Originally Posted by reed1v
Your boat should have a common ground, usually the engine
block. Try not to connect directly to the battery's negative.
I agree. The engine block,if it is connected to water
via shaft & prop, is the better point for drain. The reason is that the purpose of the drain is to connect the cabinet/ckt board RF ground as close to earth(water) as possible & avoid having ANY current
in this drain connection. The Neg power wire from the eqpt. to battery carries the same current
back to the bat. neg. as the Pos. wire carries to the eqpt. from the bat. Pos.-(what goes out must come back).
The drain connection is to shield it's eqpt.(farraday cage) from induced electrical
interference that may be radiating throughout the boat from other eqpt. AND to provide a clean path to earth for any static buildup from antennae.
Your Bat. Neg. wire carries current, so it is not "clean" as there are electrons "bumping" thru it.
Having said all this,99% of the time on recreational sized craft,there is little to no "electrical noise".,so you can get away with connecting the drain/cabinet earth to the Neg Bat terminal,the Neg power lead at the eqpt. or , often,not connecting it at all.