Lowering the speed does not change range. It decreases fuel consumption, but the range is solely determined by the fuel efficiency of the engine and the fuel fraction of the vessel.
I proposed something similar to this a month or so ago, and would definitely like to see some form of range scaling with speed. A strict fluid interpretation would have drag (and thus, probably, fuel consumption) scaling with the square of speed. I'm not sure that works terribly well, but some more complex function might be a good choice.