For the submarine challenge I had the idea for one that had wings designed for underwater with the propulsion units built into them. I had been thinking about designing a modern jet with the engines inline with the wings but when this challenge came up I decided I could do the same with the sub. Below is the description I wrote for the 3D Warehouse:
This submarine is modeled to resemble a fish. Its propulsion system features large propellers contained in ducted cowlings that are designed to create a vortex enabling the sub to travel at high speeds through the water. Each propeller housing is attached to a wing that pivots up or down to ascend or descend in the water. When the wings move in opposite directions they allow the submarine to roll and turn much like an airplane does. Varying the speed of each propeller also assists in turning mobility.
Take a look at the gallery below or download the model.