Electric Health

Maintenance Q & A — August 2003
By Capt. Ken Kreisler

Electric Health
The correct way to read a hydrometer, a cure for cavitation erosion, and more.
 More of this Feature
• Reading a Hydrometer, Cavitation
• Dieseling, Vibrating Outboard, and more
• PMY Tries... EZE-LAP

 Related Resources
• Maintenance Q&A Index

What is the proper technique for testing a battery with a hydrometer? B. L., via e-mail
The battery's state of charge is determined by measuring its specific gravity, which is a measure of its concentration of sulfuric acid. The specific gravity of a battery ranges from 1.120 to 1.300, depending on its state of charge.

Before testing, make sure the battery is fully charged and the electrolyte is at the proper level. If you must recharge, wait at least 30 minutes before taking the first reading. A deeply discharged battery's electrolyte is not mixed well and will not be so until the battery begins to reach full charge (which, because of the electrolyte condition, takes a long time) and starts to gas. If you must add liquid before charging, make sure it is only distilled water, as other water most likely contains minerals and impurities that may prevent the electrochemical reaction from reaching its full potential.

Draw enough electrolyte into the hydrometer barrel to allow the calibrated float to ride freely. Then read the specific gravity where the surface of the liquid comes in contact with the scale, allowing for the curvature of the liquid against the glass parts. Note the value, and squeeze the electrolyte back into the same cell from which it was drawn. Repeat this for each cell. The readings among each cell should not have a difference of more than .050. If they do, you will have to replace the battery.

When you're finished with the hydrometer, rinse it with fresh water to flush out any electrolyte that may be left in the tube.

During a recent haul-out, I noticed the trailing edge of my props seemed to be in the throes of some sort of corrosion. All my zincs seem fine. What could be the problem? T. R., via e-mail
It sounds like erosion due to cavitation, the rapid formation and collapse of bubbles just behind the prop blades. As the prop blades spin, air pockets can form across the blades. When these bubbles collapse, water hits the blades with such force, it's as if thousands of tiny hammers were at work. The result is erosion at the trailing edge of the blade that you might mistake for galvanic corrosion.

Usually this is caused by a blunt or squared front edge and is more often than not the result of a nick or dent. Rounding that edge out can cure this. If there's not too much damage, your local prop shop can easily recondition your wheels with a file or sander.

Next page > Dieseling, and more > Page 1, 2, 3

This article originally appeared in the July 2003 issue of Power & Motoryacht magazine.

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