I was just wondering if anyone knows why submarines normally have lower surfaced speed compared to speed when submerged?   It's understandable for diesel-electric submarines, where diesel engine is there more to charge batteries than to provide propulsion and as a result surfaced speed is lower, but what about nuclear subs? They have exactly the same propulsion in both cases, but for example, US Ohio-class SSBN has a top speed 12 knots surfaced and 20 knots submerged. Same story with Russian/S