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Super Cavitation Tip Shafts

popgun pete

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Jul 30, 2008
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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.

What I don't know is the best place for the fine grooves to be located, but if the attachment screw 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 tyre tread.
replaceable tip super cavitation shaft.jpg
 
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popgun pete

popgun pete

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Jul 30, 2008
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Something like this done by manipulating some available images.
replaceable tip 4.jpg

replaceable tip 5.jpg

Note that the series of grooves could be deeper or more numerous and in the case of a pencil tip also on the conical section of the tip. The other advantage is if you hit a rock accidentally then you can renew the tip without grinding away the end of the shaft.

The Russians have developed super cavitation spears for diver operated handguns and assault rifles, however we don't have that level of power of propulsion in a speargun. But any increase in velocity and range even for a short segment of flight length would be worth it and we are not talking any million dollar development project here, instead it involves making a few spears.
waterflow projectiles.jpg
A bullet (top) and a longer projectile (bottom) moving through water.

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
diver operating assault rifle.jpg
 
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popgun pete

popgun pete

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Jul 30, 2008
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The Russians and others have produced small arms that use cavitation to extend underwater range, mostly by high gun power (big cartridge cases) to drive the projectiles at high velocity and others are miniature rockets. One pistol fires about five rounds, but reloads are done at the factory as the magazines need special handling to refurbish them for waterproofing. Spearguns don't have that power (with a few exceptions) so here we are looking to use the spear nose form to change the flow and create some cavitation effects. It may not be a dramatic improvement, but every gain counts when shooting underwater is all done within 30 feet.

As for underwater rockets there was the MBA "Lancejet", but it was not really a spearfishing device.
Gyrojet-Lancejet R.jpg

gyrojet-pistol.jpg

Gyrojet pistol Mark I Model B survival kit.JPG
 
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popgun pete

popgun pete

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Back in 2007 this idea was being kicked around for an anti-cavitation tail on a hydropneumatic gun's spear. Here a sliding "O" ring creates a valve in the spear tail to allow a water pumping action with the spear in the gun's inner barrel.

The idea here was to eliminate suction in the spear's wake as it was then thought that this would allow the shaft to move faster by preventing a low pressure zone behind the spear tail being overwhelmed by a high pressure zone at the spear tip. Pressure at depth acts on all surfaces of the shaft and the effects all cancel out except for the front to rear pressure differential. It had been noted that gun range fell off at depth with all types of guns and this was attributed to two possible factors; the front to rear pressure differential that grows greater with increased depth and the earlier collapse of the cavitation sheath at increased depth during the flight time of the shaft.
anti-cavitation spear tail.jpg
 
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popgun pete

popgun pete

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Jul 30, 2008
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Another detachable screw on tip, type X with increased circumferential grooves, for both pneumatic and band gun one-piece Tahitian or Hawaiian spears. Double floppers are another possibility, either on one pivot pin or two pins spaced at a certain separation along the shaft.
replaceable tip super cavitation shaft X.jpg

replaceable tip super cavitation shaft.AX.jpg

replaceable tip super cavitation shaft.DXjpg.jpg
 
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popgun pete

popgun pete

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Jul 30, 2008
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Russian company "Taimen" now offer a replaceable tip one-piece spear for their gun as distinct from the usual detachable tip threaded end spears. The "Marlin" screw on tip is claimed to spin the shaft as it travels through the water, I actually have one of these “Marlin” tips, but I have never used it. A concern is that it may snap where the cut-outs are located if it hit something harder than intended.
tahitian replaceable tip.jpg

garpun-nakonechnik_01_9df605dc653720a7f16b29f5c443551d.jpg

tahitian screw tip spear.jpg

Marlin tip.jpg
 
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popgun pete

popgun pete

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Found this, but no image.

Supercavitating projectile with propulsion and ventilation jet

US7347146B1
Inventor: Thomas J. Gieseke)

Abstract
A supercavitating projectile is disclosed and includes a combustion chamber, a forward-directed jet nozzle and a comparatively larger gas duct/rear-directed jet nozzle. The combustion chamber is filled with a propellant having a hollowed core. The core serves as a pathway to fluidly allow combustion gases to the jet nozzles. In operation, the propellant combusts to form gasses forced forward through the forward-directed nozzle to generate a virtual cavitator in the form of a ventilation gas bubble. Combusted gasses are also forced out the rear-directed nozzle forming a propulsion jet. The projectile therefore uses the rear-directed jet to maintain a cruise velocity approximate to the launch velocity and employs a source of ventilation gas using the forward-directed jet for supercavitating of the projectile.

Found them here. "CAV-X" ammo with tungsten alloy.
https://www.tungstenheavypowder.com/...ng-ammunition/
http://www.defensereview.com/revolut...tos-and-video/
Cav-x_display.jpg

machined_cav-x.jpg

CAV-X ammo.jpg

CAV-X Bullet tip.jpg
 
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popgun pete

popgun pete

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Here is the tip detail that I wanted to see, the cavitation bubble generator and maintainer after the initial gas injection.
CAV-X Projectile detail.jpg

In a firearm the CAV-X cartridge gases are partially sent forwards to clear a pathway in the barrel for the projectile as well as ejecting to the rear for propulsion, but in a speargun the projectile tip is already outside the gun. That means the only part of the CAV-X cartridge that we are interested in is the cavitation bubble creating and maintaining bullet tip. As a spear shaft is a long projectile compared with a rifle bullet we may need more cavitation grooves on the tip than the bullet does, plus the bullet is initially driven at much higher velocity than a spear when the cartridge propellant burns.
CAV-X Projectile.jpg
 
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popgun pete

popgun pete

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Actually this topic in a way is in a reverse order as I have seen the "Black Sea" gun's shot cavitate. When Fernando Abella was chief guinea pig/test pilot of the "Black Sea" gun and first shooting the 1,500 psi monster killer we could see on his Tuna hunting video shots the fuzzy bubble curtain surrounding the departing shaft. I say guinea pig as initially I was never sure that the gun would not blow up, however a similar gun had been previously burst tested and it looked like a factor of safety was there.

Now since I last wrote I have seen the US patent diagrams, there is more than one patent and I have read the others as well. The “CAV-X” bullet looks like it has a solid nose, but it actually has a micro-jet that blows out the point of the tip. I had thought maybe a ring of gases blew out the expanding shoulder of the cartridge case where it necks down on the bullet, but maybe not, however the case has a cylindrical neck on the larger ammunition rounds. You can see the patent diagram schematic below which I have fully annotated.

So basically the “CAV-X” round is a mini rocket with its own on-board propellant which when launched from the gun burrows its way through any surface interface to take out the target. In military use this is designed to knock out incoming high velocity torpedoes by being fired from either a fully depressible in elevation deck gun or dedicated underwater revolving gun turrets. For sure this will be part of US Supercarrier defences against underwater super cavitating missiles such as the SHKVAL.
suprrcavitating round.jpg

CAV-X Projectile detail2.jpg
 
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popgun pete

popgun pete

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There must be slight differences in the projectiles and ammo cases for different calibers with respect to details other than the simple scaling of the dimensions. Here the 7.62 mm NATO round is compared with the projectile schematic which is probably for a much larger caliber weapon.
CAV-X Sectioned.jpg
 
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popgun pete

popgun pete

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With a series of cavitation inducing grooves machined into the tip any resultant fragility may be a concern, but if the tip just screws onto the integral shaft body then it does not matter if you bust one, you just unscrew it and replace it with another tip. How this idea was first stimulated was some years ago a spring stainless steel Tahitian tip spear was sold with an additional screw thread behind the sharp tip, which meant that you could screw a standard replaceable flopper equipped tip over the top of it (from memory it was a Sea Hornet 17-4 PH shaft). The downside was you then had two floppers, one (or maybe even two) on the screw threaded tip itself and one pivoted on the shaft and they did not necessarily align or oppose each other. I wondered at the time if the sharpened threaded tip cavitated, but I never tried one out and eventually forget about it. Then when I worked on the "Black Sea" guns the shaft cavitation issue came up again as the shafts flew so fast out of the gun, but the emphasis then was on the suction forming behind the spear tail and the thoughts of a new tapered tail to defeat it, seen here earlier.

Now I have lots of spears and many are Tahitian type which came with guns as standard equipment, but those spears don’t last long tip-wise where I do most of my diving. Repeated repointing whittles them down in length. That made me think of replaceable end tips, something which was common in the sixties and now I hope that it will become common again, but with the added lure of super cavitating shafts. What causes the cavitation is a sharp boundary change or edge, the water cannot flow smoothly over the edge so vortices form as the flow passes over the gap. This turbulence can cause a localized stream or flow detachment and the resultant rarefactions produce a bubble which is regenerated as the flow continues to break up over the discontinuities in the surface of the projectile. As I stated earlier speargun shooting ranges are usually up to 30 feet, so any speed improvement is worthwhile in that zone
 
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popgun pete

popgun pete

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New version with cone added to provide flow "shadow" for flopper on the shaft.
replaceable tip super cavitation shaft XZ.jpg

replaceable tip super cavitation shaft XZZ.jpg

Also non pencil tip version with flutes on tip or a tri or quad cut.
 
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popgun pete

popgun pete

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Here is a flopper retention method used in the past that could also be used on such tips. Wire loop segments cut from a spring steel have tension as their locating ends are drilled out of side to side alignment.
flopper pivot ring keeper sketch 2.jpg

double flopper pivot ring keeper sketch 2.jpg

Rings (grooves) now added.
double flopper cavitation point pivot ring keeper sketch 2.jpg
 
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