Seeing as its in a sub, you shall be excused
The old torpedoes had a petrol engine in them, fed with air by a bottle of oxygen, and the exhaust went out through the middle of the prop, like a modern outboard's exhaust does. These were quite large and so an RC sub powered in this manner would not fit into the back of your car, even if you had a transit van. :)
I'd quite like a sub actually, but they're far too expensive for me (a student, says it all
) and our lake is also quite smelly and brown, so I wouldn't be able to see it if it did dive, and if I did dive it the water would probably dissolve all the internals or something!
Andy