U.S. Navy Submarine First In World Fitted With Silent Caterpillar Drive - Naval News

The Swedish Gotland subs uses the Stirling engine. No vibration and no need to surface to recharge batteries and they do not use air. This is why in war games it was able to get through the protection provided to US aircraft carriers and sink it.

PO1 William "Chip" Nagel Instead of a traditional propeller at the stern, the new propulsion will be entirely within the submarine’s hull. According to British experts the only external clues are likely to be the water intake doors in the bow.