The ‘sail’ of a submarine is the raised portion above the pressure hull. What’s in it? I know the periscope, antennas, sensors, and whatnot are in there, as well as a tunnel joining the observation platform to the pressure hull. But since it’s not part of the pressure hull, why isn’t it crushed at depth? Is it vented to the sea, so that it fills up with water? If so, how are the devices inside protected? If not, is it a limiting structure as to how deep a sub can dive?
Fixed typo in title.
Colibri
General Questions Moderator
The sail is open to the sea, and in fact, in many sails you can see loose hinged panels (flappers) which permit water to flow in or out so that it doesn’t retain water when the submarine is surfaced. The sail contains the gear for the periscope(s), floating antennas, electronic warfare masts, and the gear for the fairweather planes. Anything that can’t be exposed to sea water is sealed and pressurized to withstand pressure at depth. The limiting structure on a boat is invariably the pressure hull(s), which has a characteristic operating depth and crush depth depending on its material strength. The Soviets built some pressure hulls out of titanium for strength at extreme depths (as well as a significant ability to avoid detection by magnetic anomaly sensors) but virtually all submarine hulls are built out of high strength corrosion resistant steel.
Stranger
Thanks. This has been bugging me for a while, and the other submarine thread reminded me of it. In movies the sail is never mentioned. There are either people topside, or else inside of the pressure hull. I’ve always wondered how it withstood the pressure, unless it was allowed to fill with water. I’ve never seen the flappers in films, nor recall reading of them in novels. So now I know. 
You know, if you ever get to Connecticut, they have the Nautilus, the first American nuke sub bolted to a pier and rigged for tourists … I think I have toured it a couple of dozen times by now …
It’s actually more correct to state that most of a modern U.S. submarine’s sail is flooded when submerged and exposed to sea pressure.
If you were standing in the submarine control room looking up into the sail, there is a hatch (in the pressure hull) at the bottom of the sail that leads to a small compartment. This compartment extends up into the sail outside of the pressure hull. There is another hatch at the top of this compartment that leads to the rest of the sail. While the portion of the sail outside of this compartment is open to sea, this compartment is maintained at the same pressure as the rest of the submarine interior (approximately 1 standard atmosphere). The purpose of this setup is simply to give some redundancy to the lower hatch. You can still submerge so long as one of the hatches could be locked shut.
There is a similar setup for the two other main access hatches in the submarine, but the main purpose of the setup for these hatches is to provide an airlock for an escape system. These other two points of egress are therefore referred to as the forward and aft escape trunks.
Under normal submerged operation, the upper and lower hatches are kept shut. However, it is possible to open the lower hatch and enter the sail compartment (and the escape trunks) during submerged operation.
There’s a cross-section in which you can see this compartment here (as well as the two escape trunks) for a 688-class submarine:
Neat schematic of an escape trunk here:
http://www.pbs.org/wgbh/nova/subsecrets/spritru2lo.html
Why does a sub need the sail, anyway? Couldn’t all that gear be stored in the hull? Wouldn’t a smooth, torpedo-shaped vessel be stealthier?
Guessing it works as a conning tower and control deck when surfaced.
Most submarines have water rushing over the mail hull when underway (especially in heavy seas I would think). If there was no sail people could not be above deck underway for fear of being washed off.
The sail serves as a bridge and conning tower when surfaces, a mounting point for the fairweather planes (which gain more leverage by being mounted up above the centerline) and a place to store periscope and EM masts without having to have a large mechanically sealed opening in the pressure hull. You could do without it (most modern attack submarines don’t mount fairweather planes on the sail so as to facilitate under-icd operations) and in fact there have been several proposed designs that reduce or eliminate the sail. Most unmanned underwater vehicles (UUVs) don’t have a sail, just one or more single masts. However, for a large manned submarine having a sail makes functional sense, especially since modern subs have little or no keel.
Stranger
Actually, for submarines with optical periscopes (since the new Virginia-class subs have electronic photonic masts), there is a relatively large mechanically sealed opening in the pressure hall for each periscope.
The sail does indeed give the periscopes an area to retract into, along with the other masts and antennas.
In addition, when a submarine is at periscope depth, you can’t allow the main hull to get too close to the surface, or the Bernoulli effect create a lift effect that can cause the submarine to broach the surface. You counteract this by keeping the submarine a few feet deeper, but then the masts and antennas would have to be much longer, increasing the chance of damage or breaking them off. The answer is to build a sturdy platform from which the masts and antennas can extend from–which is the sail.
Finally, you need the sail to operate on the surface, as others have stated. At high speed on the surface, water does indeed sweep over the whole main hull. Even at low speeds, waves routinely wash over the whole hull out in the open ocean. For this reason, Navy personnel have to wear safety harnesses if they need to go on the hull outside of protected waterways.
For diesel-electric subs the sail also houses the snort (snorkel) with the same structural benefits robby mentions.
Nuclear subs have snorkels in the sail as well.