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12-07-2018, 06:24 PM | #1 |
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Super Cavitation Tip Shafts
When a spear is fired from a gun it leaves at its maximum velocity, after that hydrodynamic drag and towing the shooting line to the target slow it down while the force of gravity pulls the shaft towards the bottom. Except for judged lobbed or parabolic shots the most useful part of the flight to the target is the linear trajectory travel zone. In addition to the spear tip driving through the water and the suction created behind the spear tail the wetted length of the shaft surface area adds to hydrodynamic drag. If a cavitation sheath can be created early in shaft flight, which takes considerable gun power, then the drag on the wetted shaft surface area is reduced thereby prolonging the flight at higher velocity and the length of the linear trajectory travel zone.
Integral tip shafts have low drag, but have poor durability when striking rocks, but it seems to me an old idea can be revived that will both minimize this problem by using replacement tips and assist in creating a cavitation sheath. The small rear step in tip diameter could serve as a vortex or turbulence generator to produce a cavitation sheath along the shaft length and possibly some rings grooved into the replaceable tips would help create this effect. The sheath will collapse eventually, especially at higher pressure with increased water depth, as the spear velocity falls, but it may buy some extra linear travel distance and decrease the reaction time available to the fish for avoiding the shaft. Last edited by popgun pete; 12-08-2018 at 09:11 PM. |
12-08-2018, 05:12 AM | #2 |
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Re: Super Cavitation Tip Shafts
What I don't know is the best place for the fine grooves to be located, but if the attachment thread makes the threaded wall thickness too thin then they may be better placed forwards on the replaceable tip. This vortex-cavitation idea was canvassed once before, but the series of ring grooves then ran along the shaft which would be like shooting a very long rat tail file from your gun! It could ream the gun’s shaft guide track if the fluid squeeze film collapsed which the grooves may just do being like the sipes on a tire tread.
Last edited by popgun pete; 12-08-2018 at 06:59 AM. Reason: typo |
12-08-2018, 11:59 AM | #3 |
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Re: Super Cavitation Tip Shafts
Por fin un tema muy interesante para mi en este foro.
Es un inconveniente grande no saber el idioma ingles y escribiré en español. Tengo años estudiando este fenómeno y desde mi gran ignorancia diré : 1ª que esa punta jamas supercavitara.. 2ª que la velocidad de una varilla jamas superara la velocidad de contracción de la propia goma en vació. 3ª puede abrirse una puerta en este fenómeno físico con los fusiles desmultiplicados que pueden duplicar la velocidad de salida de la varilla, pero esta varilla tendrá que ser: de poca masa yo calculo que no mas de 40 cm de longitud y con punta mas o menos plana y con una forma muy determinada, impulsada a una gran velocidad capaz de ejercer una gran presión inicial sobre el agua capaz de generar una burbuja que la envuelva. |
12-08-2018, 02:30 PM | #4 |
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Re: Super Cavitation Tip Shafts
Interesting Topic. We had some good discussion about this in a thread once before communal pantywad got the thread closed down.
Supercavitation works best/easiest with smaller length objects such as Bullets. The idea is to encapsulate the entire projectile in an air bubble so that it moves through water at higher velocity. (I don't really understand that part) The ones which I studied used a compressed air within the projectile to form the bubble. Super Cavitation Bullets can shoot underwater, from the air into the water, or from the water into the air. When it comes to Speargun shafts, it may be a little different. When we are doing Video testing for Shaft flight we like to see No Cavitation. In our tests cavitation (bubble stream) points to instability or drag from something like a Shooting line connection. One thing which might prove interesting is using some kind of surface treatment for shafts to make them hydrophobic and possibly decrease their drag and increase their speed. Surface treatments are used in sailing for example on hulls to do just this. If we could increase shaft speed by 10-15% might be a big gain on longer shots. Hope to see you explore the Super Cavitation concept. There's some good stuff coming from military, mostly submarines. Ie, the concept of the super cavitation bubble for submarine or missile travel.
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12-08-2018, 02:32 PM | #5 |
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Re: Super Cavitation Tip Shafts
The point itself will not super capitate as you say, but the shoulder behind the tip will. My "Black Sea" hydropneumatic gun will shoot spears at extremely high velocity and I believe this effect occurs. Part of my suggestion is based on the difficulties I have in damaging integral tip spears and then I remembered the replaceable tip spears that I received with my first Aquatech gun. These have a single flopper on the shaft and a screw on pencil point tip. The screw on tips don't need to be pencil points as can be seen here on the photos. These are Pelengas spear tips, but others also manufacture them. Due to the need to have a certain thread size on an integral tip spear for strength, such as Hawaiian or Tahitian spears, the replaceable tip head would probably need to be slightly larger than the shaft diameter, hence I thought about the ring grooves which I had seen demonstrated on a grooved sphere which moved through water at higher speed due to the series of concentric ring grooves cut into the advancing face, i.e. facing the direction of water flow than a smooth surface sphere.
Last edited by popgun pete; 12-08-2018 at 02:54 PM. |
12-08-2018, 02:45 PM | #6 |
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Re: Super Cavitation Tip Shafts
Something like this done by manipulating the images.
Last edited by popgun pete; 12-08-2018 at 03:29 PM. Reason: improved the image |
12-08-2018, 03:11 PM | #7 | |
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Re: Super Cavitation Tip Shafts
Quote:
https://en.wikipedia.org/wiki/APS_underwater_rifle http://arsenal-info.ru/b/book/2921652036/32 http://weaponland.ru/publ/specialnoe...cov/15-1-0-305 Last edited by popgun pete; 12-08-2018 at 03:36 PM. |
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12-08-2018, 03:25 PM | #8 | |
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Re: Super Cavitation Tip Shafts
Quote:
It is a great inconvenience not to know the English language and write in Spanish. I have years studying this phenomenon and from my great ignorance I will say: 1st that that point never supercavitara.. 2nd that the speed of a rod never exceeded the speed of contraction of the rubber itself in vacuum. 3rd can open a door in this physical phenomenon with the rifles demultiplied That can double the output speed of the rod, but this rod will have to be: of little mass I calculate that no more than 40 cm in length and with a more or less flat point and with a very determined shape, driven to a high speed Able to exert a great initial pressure on the water able to generate a bubble that wraps it. Por favor, lea este http://arsenal-info.ru/b/book/2921652036/32 Last edited by popgun pete; 12-08-2018 at 06:39 PM. |
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12-09-2018, 02:42 PM | #9 |
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Re: Super Cavitation Tip Shafts
Hi Pete,
Yes, we are talking more about Non Super Cavitation, more like Cavitation enhancing Tips, I can't remember whether those rings need to be on the angle of the point itself, or behind it. Simple enough to test as you say. But going back to Air guns, I wonder if there's not a way to do it. The idea is to compress some air into the tip so that it can release over some time from the front and allow for an encapsulation.
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12-09-2018, 03:04 PM | #10 | |
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Re: Super Cavitation Tip Shafts
Quote:
http://aquatech1.narod.ru/index1.html "Testing of Speargun Performance using different mass, diameter spear shafts and shooting line drag." http://aquatech1.narod.ru/eng7.html |
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12-09-2018, 05:28 PM | #11 |
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Re: Super Cavitation Tip Shafts
I've been watching this conversation and thinking about it. Yes, the Russians have proclaimed to have a torpedo that creates a cavitation at the nose that allows it to travel much faster.
At what cost though? Take for example the advances in aircraft speed. Air is really just another liquid we're flying around in. Consider the nimble bi-plane vs. the current tech that allows us to travel at much greater speeds. There is a loss of nimbleness there as well as a huge cost to increase flight speed. IMO, I believe the physics ROI outweighs the idea. BUT, hey...carry on, interesting conversation.
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12-09-2018, 06:28 PM | #12 |
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Re: Super Cavitation Tip Shafts
sorry meant to attach this pic before.
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12-09-2018, 10:01 PM | #13 | |
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Re: Super Cavitation Tip Shafts
Quote:
Last edited by popgun pete; 12-09-2018 at 10:34 PM. |
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12-09-2018, 10:03 PM | #14 |
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Re: Super Cavitation Tip Shafts
Yes, that is the sort of thing I am talking about. The SHKVAL torpedo works differently as gas jetted out of the nose provides the sheath for the rocket propelled torpedo. Note that the nose plate can tilt.
https://en.wikipedia.org/wiki/VA-111_Shkval https://www.rbth.com/defence/2014/05...ble_36941.html Last edited by popgun pete; 12-09-2018 at 11:12 PM. Reason: added another reference |
12-10-2018, 01:54 AM | #15 |
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Re: Super Cavitation Tip Shafts
Pete, Yes, that is the Tip I'm talking about which was developed both for underwater missiles, (high speed torpedos), but also with the hope of making extremely fast Submarines. I'll try to find it again, but there was a good article on Super cavitating Small arms bullets. The rig and ring placement had the function of redirecting the airbubbles so they encapsulated the projectile. Bullets could reach 50'+ underwater when fired from an aircraft, or the reverse, could hit target above water if fired from below. Ohh. found it. you'll be on the edge of your seat. http://www.military.com/video/ammuni.../5389871935001
Another thing to consider is the "Shaft Sauce" applying a chemical treatment to the shafts making them more slippery. I do this on my own shafts.. I need all the help I can get and I have the stuff sitting there so why not.. but I have no proof that it increases shaft speed.
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