw for a motor is 5 amps; its start-up current draw might be as high as 30 amps. A motor that draws 50 amps continuously could conceivably draw as much as 300 amps on initial start-up. Granted, this start-up current draw is usually measured in milliseconds. None-the-less, the power-providing equipment has to be capable of delivering the needed current just to get things going.
In our search of DC-DC converters, we found no such equipment available. Batteries as power-storage devices, with high current discharge capability-typically measured in either CCA (cold cranking amps) or MCA (marine cranking amps)-are the only way to power your system.
Sealing LPG Fittings
Planning to change my propane regulator and fittings, I began looking for the best sealant to use on the brass NPT fittings. After a half-hour on the Internet, and talking to a couple of chandleries and other boaters, I was surprised at the varied information I received.
What do you think is the best way to seal the brass fittings for marine use? I tried the “use nothing, brass NPT will self-seal” approach and can say that, in my experience, it does not.
Frank Tansley
Grace, Hans Christian 41/T
Ventura, Calif.
According to Trident Marine, maker of marine hose and LPG systems, the only self-sealing NPT fitting is the 45-degree SAE flare fitting. For others, the company suggests Teflon tape, a Teflon thread compound, or Loctite.
Brad Clarke at Force 10, which manufactures top-rated LPG galley stoves (July 2007) and marine grills (June 2006), said you must be sure to use a high-temperature acrylic sealant. He recommended Loctite 567, a smooth and creamy, one-part thread sealant. To apply, clean the fittings with a compatible solvent. Distribute a 360-degree bead of the 567 to the male fitting and put the fittings together. Properly sealed fittings will seal instantly. A minimum 24-hour cure time is required.
For more on Loctite and its application, visit the manufacturers website,
www.loctite.com
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