Re: electrical grounding to aluminum hull
Several answers here.
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Bill: Yes the capacitor is there for that purpose, ignition interference from you or another boat; anything electrical/electronic. However, any interference caused by hull disturbances (bubbles) will not be affected.
Now, chasing noise (EMI) is an exacting science and basically every system has to be "tuned" to get rid of it.
So for your boat, If you have a shielded power cable from the sounder containing: a (usually red) power wire, and (usually black) return wire for your depthfinder, put the ground and shield together on your - buss plate with one lead (as short as you can get it) of the capacitor. If no shield, it will do better than without the cap, but not near as effective. The other lead of the capacitor goes to the red lead and obviously they are on the + buss. This would be an optimum installation.
What the capacitor does is to short circuit high frequency signals. Since it's connected across the power lines and the lines (up to the sounder) are shielded, most noise coming up through the boat's wiring is not allowed to continue up the wire to the sounder; it gets shorted out. The capacitor has no effect on your dc power so don't worry about it. 8)
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swist: The plate on the outer side of your hull may be for your VHF antenna to insure a good reflective field off the water. Otherwise your engine, while in the water serves as a reference for your boat to the water and some people use it for that purpose; CB's especially.
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Chesie, the first ground is "electrostatic" to prevent what I was referring to when I talked about static discharges....it's for no-fire safety and prior to removing the fill cap, you should touch the fuel nozzle to one of the screws mounting the filler tube to the boat which should pick up this ground (underneath) and discharge any static electricity. Otherwise, if here is some, and usually there is as you were rolling down the highway, or running across the water, you could have a spark when the nozzle touched the filler tube and possibly a resultant fire.
The other line is to insure that you can run your fuel gauge. Since fuel sensors only require milliamperes to operate, and since it is an electrical device as compared to an electronic device, you can probably do that without worrying about it.
HTH
Mark