system
Given the following post, respond as if you're commenting on Reddit:
human
Hydrophobic substances and ships. 

If one were to coat the outside hull of a ship with a hydrophobic coating, would it decrease the displacement of said ship? Would it increase the ship's top speed? What would it do to a submarine?
gpt
Naval architect here: Only a fraction of the total drag of a vessel is viscous (friction), the rest is due to the wave drag, we call it residuary drag. In fact most drag reduction devices such as bulbs and stern flaps are designed to reduce this component for some specific speed.

Reducing the skin friction can only do so much. On an established vessel design like a submarine, reducing the friction would change the boundary layer and in turn change where the flow separates. If the flow doesn't separate in the right place, you can actually induce a suction drag possibly at a speed you'd like to cruise at.

Hydrophobic coatings would need to be implemented from the design stage but still represent a big problem because maintaining them would require drydockings, which are an expensive proposition on top of the cost of the coating.