I don’t know much about the kind of giant motor that would be used in a ship, but as a general principle: the load that a motor presents to the drive electronics does not resemble your house in the slightest.

To power your house (or, more generally, supply vaguely sine-wave like output at a constant voltage), you need a converter that will convert DC at the battery voltage to AC at the desired voltage. If a buck converter is used, for example, the AC voltage can only ever be lower than the battery voltage. If you use a cheap square wave inverter, it’s possible that the output and input voltages must actually be equal.

A motor, though, is a highly inductive load, and large motors will and do operate from truly gnarly supply waveforms as long as the current waveform is approximately correct. Industrial VFDs (variable frequency drives) do unspeakable things involving switching a DC bus voltage across the motor via H bridges at tens of MHz, which is a horrible thing to do the the wiring between the drive and the motor if it’s not extremely short. (There are, recently, some guidelines that specific types of wire with twisted conductors, better than average insulation, and high quality shields should be used to improve tolerance of the fact that rather impressive standing waves can appear in the wiring if the wiring is a quarter wavelength or longer.). I can easily imagine designing a VFD that works just fine over a respectable range of DC input voltages by adjusting its duty cycle accordingly.

One way to think of this is that a VFD looks kind of like a buck converter where the inductor is free in the sense that it’s already right there in the motor. If it’s designed right, it will handle the battery’s full voltage range, and the inductor will still be free :)

> I can easily imagine designing a VFD that works just fine over a respectable range of DC input voltages by adjusting its duty cycle accordingly. [...] I can easily imagine designing a VFD that works just fine over a respectable range of DC input voltages by adjusting its duty cycle accordingly.

I imagine it's not the waveform or current that matters so much, as the voltage. These motors would be powering massive blades encountering incredible resistance, so you need megavolts to move them, with an input voltage all the way down to near zero.

> H bridges at tens of MHz

Imagine the MOSFETs on this thing! Do they have something that scales up to MV? That sounds like an engineering challenge in itself.

Full disclaimer: electronics is not my wheelhouse, though I have played around with motor controllers.