What sort of ships????Are you saying that some of them, but not most, have shore supply? The reason that the mains frequency is much less than 400Hz has to do with the distance that the power has to be transmitted, given the impedance of the great length of power lines.
Bulk carriers, container carriers, oil tankers, roll on/roll off carriers ..... Real ships of the merchant marine. And NO very few of
them have "Shore Supply" facilities.
They use common mains voltage and frequency so that they can purchase "off the shelf" electrical equipment and not need specially made items (This is called Common Sense)
But conductor distance wouldn't be a major issue on ships, and actually, plenty of 400Hz equipment is available given its commonality in aircraft and airport equipment, and in some other places too.
Also, 400Hz equipment isn't that much different from standard frequency equipment. For example, if it uses D.C internally, and at a lower voltage than the mains, as is the case with things like desktop computers, amplifiers, etc, running it on 400Hz could be done just by changing the transformer. Appliances containing 50Hz motors could likely run on 400 Hz simply by replacing them by motors with 8 times as many poles.
In addition, I suspect that a 400Hz transformer would offer a greater performance to cost ratio than a 50Hz transformer, even though the former are produced in smaller quantities.
I am a fully qualified Special Class Electrician and Armature Winder Myrtone, so I Do know what I am talking about.Your comments are valid for synchronous motors, because the rotor must turn in synchronism with the field of the stator. But I do wonder if voltage alone can affect the speed of an induction motor, frequency being equal. And A.C motors do seem to produce back E.M.F. While the frequency supplied to a synchronous motor with a given number of poles does determine the speed of the rotor, it also determines the amount of back E.M.F, and the field of the rotor would add to that. Induction motors too would produce back E.M.F owing to the rotating magnetic field. So the voltage of the supply to the motor must be varied whenever the frequency is varied.
AC motor speed IS determined by the number of poles and the frequency, not voltage ratios