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Fanjet-propelled sub chaser

Stealthy
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Submarines are hard to detect when submerged ... that's one of their main assets, intrinsic stealth.

There are hunter-killer subs, but these are also hugely expensive.

A cheap surface vessel that could hunt down subs would be an advantage. But surface vessels give themselves away by hull noise, particularly the hydrophone effect (noise) of their screws.

A sailing vessel would have the advantage of low intrinsic noise, but maneuvering is limited by wind direction and strength. It would be inconvenient if the enemy elects to attack on a calm day ...

So ... make your own wind. A smooth, non-magnetic hull (using light nonferrous alloys, and plastics) with retractable screws, and above deck level a couple of powerful high-bypass vectorable fanjets - a well proven technology. Hovercraft are propelled by exactly that system.

It would be possible to put the gas turbines inside the hull to keep the centre of mass low, and control the metacentric height, but there's the issue of power coupling up to the fans. There are advantages and disadvantages of both approaches, best established by building some prototypes.

The vessel can move by using efficient submerged propellers when off duty, but can then switch to stealth mode, retract the screws and use the less efficient but hydrophonically quiet fans to move itself when hunting.

The first thing the target sub hears is the whirring noise as a smart torpedo's props start up and the onboard active sonar starts pinging ...

8th of 7, Feb 15 2020

Underwater acoustics https://en.wikipedi...r_acoustics#Surface
" ... the sea-air surface can be thought of as a perfect reflector." [8th of 7, Feb 16 2020]

Underwater Hullaballoon Underwater_20Hullaballoon
almost 9 years ago [sninctown, Feb 16 2020]

[link]






       This is possibly not a stupid idea.
MaxwellBuchanan, Feb 15 2020
  

       Transferring the push without the vibration would be an engineering feat.
Voice, Feb 15 2020
  

       No, because the vibration of a gas turbine is relatively high frequency, which would be much easier to decouple from the vessel's structure using composite elastic mountings, and propagates poorly through water.   

       The sound levels inside aircraft propelled by gas turbines can be brought down to levels that permit un assisted conversation.
8th of 7, Feb 15 2020
  

       I was talking about vibration from the fans. And "permits unassisted conversation" isn't really the standard you need.
Voice, Feb 16 2020
  

       So, airboats...in the open ocean...what could go wrong?   

       Notionally, get the boat flying on hydrofoils which are acoustically isolated from the engines, and on stilts to avoid rough seas, and this might be a winner.   

       That said, from a civil liberties perspective I'd prefer to see technological developments that make submarines harder to detect. That way the gentleman farmers of the world can travel the open seas hidden below the surface, free from unconstitutional restrictions upon their intrinsic freedoms.   

       I was about to suggest an "underwater hullaballoon" but I see someone beat me to it by a few years (link).   

       A radical method of boat propulsion would be an array of giant pistons along the coast which create synchronized wave pulses analogous to beam-forming radar (water waves replacing electromagnetic waves) which propagate as barely-noticeable ripples until the waves meet to create a series of rogue waves directly behind the vessel being propelled, enabling the vessel to "surf" at high speeds, propelled by the shore-based wave stations.   

       [Max] good to hear from you although i can't believe you're agreeing with [8th], perhaps i'm having a hallucination brought on by too few gin and tonics?
sninctown, Feb 16 2020
  

       I can't believe, there's no way to image the surface wave patterns?
wjt, Feb 16 2020
  

       So hull noise isn't an issue? Its not as easy as screws, but Shirley a hull smashing into every wave at 30kt makes some noise?
bs0u0155, Feb 16 2020
  

       I wonder if an AI would eventually be able to distinguish, from noise and normal wave patterns the displacement of a deep running submarine, given enough data. Subs would not be able to hide except in weather harsh enough to obliterate detailed patterns. Submarines are quite large momentum displacement so an effect must hit the surface. The opposite could be done from below.
wjt, Feb 16 2020
  

       // airboats...in the open ocean...what could go wrong? //   

       Well, the beer cooler is way harder to retrieve if you drop it overboard....   

       // there's no way to image the surface wave patterns? //   

       Not acoustically, with any useful degree of resolution.   

       // So hull noise isn't an issue? //   

       Yes, it is- but when hunting, the vessel won't be going that fast, so as not to interfere with their own hydrophones. A sub hunt consists of several distinct phases.
8th of 7, Feb 16 2020
  

       //So hull noise isn't an issue? Its not as easy as screws, but Shirley a hull smashing into every wave at 30kt makes some noise?//   

       How about adding a semi soft hull that would absorb the impact of the waves without making noise?
doctorremulac3, Feb 16 2020
  

       Energy absorbtion. It would add hugely to drag.   

       It might be worthwhile in this specialist application, particularly if the flexible areas could be fully pressurized when not required, so as to become rigid.
8th of 7, Feb 16 2020
  

       Obsolete for many Western navies, yes. For less developed nations whose opponents still use traditional diesel-electric boats or their derivatives - even with AIP - there's still a market.
8th of 7, Feb 16 2020
  

       Could not trained tuna fulfil this role more economically, at least in warm waters? And in colder seas, the cod might prove serviceable.
MaxwellBuchanan, Feb 16 2020
  

       Corvettes are an expensive item.   

       // fanjets are inefficient at the speeds of a surface ship //   

       Only if you want to travel at 30 knots. For a hydrophone search pattern, 8 to 10 knots is the upper limit - above that, turbulence degrades acuracy.   

       // and very, very loud //   

       ... which won't propagate below the water's surface, particularly if the powerplant is carefully isolated from the hull.
8th of 7, Feb 16 2020
  

       Use an ekranoplane flying at minimum speed. The hydrophone is dropped on a long cable which unspools at nearly the speed of the 'plane, for a net speed of a few knots. When the cable runs out, reel it back in and start again.
MaxwellBuchanan, Feb 16 2020
  

       Ducted fans are an option, but they're bulky, and if the vessel is to be steered by vectored thrust, the smaller the size and mass of the jet, the better.   

       It's possible to have vanes in the exhaust, but the simplest method is to swivel the whole nacelle. There aren't that many controls and services to pass through the pivot, after all, and it's only got to turn 45 degrees each side of centreline.   

       A repurposed JT-8 off a superannuated DC-9, still in its enclosure, would do for prototyping. A pair would be better, as then differential power could be used for steering, and would give redundancy. Rudders generate turbulence, so they'd be retracted along with the props.   

       They need to be mounted low down, preferably near the bow (to reduce the risk of FOD) - otherwise the thrust will drive the bow in, which is undesireable.
8th of 7, Feb 16 2020
  

       Only on the prototype. As a proof-of-concept we suggest grafting the jets onto an existing hull - of a sailing vessel, like a large yacht - and see how well it works.   

       Do you mind nipping down to the marina and fetching back a suitable vessel ? Probably best if you make sure no-one sees you, though.
8th of 7, Feb 16 2020
  

       How big/heavy are these hydrophones?
MaxwellBuchanan, Feb 16 2020
  

       The actual transducers aren't very big. They vary according to application, from the size of a regular microphone capsule up to a soft drink can. For marine applications they're usually fairly solidly packaged, though - mostly a semi-armoured casing.   

       // One that looks nice - and not too expensive //   

       You've got it. About 20m long, GRP hull, single mast ... twin engines as backup for the jets.   

       See if you can get one with a jacuzzi. Oh, and decent satellite TV ... air conditioning ... and a bar, of course. And cabins for the servants.
8th of 7, Feb 16 2020
  

       //The actual transducers aren't very big. // In which case, why not tow them from self-piloting dirigible drones?
MaxwellBuchanan, Feb 16 2020
  

       *slowly rips open a bag of popcorn* Don't mind me.
21 Quest, Feb 17 2020
  
      
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