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Any submarine of my experience uses water for ballast to achieve whatever degree of buoyancy is required to submerge or surface. It might be possible that a sufficently tough hull would work almost as well against pressure as against vacuum, but I haven't done any math yet.
Yep, that's why submarines are designed with a density close to that of water. It would take a HUGE amount of ballast to sink a surface ship. With a sub being almost the same density as water, it just takes small ballast tanks to change from floating to sinking.
Hence, if a starship was designed to be submersible, it should have a similar density to water, but if it is only designed to land ON water, then its density may be nearer 0.5.
IIRC, water pressure is roughly 1 atmosphere per 10m, and obviously it's working in the opposite direction. Most starships are not designed with an optimal submersion shape and a double hull, so I think you would need quite a bit of armour to make it submersible.
Yeah, thinking as I type, making a starship float is relatively straightforward, and probably standard practice. Making it submerge to any appreciable depth might require a special hull construction.
In which case, standard starships may have a density of 0.5, and submersible starships would need to be built on a submersible hull with a density of 0.9.
Does that sound reasonable?