Submarines

[911newbie]

Follwoing a discussion which developed in the Nimrod thread about detection methods for submarines and alternative methods / materials for their construction.

I suggested aluminium and titanium as paramagnetic materials which would have much lower magnetic anomaly signals, and Hidetheelephants pointed out that welding would be an issue for both of these materials...

Yes, thinking about it the welding issue with Aluminium is the killer. I can imagine the meeting with the boat yard people if that idea came up .... 'You what ?!'

So the Russians made titanium subs eh ? Well hats off to them for trying. Mind you, Aerospace has been welding titanium for some time now so perhaps those problems are less significant now. There will likely be some differences between welding thin sheet and welding 100mm thick plates of course.
You'd get less corrosion too.

How about composite subs, like the Columbian drug smugglers. What are the limits and benefits ?

Limits - Corrosion/organic life issues with the polymer matrix materials, reliability of glued joints... ?

Benefits - potentially many fewer joins in the main structure, lower density so more payload, perhaps some more design flexibility, zero existing infrastructure for construction or support... ?

[scubadude]

911newbie said:

and welding 100mm thick plates of course.

I think you may be overestimating how thick submarine hulls are, military subs don't go very deep... the Russian Titanium Alfas alledgedly being the deepest diving, although questions about the materials quality have been asked.

FWIW, WWII subs hulls where less than 1 inch thick and could dive to 400ft.

[Tango13]

Get a copy of "Blind Mans Bluff" by Sherry Sontag and Mr & Mrs Drew. It talks of a scientist working for the US Navy who wanted to build a submarine from glass!

[Boosted LS1]

^ That was one of those nu clear submarines!

[stuttgartmetal]

badum, ser-mash

[Fittster]

I thought there was a concept to make concrete submarines?#

http://www.orbitalvector.com/Aquatic/Concrete%20Su...

Edited by Fittster on Wednesday 16th February 16:43

[911newbie]

Well it seems composites are being used in submarines already -
http://www.goodrich.com/Goodrich/Businesses/Engine...

[Odie]

subs are made from alu, Ti, ceramics and composites already.

And i believe your refering to the Magnetic Anomaly Detection method, but this isnt what it seems, this is the measurement of the earths magnet field being changed by a large object in the water. No matter what you make it from its going to affect the magnetic field.

The most popular way they detect subs is with passive sonar (ie listening) so subs need to be silent in operation and also not cause cavitation (hence the odd shaped impeller the attack sub that grounded had) , this is a far far greater problem than what to make it from.

[Engineer1]

I strongly suspect that submarines are a lot more complex and technical in their materials choices than just using simple steel.

One point i have seen raised is you don't want too stealthy a submarine as it could become a "hole" in the sea.

[dvs_dave]

Odie said:

.....the odd shaped impeller the attack sub that grounded had....

Otherwise known as a pump jet propulsor. They're basically a ducted propellor along with fixed stators so that the water being forced out the back is a linear flow and as free from turbulence (and noise!) as possible. They can also run at much higher speeds due to the pressure created within the tapered cowling which prevents cavitation. Pretty clever design, although I'm not sure how efficient they are?

Here's some pics of one on a Russian Sub.

Edited by dvs_dave on Thursday 17th February 02:14

[HoHoHo]

dvs_dave said:

Odie said:

.....the odd shaped impeller the attack sub that grounded had....

Otherwise known as a pump jet propulsor. They're basically a ducted propellor along with fixed stators so that the water being forced out the back is a linear flow and as free from turbulence (and noise!) as possible. They can also run at much higher speeds due to the pressure created within the tapered cowling which prevents cavitation. Pretty clever design, although I'm not sure how efficient they are?

Here's some pics of one on a Russian Sub.

Edited by dvs_dave on Thursday 17th February 02:14

Gotta love them Ruskies

They'll give anything a go!

[911newbie]

Gents, I'm certainly not coming at this from any point of expertise on subs in particular. So if my comments sound naive - they are. I'm sure you're correct in that the materials used in sub construction have had lots of money and clever people thrown at them.

I'm just having fun thinking about the problem of detection.

I'm having a little trouble with this statement -
"...the Magnetic Anomaly Detection method, this is the measurement of the earths magnet field being changed by a large object in the water. No matter what you make it from its going to affect the magnetic field."

It seems pretty clear that some materials, eg ferromagnetics, are going to affect the state of the Earth's magnetic field more than others, eg polymers or paramagnetics like titanium.
If polymer composites have pretty much the same effect upon the Earth's magnetic field as water does it is from this point of view invisible, no ?

It seems the permeabilities of paramagnetics, diamagnetics and frees space are pretty similar in comparison with ferromagnetics -
http://en.wikipedia.org/wiki/Permeability_%28elect...

I do like the ducted fan approach in the pictures above - but how on earth do they get laminar flow out of that thing ? For starters its coming out of a rotating fan, then there's the speed of it. The transition Reynolds number for water in a pipe is circa 5 x10^3 right. Which probably equates to a fairly low speed...

[dvs_dave]

The rear casing has stators on it to straighten the flow out, but it won't be completely laminar. It will however be much smoother than the flow from an open prop. Couple that with the greatly increased resistance to cavitation (which is very noisy) and it's a pretty good solution. I'm no expert but I don't think any modern subs use anything different, and many older ones have since been retrofitted with pump jets.

With regards the magnetic anomolies, I think the problem will always be that whatever you make a sub from, it'll always be full of air and large pieces of metal which will obviously alter magnetic fields differently than the surrounding water will. Consequently you'll theoretically always be able to detect them this way. Just my thoughts.

[Mr Dave]

doogz said:

I don't know much about subs, but i can tell you, fairly large chunks of the T45 Destroyers are aluminium.

Really? I thought they were moving away from Aluminium as it burns as seen in the Falklands?

911newbie said:

I do like the ducted fan approach in the pictures above - but how on earth do they get laminar flow out of that thing ? For starters its coming out of a rotating fan, then there's the speed of it. The transition Reynolds number for water in a pipe is circa 5 x10^3 right. Which probably equates to a fairly low speed...

Surely they get laminar flow out of it by employing lots of clever people with very expensive calculators?

[911newbie]

@Mr Dave_ As Scotty said, whatever money and clever people you throw at a problem, "you canna break the laws of physics"... or something like that.

[Caruso]

Welding aluminium on an industrial scale is not as hard as people think. It's a different technique from Mig welding steel, but there are ways to weld aluminuim that you can't do with steel. But on a sub there's really no benefit from using aluminium as weight is not an issue.

[anonymous-user]

I'm pretty sure that most modern subs "actively" cancel their magnetic signature, using the modern equivalent of a "degaussing ring" ??

http://en.wikipedia.org/wiki/Degaussing

[Condieboy]

dvs_dave said:

The rear casing has stators on it to straighten the flow out, but it won't be completely laminar. It will however be much smoother than the flow from an open prop. Couple that with the greatly increased resistance to cavitation (which is very noisy) and it's a pretty good solution. I'm no expert but I don't think any modern subs use anything different, and many older ones have since been retrofitted with pump jets.

With regards the magnetic anomolies, I think the problem will always be that whatever you make a sub from, it'll always be full of air and large pieces of metal which will obviously alter magnetic fields differently than the surrounding water will. Consequently you'll theoretically always be able to detect them this way. Just my thoughts.

An interesting point about magnetism and submarines, is that the dry docks in which they are refitted are roughly aligned with the planets magnetic field, and after a couple of years of having the dockies bang the st out of the thing, the boat will come out more magnetic than it went in.