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Vacuum powered speargun

popgun pete

Well-Known Member
Jul 30, 2008
3,377
744
153
Australia
Having mentioned the all-reservoir-tank-and-no-inner-barrel "Vlanik" speargun in the recent speargun compression ratio thread, I thought that I would complete the list of pneumatic speargun types by describing another speargun which also has no inner barrel and in a sense no air pressure tank either. This is the vacuum speargun which is powered by a relative vacuum formed inside the gun body when the gun is muzzle loaded. The gun has a large diameter body tube (internal diameter is 40 mm) to provide the cross-sectional area necessary to obtain some useful propulsive force from the pressure differential acting on the sliding piston that will propel the spear from the gun.

The spear (8 mm diameter) inserts through a muzzle seal and drives a large diameter piston back (same size as the inner diameter of the body tube and carrying a single large diameter rubber cone type seal), pushing it away from the muzzle and moves it towards the rear end of the gun which is open to the surrounding water. Water sitting behind the piston is pushed out of the rear end of the gun while a relative vacuum forms in the space opening up between the piston's front face and the rear face of the muzzle bulkhead. That means the spear is sitting in a vacuum inside the gun body when the gun is cocked and ready to shoot, the gun body being in a sense one big barrel. You can think of it as a scaled-up "Mamba" system, but without the rest of the gun.

The problem with this intriguing design is the large contact area of the piston seal and the need to have a very smooth finish on the interior of the gun body/big diameter barrel, otherwise sliding friction will absorb too much of the gun's power. Especially as abrasive particles like sand and grit can easily enter the gun and damage the finish on the internal bore of the big diameter barrel. Any loss of vacuum inside the gun and the gun will lack any shooting power, plus water will fill the gun body if it has a bad leak when the gun is cocked. After shooting the gun has no buoyancy, being full of water. It needs to have some mass or it will float when cocked, which is not such a good idea! The spear has no stop diameter on its tail and is attached to the gun with an auto-coil type shooting line affixed behind the spear tip.

Basically a quirky gun that explores yet another variation on the differential pressure theme which is used to power all pneumatic spearguns, regardless of the type.

You can read about this very interesting gun on Apoxy.ru, a thread is posted there based on an article by L. Volkov from "Sportsmen - Submarine", issue number 69.

The spearfishermen of the former Soviet Union explored nearly every speargun design imaginable, no doubt something to keep them busy in the months while they waited for the ice on the local river or lake to melt!
 
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tromic

Well-Known Member
Aug 13, 2007
1,615
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Croatia
I like this concept Peter! Very simple and clever solution! Deeper you go you will have more power in the gun. At 10 m almost 2 x more. No need for pump. No need for lubrication. Lower noise then regular pneumatic gun. I would use it with the latest tomba so I would have a slider on the shaft, but... maybe better without the slider to have lover noise. I would like to try it!
Thanks for the info.
Cheers, tromic
 
Last edited:

Old Man Dave

Offline
Feb 19, 2005
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Guernsey, Channel Islands, UK.
Interesting as always Pete.

Seems to me another limiting factor is that the propulsive force is atmospheric pressure and this is a set value. Although as pointed out this gun has a force that increases at depth never the less pressure at the surface its approx 15lb/sq in. With a 40mm diameter piston/barrel this is an area of 2 sq in and therefore gives 30lb of thrust.

Most modern air guns seem to have a thrust of at least double that, although absolute power is not of course a direct measure of performance and in this case with no water to move in front of the piston efficiency would be good.

Interesting idea and as I've said before shows there's nothing new in design.

Dave.
 

tromic

Well-Known Member
Aug 13, 2007
1,615
180
153
Croatia
Easy to load. Just 13 kg at the surface. At 20 m performs like regular pneumatic gun at 39 Bars!
 

willloomy

Well-Known Member
May 7, 2008
199
23
58
Hawaii
okay so i think i figured this out. the power of the gun depends on two things, the depth of the gun when the shot is taken and how far away the piston is from the muzzle when unloaded. the page says it takes 13kg to charge the gun and 13kg-force is approximately 127 newtons. after a little algebra you get that the difference in pressure at the is 101325 PA or 1 atmosphere so there was no space between the muzzle and the piston. After a little work I came up with a few equations that give you all kinds of good data about the power to depth ratio
a little graph about the power in kg/f (kilogram force) as it relates to depth
keep in mind this is potential energy, much of it is lost due to friction, the shafts momentum ect.
 
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popgun pete

popgun pete

Well-Known Member
Jul 30, 2008
3,377
744
153
Australia
Note that the spear sticking out through the muzzle seal is also exposed to ambient water pressure and that will cancel out the contribution of the water pressure at the centre of the piston acting over an equivalent area (to that of the spear) at the rear end of the gun. The cross-sectional area of the spear shaft represents 4% of the cross-sectional area of the sliding piston (for spear diameter 8 mm, piston diameter 40 mm). Effectively the ambient water pressure to drive the spear from the gun relies on the remaining annular area of the sliding piston that is facing a vacuum inside the gun, not the whole cross-sectional area of the rear of the piston.
 
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popgun pete

popgun pete

Well-Known Member
Jul 30, 2008
3,377
744
153
Australia
Just found a reference to a vacuum powered speargun and here it is. The builder has made a nice job of it and the big diameter tank tells you what it is as he demonstrates the action in this video made during an interview. Any foreign material getting into it could jam the large diameter sliding piston, but apart from that it should work OK. Note the very big trident!
vacuum powered gun.jpg
 
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popgun pete

popgun pete

Well-Known Member
Jul 30, 2008
3,377
744
153
Australia
One problem with this vacuum powered gun is that water can access the front end of the gun after the shot and if the piston is not fully forwards in the body tube then water will occupy the volume in the gap between the piston's front face and the rear of the front bulkhead thus shortening the operating stroke of the gun. If the front end of the gun progressively takes on more water with each shot the operating stroke of the piston will then shorten even further. One way to overcome this reduction may be already demonstrated in the video when the spear is shown being pushed in from the rear end of the gun as this would drive most of the water out of the front end of the gun by pushing the piston right up to and contacting the front bulkhead.

After the completion of the dive it would be necessary to wash both ends of the gun and then let it completely drain and dry out. Even so it does have the attraction of no spring or air pump, but the two seals in the muzzle and on the large moving piston respectively would need to be kept in good condition as would the surface finish on the bore of the body tube to maintain low sliding friction.
 
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popgun pete

popgun pete

Well-Known Member
Jul 30, 2008
3,377
744
153
Australia
A bit of research has revealed that these guns are not very accurate as the big piston moving in the barrel tends to throw the gun around, including when it hits the muzzle at the end of travel. They were intended for spearfishing on scuba where the gun could be used at depth and make use of the higher ambient pressure, however they never lived up to their expectations and were no good against big fish. This may explain the big trident as that would increase the chance of a hit if the gun was not particularly accurate.
 

Vlanik

Well-Known Member
Jan 19, 2009
111
3
58
Sothi
[QUOTE = "popgun pete, post: 985111, member: 18674"]
Это имеет тенденцию к разбрасыванию оружия, когда оно попадает в дуло в конце хода. Они были предназначены для подводной охоты в акваланге, где можно было использовать только глубину и высокое давление окружающей среды, однако они никогда не были оправданы своими ожиданиями и были бесполезны против крупной рыбы. Это может объяснить большой трезубец, потому что это увеличит вероятность попадания, если пистолет не был особенно точным.
[/ QUOTE] Rave
 
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