Well as you could see from the numbers going from cruising at 10 knots to 15 Knots doubles the required energy input and that is for an absolutely perfect system where there is no waste converting the chemical energy of the fuel into kinetic energy and zero hydrodynamic drag or other factors increasing the energy requirements. In practice a good diesel engine is about 50% efficient(and no you can't do much better though explaining why gets into yet more physics) and you can reduce drag but not eliminate it, though I suspect your proposal for more guns will eliminate any gains from tweaking the hull shape. Look at the shape of modern attack sub hulls versus their WWII counterparts. They are streamlined, have no exterior attachments and of course have practically limitless power courtesy of nuclear plants.Thanks for the lesson in torpedo design. I obviously don't know enough.
I was actually not trying to reduce fuel capacity. I was accepting there would be a range reduction consequent to increased fuel burn to produce a higher cruising speed. (More power requires more fuel.) I hoped to mitigate it somewhat with better hydrodynamic qualities (greater fineness). Was that estimated reduction too low?
And before it occurs to anyone, yes there are chemical fuels that offer greater energy density than diesel but they tend to be toxic, explosive, toxic and explosive, or cryogenic so they really aren't an option.
ETA: Just to re-emphasize those numbers from the Kinetic Energy equation represent the absolute best case. All real world systems fall far short of that perfect system and the goal is simply to get as close as possible. There is no tweak or modification that can take you below the numbers generated by that equation.