I'm not sure what the second drawing on the right represents but I think that it is the profile view of the sub. It also looks like that you are going with the classic two-prop steering system and a central prop for thrust, but I think that you could do without it. Use the two props on the side for steering and thrust, thus eliminating the need for an extra motor. If using this method, then I can agree that it looks a lot like the Megatech/Nikko/Mako sub in terms of steering. Is the motor/prop located at the top of the sub (in the second drawing) for diving purposes? I think this plan could work, you can use a PVC pipe for the main hull and maybe running the motors in the wet, or putting them in separate containers outside the hull. If you need more advices I'll be happy to give you some.