Submarine noise level reduction, stealth question

[FusionJim]

I've found online some failed projects that some navy subcontractors have done in the past with regards to creating a truly silent motor for submarine propulsion. IIRC it was General Atomics that tried a homopolar motor for exactly this purpose.
I don't know for sure but it seems to me that currently subs use AC machines for generators and propulsion, stuff like permanent magnet synchronous motors , DC machines that are essentially commutated DC motors, etc.

One of the greatest feats of a modern nuclear sub is to get close to enemy shore without being detected.
So it got me interested , does anyone know what in fact are the still the biggest sources of sound/vibration/noise that are still present in modern state of the art nuclear subs?

Would it be a large gain if there were real DC generators and motors (not AC machines that simply have their output rectified) ?
Such motors and generators could be electrically silent and therefore also mostly mechanically silent, given most of the vibration in an AC machine comes from the current in the windings , assuming very good bearings and balance of the rotary components.

Also I read (is it true?) that because electrical motors normally don't run at such low RPM as is needed for a prop shaft they use gearboxes?

 

[Baluncore]

One of the greatest feats of a modern nuclear sub is to get close to enemy shore without being detected.

That is what the navy wants their own countries' population to think. It also plays on the enemy's paranoia. Keeping the location of submarines secret, is all part of the misinformation game.

So it got me interested , does anyone know what in fact are the still the biggest sources of sound/vibration/noise that are still present in modern state of the art nuclear subs?

When running quietly, it is the propellers, because they are outside the hull.

Also I read (is it true?) that because electrical motors normally don't run at such low RPM as is needed for a prop shaft they use gearboxes?

Because the product of RPM and torque is power, and the motor shaft has a diameter determined by the maximum torque transmitted, faster motors can be smaller than slower motors in the same power range and application. That then requires a reduction gearbox to turn the big and slow, hopefully quiet propeller.

If gears are helical cut, and super-finished, the gearbox can be close to silent. Those techniques were developed for use in Rolls-Royce cars.

 

[Flyboy]

Afaik, modern nuclear subs still use steam turbines directly driving the screw for propulsion. There have been multiple attempts at a hybrid power train, but it’s been louder, more maintenance intensive, and bulkier than the direct drive system.

The biggest noise source is still cavitation, but it’s generally accepted that a modern US Navy nuke boat at low power and making turns for, say, 5 knots, is literally quieter than the ambient background noise in the open ocean.

 

[Ranger Mike]

When I was Product Manager at a gage manufacturing company in the 1980s we sold an andrometer. Made by Sheffield in 1940s. Product line sold off in 1991. Andon - Angular deviation of the radial measurement. It is used to measure bearing noise. US Navy used it to find quietest bearing available. It got copied by the rising sun people. You need quiet bearings in a sub.

 

[Astronuc]

One of the greatest feats of a modern nuclear sub is to get close to enemy shore without being detected.
So it got me interested , does anyone know what in fact are the still the biggest sources of sound/vibration/noise that are still present in modern state of the art nuclear subs?

Moving things would generate noise. The technology and applications are not disclosed in the public domain for obvious reasons.

One may read - Blind Man's Bluff: The Untold Story of American Submarine Espionage
https://en.wikipedia.org/wiki/Blind_Man's_Bluff:_The_Untold_Story_of_American_Submarine_Espionage

 

[FusionJim]

Afaik, modern nuclear subs still use steam turbines directly driving the screw for propulsion. There have been multiple attempts at a hybrid power train, but it’s been louder, more maintenance intensive, and bulkier than the direct drive system.

The biggest noise source is still cavitation, but it’s generally accepted that a modern US Navy nuke boat at low power and making turns for, say, 5 knots, is literally quieter than the ambient background noise in the open ocean.

Hmm , I've seen industry companies selling large DC motors advertizing them publicly as for use in submarines like the Siemens permasyn type, but i thought their mistly for diesel subs, so your saying nuclear subs drive the prop directly from turbine vua a reduction gearset? Ni electric motor on the shaft at all? IIRC there should be one as in case of main power loss to drive from the batteries

 

[Flyboy]

Hmm , I've seen industry companies selling large DC motors advertizing them publicly as for use in submarines like the Siemens permasyn type, but i thought their mistly for diesel subs, so your saying nuclear subs drive the prop directly from turbine vua a reduction gearset? Ni electric motor on the shaft at all? IIRC there should be one as in case of main power loss to drive from the batteries

It's possible that technology has advanced to permit that for a larger fast-attack or missile sub, but to my knowledge the current production Virginia-class SSNs still make use of direct drive systems. Per public sources, the Virginia-class uses two 15MW steam turbines for propulsion as well as an unspecified "secondary propulsion system" which is likely a deployable electric motor pod for maneuvering assistance at low speeds.

The upcoming Columbia-class SSBN is expected to use a turboelectric system of unspecified configuration, and it's likely to be kept relatively classified for some time what exactly that system entails.

Smaller diesel-electric subs like the German Type 212/214, Swedish Gotland-class, Japanese Soryu-class, and a few others do use electric motors directly attached to the screw, but the designers and operators are publicly very cagey about what exactly those motors are. They're also significantly slower and shorter ranged than the SSNs and SSBNs, but can be incredibly quiet and hard to detect, especially in the littoral/coastal waters they were designed for.

 

[OmCheeto]

Hmm , I've seen industry companies selling large DC motors advertizing them publicly as for use in submarines like the Siemens permasyn type, but i thought their mistly for diesel subs, so your saying nuclear subs drive the prop directly from turbine vua a reduction gearset? Ni electric motor on the shaft at all? IIRC there should be one as in case of main power loss to drive from the batteries

It looks as though wiki confirms that there is an electric motor on the shaft:

https://commons.wikimedia.org/wiki/File:Diagram_Submarine_Reactor_&_Steam_Plant.jpg

 

[Flyboy]

Interesting. It’s not going to provide a lot of power, and I can’t think of any situation where there would be enough of an advantage to use batteries and that auxiliary motor instead of very low output on the reactor.

…y’know, I take that back. I can think of a situation where that would be useful. Docking/departing.

 

[RicoGerogi]

Modern subs run quiet because of electric drives, most probably with PMS motors. Some could use gearboxes to slow the prop, others go direct.