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Subject: Do submarines still need sails?
cheshirematt    7/13/2010 9:51:07 AM
"Aesthetically," I like sails on submarines. But looking at torpedoes, and the US Navy's ASDS, I had to wonder how essential they are. As best I can tell, sails: - store the periscope... - and other equipment like radar and radios - help keep the submarine upright? (I'm not sure of this one) On the downside: - they have to generate some drag (more water to push aside). - they make the sub taller, so you have to dive deeper to hide. But on modern (Virginia class) subs, the periscope has been replaced with a photonics mast, which doesn't have to run through the pressure hull. So ould a submarine get by without a sail, and (for example) moiunt the photonics mast, radar/radios, and snorkels in the ballast tanks? Or does the sail serve some other vital role I'm missing? I'm guessing the end result would look a bit like the ASDS: http://www.navy.mil/navydata/cno/n87/images/seal2.jpg Only quite a bit bigger.
 
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earlm       7/14/2010 12:29:30 AM
Look at the relative size of the sail on a Skipjack vs a Virginia.  On the latter it probably makes up 10% or less of the drag.  It also makes life easier for the crew on the surface and it is somewhere to store the scope etc.  Very seldom do you see weapons optimized in one sphere (drag) at the expense of ease of use.
 
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VelocityVector       7/14/2010 12:12:51 PM

Or does the sail serve some other vital role I'm missing?

You both mentioned most of them.  Upright is important insofar as the sail prevents uncommanded rolling.  Without a sail acting much like an airplane vert stab, a boat would be a more difficult if not outright dangerous platform in which to work relatively-speaking.  Uncrewed torpedos and uuvs don't need to address this concern.

v^2

 
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WarNerd       7/15/2010 4:26:22 AM
You also need the sail for surface operations at high speed or in heavy seas to keep wave action from flooding the sub.
 
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cheshirematt       7/15/2010 5:05:52 AM

You also need the sail for surface operations at high speed or in heavy seas to keep wave action from flooding the sub.

I'd had that thought... without the sail location there's nothing to keep crew from being swept off the ship/water flooding into hatches. My first thought is "stay under water then," but I can imagine times where you'd need to take on supplies (or get injured crew off the boat) and you don't get to choose when/were you do that.
 
I'm still not sure how the sail keeps the boat stable, since subs seem to stay upright on the surface (with the sail clear of the water) just fine. And underwater, the sail is flooded, so it should want to "sink," which would seem to make the sub more likely to turn over than not. But I'm not an engineer, and I do know that subs keep right-side up just fine, and all of them use sails, so I'm pretty sure those who are engineers know what they're doing :-).
 
That, and thanks for the replies!
 
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VelocityVector       7/15/2010 11:43:38 AM

A sub is ballasted by placing its heaviest components near the lowest decks, and the tanks are oriented to keep the boat upright no matter what these are typically filled with.  A sail is light-weight in comparison, filled with atmosphere when surfaced or filled with neutral seawater when submerged, and its height and mass are restricted to some function of a boat's diameter.  When the sub is running below surface the water flow past a Skipjack-type sail can actually provide a lifting force which opposes the direction of a roll as with a yacht's keel/sail combo.  Sails evolved from the cons which had been adopted to help protect extensible periscopes and snorkels.  Obviously they also help keep the boat from flooding when surfaced and fulfill the other advantages mentioned as well as enable storage outside a boat's pressure hull and provide for beneficial acoustic sensor location.  0.02

v^2

 
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doggtag    speculation....   7/15/2010 12:38:39 PM
....perhaps,
another importance of the sail,
is that it actually means the sub can ride deeper in the water when using its various periscopes/masts.
Think about it: if I have say a ~5m high (or greater) sail height,
that effectively allows me a somewhat longer periscope/mast reach, as combined with the hull depth/height and sail internals, I could have a longer 'scope/mast stored inside the boat.
Plus, that longer height means my 'scope/masts don't have to be telescoped into as many shorter sections than if I had a smaller fin (barely a hump).
That higher sail means my large hull isn't as close to the water's surface, where I could create a wake or surface eddy that could be more easily tracked (especially visibly),
plus it lowers my bow sonar a little farther under any noise from surface chop (small waves and breakers).
 
Also remember that, at least in the USN, the Los Angeles class was said to have featured its horizontal planes (which could be turned to the vertical when breaching thru ice/polar caps) on the sail
because that limited the noise of drag/water flow across the planes
from interefering with the hull-mounted hydrophones.
 
The newer subs, they feature newer generations of sonars and software that aren't as affected by such minute acoustic disturbances.
 
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VelocityVector    doggtag   7/15/2010 2:32:38 PM

Good observations, especially The newer subs, they feature newer generations of ... software that aren't as affected by such minute acoustic disturbances.

Downsides to Skipjack-type sail include vulnerabilty to collision (polar ice, trawler nets/lines, wildlife, uncharted manmade structures and other ships) and, I believe without actual knowledge, increased detectability by active sonars, lidars and mad (the last because I presuppose the magnetic properties of the sail differ over time and are not linked to those of the main hull obviously; new-order Virginias will address this via active means on the main hulls but as to sail???) what with perpendicularity in abeam aspect.

A thing I find interesting as an amateur is that the US has considered what I deem to be a Russian-style squat sail (ice) for Virginias and some recent Russian designs appear to be moving closer to Skipjack sail shape -- go figure ;>)

v^2

 
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cheshirematt       7/15/2010 4:14:57 PM

A sub is ballasted by placing its heaviest components near the lowest decks, and the tanks are oriented to keep the boat upright no matter what these are typically filled with.  A sail is light-weight in comparison, filled with atmosphere when surfaced or filled with neutral seawater when submerged, and its height and mass are restricted to some function of a boat's diameter.  When the sub is running below surface the water flow past a Skipjack-type sail can actually provide a lifting force which opposes the direction of a roll as with a yacht's keel/sail combo.  Sails evolved from the cons which had been adopted to help protect extensible periscopes and snorkels.  Obviously they also help keep the boat from flooding when surfaced and fulfill the other advantages mentioned as well as enable storage outside a boat's pressure hull and provide for beneficial acoustic sensor location.  0.02


v^2



I figured ballasting was part of it, but I hadn't thought about lift... I have learned something today, thanks!
 
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