In early submarine designs we see that a surfaced submarines are faster, while submerged submarines are slower, but gain the obvious benefits of stealth and being harder to target (requiring specialised weapon systems). This seems intuitive, as a surfaced submarine will only have part of its hull in the water, leading to lower drag, in addition to better propulsion options (diesel vs. battery), esp. in early models.

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> Example:
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> * [Type VII submarine from WW2 Germany](https://en.wikipedia.org/wiki/Type_VII_submarine): 17.7 knots surfaced, 7.6 knots submerged
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However, in later models, submarines are actually ***faster when submerged***, which seems counter-intuitive to me (again, more submerged hull should lead to higher drag, while modern propulsion methods should be equally usable while on surface or submerged).

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> Example:
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> * [Type XXI submarine](https://en.wikipedia.org/wiki/Type_XXI_submarine), the reversal is already present in later WW2 models: 15.6 knots surfaced,
> 17.2 knots submerged.
> * [Typhoon-class submarine](https://en.wikipedia.org/wiki/Typhoon-class_submarine), but it seems the trend also holds for modern nuclear subs too: 22.22 knots surfaced, 27 knots submerged
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**What changed to make submerged submarines actually faster?**