Peter B wrote:
*If the wiring was that poor, you would think it would warn of that by shorting out fuses etc, would it not?
What generally happens is that the AC safety ground (not neutral) is not connected to the boat's common DC ground.
If there is an AC electrical fault that connects a "live" wire to some piece of hardware like an engine*(or a metal hull) that allows either the hull or the propeller shaft to be energized, those parts become part of the "live" circuit. From then on, any conductor that bridges the ground and the energized parts will complete the electrical circuit.
There is not usually enough current flow in these cases to trip a breaker. But, there is a high enough potential to establish an electrical field around a metal hull or in the vicinity of through hulls or shafts and rudders.
Picture a*hemisphere radiating out from a metal hull underwater, the closer you get to the hull, the stronger the voltage, sort of like the power radiating from a radio transmitting antenna.*The*gradient of the field surrounding the boat can be measured as volts per centimter, amps per centimeter squared, and watts per cubic centimeter. The conductivity of the water is measured in Siemens per cm that is what determines the risk of electrocution.
In salt water the conductivity of the water is enough to minimize the gradient of the field, the*leakage current is dissipated over a large area and volume of water with the result that a swimmer is unlikely to get zapped with 120V at his head while*his feet are at or near ground potential so little current will flow.
In fresh water however, the low conductivity creates a strong gradient, it keeps high voltage near the*hull.*Picture the boat hull*as a wire and the water*as the insulation, if you puch through the insulation, at some point it will not be enough to stop you from getting zapped. A swimmer could have his arms or head near the hull in a high voltage gradient while the feet are near ground potential and a current will flow through the comparitively good conductors inside his body (including the heart and muscles) and out the feet. Remember it only takes about 50 milliamps to stop the heart and much less to paralyze the muscles and drown the unfortunate swimmer.
All that was probably more than you really wanted to read but the bottom line is to make sure that*all AC safety grounds*are connected to the common DC ground and if you are worried about galvanic corrosion, use a galvanic isolator, make sure your shore power ground is intact and in good condition, and do not disconnect the AC ground to the dock. All this applies as well to boats on the hard. You could touch your prop and get killed if you have a ground fault on the AC system.