Colin,
Did not have to go too far back. The following is extracted from "Electricity Applied to Marine Engineering" published as recently as 1966. Well. 1966 is recent to me....
Quote
The Single-wire System
Although this system is considered to be obsolete for use on larger ships Lloyd’s Rules for Electrical Equipment states that ‘distribution systems employing single-wire with hull return may be submitted for special consideration’. The system is therefore described below
In this system the hull of the ship is used as one of the supply leads, only one cable being used to carry the electricity between the generator and the consuming apparatus, lamps, motors, fires, etc. The principal thing in favour of this system is cheapness. From an electrical point of view it is in many ways undesirable for the following reasons.
A failure of the insulation on the live conductor at any point will, or should, cause the protective gear to function, that is either a fuse blows, or a circuit breaker comes out causing an interruption of the electric supply. If the protective gear failed to function there would be a dead short circuit on the generator.
For this reason all fittings used in damp places should be watertight. Where cables pass through bulkheads, insulated bushings must be used. Also, because of the magnetic effect of the current, a single live cable must not be laid within 30 feet of steering or standard compasses. A person touching any live cable, or live portion of a switch or other piece of apparatus, immediately gets a shock to earth. For these reasons only low voltages should be employed. To avoid electrolytic corrosion, the positive pole of the generator should be earthed. If the negative pole is earthed and there are any leakage currents due to damp, the armouring of the cables will be eaten away and deposited on the hull of the ship.
The question is sometimes asked as to what would happen if two ships lying side by side in port, one with the positive earthed, the other with the negative earthed, were to swing against each other and come in contact. The answer is precisely nothing. They are in electrical contact all the time via the sea water. Unquote.
As I suspected it was used to keep costs down, Shipowners don't change.
Cheers,
Barry M