# Can a submarine turn upside down?

**URL:** <https://boards.straightdope.com/t/can-a-submarine-turn-upside-down/680123>\
**Category:** Factual Questions\
**Created:** [January 30, 2014, 11:50am UTC](https://boards.straightdope.com/t/can-a-submarine-turn-upside-down/680123 "2014-01-30T11:50:07Z")\
**Posts on this page:** 1\
**Showing post:** 49

<div class="post-metadata">

**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\
**Post date:** [February 1, 2014, 6:50pm UTC](https://boards.straightdope.com/t/can-a-submarine-turn-upside-down/680123/49 "2014-02-01T18:50:57Z")

</div>

> [@Submarine ends upside down, how will it go up?](https://boards.straightdope.com/t/submarine-ends-upside-down-how-will-it-go-up/151431/5):
>
> …I asked the same question, and a former submariner replied:
> 
> > [@Submarine ends upside down, how will it go up?](https://boards.straightdope.com/t/submarine-ends-upside-down-how-will-it-go-up/151431/5):
> >
> > Bouyancy for modern submarines requires that the ballast tanks be trimmed as necessary with air and the tanks have valves at the tops to keep that air in, however the bottoms of the ballast tanks are open to the sea (consider that the water which displaces that air does have to come from somewhere, ya know?)
> > 
> > A submarine which (for whatever bizarre reason) should happen to become fully inverted would have the air in the ballast tanks empty out almost instantly and any attempts to pump more air to replace the lost bouyancy would simply allow that additional air to escape as well. Without the support of this air the submarine would descend to the bottom (or more likely, would sink to crush depth and the hull would collapse under the intense pressure).
> > 
> > And before anyone comes up with any bright ideas about using the engines to bring the sub rightside up i would point out that a nuclear reactor subjected to these sort of gymnastics would have scrammed rather promptly leaving the sub powerless and sinking into the darkness (not that the sub’s control surfaces are made for the challenge of flipping a submarine back over from an inverted position anyways). Regardless, an upside down submarine is effectively a doomed one.
> > 
> > wolf189 (former submariner)

**Wolf189** ’s conclusion that an inverted submarine is effectively doomed is correct; however, his statement that modern submarines are trimmed with the main ballast tanks (MBTs) is not. As I stated back in [Post #9](http://boards.straightdope.com/sdmb/showpost.php?p=17060063&postcount=9), the MBTs are the only ballast tanks open to sea on the bottom, and when a submarine is submerged, they are completely full of water. The submarine’s buoyancy is trimmed with the variable ballast tanks (VBTs), which are sealed tanks not open to sea.

Indeed, the valves on the top of the MBTs are periodically cycled (opened and shut again) periodically when submerged to ensure that they stay full of water. (A bubble of air in the tanks might have worked its way out of a corner, or some air from the high pressure air tanks might leak by, displacing some water).

Also, if the MBTs were damaged while the sub is submerged, you would not want a dramatic change in buoyancy. This is important for a vessel that is to be taken into combat.

In any event, I’m having trouble picturing a situation in which a submarine could invert in the first place. For one thing, like any other large vessel, a submarine has many tons of fixed lead ballast along the keel to keep it upright. Also, modern military submarines are huge vessels that maneuver like any other large marine vessel. Think “cruise ship”; not “acrobatic airplane.” A submarine certainly can’t invert itself, and anything external that could possibly invert a large military submarine is likely to sink it in the process.

---

_[View the full topic](https://boards.straightdope.com/t/can-a-submarine-turn-upside-down/680123)._