1. draw a box
2. add engines
3. add guns
4. add armor
5. figure out how much fuel the engines need, how much ammo the guns need
6. figure out how much crew you need to take care of the guns and engines
7. add more crew to take care of the crew taking care of the guns and engines
8. figure out how much food/water/supplies the crew needs
9. find a way to stuff everything in the box you drew (you can stuff the crew anywhere, they're not that important, the guns and engines are the important parts)
10. now make the box able to float, it probably needs a more hydrodynamic shape, too, make sure the mass isn't off-center.
11. now keep looping back and forth, tweaking variables (shape, mass, armor, guns, engines, fuel, ammo, supplies, ...) until it's somewhat acceptable. Stop when the project is due.
