While submerged, submarines can travel faster than when they are on the surface. As water is more viscous and dense than air, causing more friction, how can it travel faster while travelling through a denser medium.
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A ship on the surface is continually making waves at the front and back, and this puts drag on the ship, called wave drag. This takes power to overcome. Ships do this more efficiently because they have a water-cutting shape. WWII subs looked more like ships than subs, and their surface speed was faster than their submerged speed because their surface-optimized shape shape greatly increased the drag of the water over their skin (skin drag) when submerged. It made sense because they had to spend most of their time on the surface, mainly submerging for attacks.
Modern nuclear subs can spend months underwater, and they rarely surface during a deployment. Thus, a sub’s hull is optimized to reduce the skin drag, but at the cost of a shape that greatly increases wave drag when on the surface.
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