Hi,
You basically got the idea there!
Sharpie pens could work, along with any other type of pen or marker who has a tube diameter of 1 cm or more. As for the shaft, it does not matter what kind (brass, metal, plastic...) as long as it can be threaded through the end of the pen. I included a diagram of the "X-rayed"
torp. In the picture, the blue thing at the top is actually a piece of wood with the rubber band across, glued at the top of the pen tube to keep the band in place, as you can see, the other end of the band is attached to the top of the shaft. As for the prop, I simply cut out a prop shape out of clear PVC sheet, pierced the middle with a nail, bent the blades at an angle, and mounted it at the end of the shaft!
For the launching mechanism, all you have to do is come up with some kind of stick attached to a servo disk to keep the prop from spinning, to fire, make the servo disk spin, unblocking the prop blades and releasing the elasticity. The torpedo should travel forward!!