ernest2,
A '540' will run just fine on 6 volts. Considering the size of the prospective boat, putting more than one 6 volt battery in it shouldn't be unreasonable. That would double the run time if they are in parallel, or give you 12 volts if in series (also takes the place of ballast). There are several sources of gel-cell batteries, not all of them are 'huge', and some of them offer a fairly large 'Amp-hour' rating. I think the run time will be longer than what you may expect. If your '540' draws 4 amps, and the battery(s) total 12 Ahr, that's a possible 3 hours of run time at 6 volts, or 1.5 hours at 12 volts. I don't know about you, but my attention span isn't quite that long (LOL)! Also, scale speeds are much slower than what you might think.
(I seriously doubt if a single '540' will draw that much current (4A), so run times should be even longer.) Electric power is a lot easier to 'do' than using an internal combustion engine, at least for me, and I don't have to mess with all the other 'stuff' that goes with the gas engines. Just depends on what you want to do...
- 'Doc