Hello RalphChristie and
I believe that the main reason that ungrounded systems are chosen for vessels is unique to vessel applications.
In a normal on shore distribution system the ground path from a service to the supply transformer is in parallel with the neutral. This ground system includes the grounding system at the service, the grounding at the distribution transformer and often the multiple protective earthing on a common neutral.
As low as the impedance of the ground connection is, the dynamic impedance of a properly installed neutral is typically low enough that virtually all the neutral current flows in the neutral and very little of the neutral current flows in the ground system.
Not so in marine installations. The contact of a hull with the ocean is an extremely low impedance connection.
Consider a shore power installation. There may be a feeder running the length of the anchorage. Taps will be taken from this feeder to service various vessels.
The port authority may exersize little or no control over the load balance of vessels connected to shore power.
For example consider the shore power system is 120/208 volt 3 phase and that the vessel on the end of the line uses a single phase system. This vessel has a lot of single phase loading. The resulting neutral current is approximately equal to the line current. As a result, there is a voltage drop in the long supply circuit not only in the line conductors but also in the neutral. If this vessel has a grounded system much of the current will flow through the hull rather than through the neutral conductor. If the ship-board system is ungrounded there will be no hull currents.
Now consider the adjacent vessel. This vessel may have a perfectly balanced electrical system with zero neutral current, however if it is connected to the common feeder the neutral potential will be elevated from ground because of the neutral voltage drop in the feeder and if the vessel system is grounded then hull currents will flow. This will happen as long as the neutral connection to shore power is intact even if the vessel is not taking any power from the shore power system.
Hence, ungrounded systems, isolating transformers and ground connection switches.
I agree with most of the previous posts regarding normal on-shore installations, but I understand that possible hull corrosion is the main concern, and the main reason for using ungrounded systems at sea.
Second is economics: No expenditures are required for grounding equipment or grounded system conductors.
How old is the green book and how long since it has been updated to recognise NEC requirements for equipment grounding?
respectfully