Imagine a boat: but someone added a canopy, and now it travels underwater (Fig. 1). Now you’ve got a submarine! That’s all the background you need. (Actual submarine construction may be somewhat more complicated.)
There has been interest lately in putting sails on oceangoing vessels again: this is proposed as a cheap (but somewhat mechanically complicated) way to increase fuel efficiency.
Unfortunately, because a submarine is UNDER the water, there’s no way for it to easily benefit from new sail technology.
The fix here is easy: the submarine can deploy a kite-like sail that can pull it through the water (Fig 2).
It might also be possible to deploy a sail on an extremely tall retractable persicope-style mast.
This would be a great way of increasing the fuel efficiency of non-nuclear submarines and would serve as a safety feature, notifying other vessels in the area that they should watch out for a submarine (which could help avoid a repeat of this 2001 submarine-sinking-a-boat incident).
The exact nature of the kite / sail must be determined by licensed sail-ologists: in particular, the initial raising of the sail out of the water remains an unsolved technical question of submarine sail research.
As an added bonus, a sail-deploying-submarine can be a way for a nation’s navy to brag to other countries. It basically says “we have so much military funding that we can build a super-expensive stealthy underwater vessel and then not even use the stealth feature!” The intimidation factor here cannot be overstated.
PROS: Reduces fuel usage, thus allowing submarines to be operated at bargain-basement prices.
CONS: This kind of sail will probably only work in the exact direction the wind is blowing. Can you “tack” and sail upwind with a submarine?