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Written by Staff of Quadrant Marine Institute
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Q.
Is it true that the ground plate under your boat should be attached to the negative of the 12-volt system? When I bought the boat, the plate was connected to all thru-hulls and the rudderstock, which I believe to be correct. Is it also supposed to be attached to the radar arch for lightning? On the Central Coast one year, I had bad reception on my VHF. People said to check the ground. Did they mean the 12-volt ground, or is the ground plate supposed to be attached to the VHF somewhere?
—Don Boudreau, Cortes Island
A.
Your questions cover three distinct areas involving grounding and bonding a vessel’s components and equipment. These issues are often confused, but it’s really important to understand them because mistakes can cause a lot of trouble or damage.
The bonding system you describe is designed to prevent galvanic corrosion of your underwater metal parts. It must be attached to a sacrificial zinc, not a grounding plate. All underwater metals must be connected to it to keep them at one potential voltage, therefore sacrificing the least noble metal—the zinc—and not your engines, propellers, shafts or thru-hulls.
This bonding system is also connected to your 12-volt negative ground at one point and one point only (normally the engine block).
We should note here that many boats do not use bonding at all and do just fine without it as long as zincs are attached directly to major components, such as drive legs, prop shafts and so on. Whether to use a bonding system depends on many factors. Consult an expert if you are having galvanic corrosion problems.
A lightning ground is a porous bronze plate on the bottom of the boat that is connected to the highest metal point on the top of the boat (mast on a sailboat) or radar arch (power vessel). It would be preferable to have a lightning diffuser rod installed using a large-sized wire conductor in the straightest run possible.
Again, many (if not most) boats on the B.C. coast do not bother with a lightning ground, since lightning is not nearly as common here as it is in places like the tropics or the Great Lakes. Extensive treatment of the pros and cons of lightning protection isn’t possible here.
For electronics, it’s sometimes beneficial to use a chassis ground aside from the standard 12-volt negative ground to the battery in order to reduce interference “noise” from outside sources on the boat. This can be attached to a porous ground plate on the hull using a two or three-inch wide copper strap.
This treatment is more likely to be found on larger vessels with complex electrical systems. Most smaller vessels get along without one. Check with an electronics technician if you are experiencing problematic “noise” on radios, depth sounders, radars and so on.
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