Pneumatic speargun without piston

[Vlanik]

Пистолет 20 сантиметров...
Щука 4 килограмма...
Дистанция стрельбы 1.5 метра...
-------
The Gun 20 centimetres...
The Pike 4 kilograms...
The Distance of the shooting 1.5 metre...

 

[popgun pete]

Yes, I understand. I want one



Thanks for the link. I copied the text from the post into google translate. It gave me a more easily understandable translation than the one above. Interesting reading.

Спасибо за ссылку. Я скопировал текст с поста в Google Translate. Это дало мне более легко объяснимо перевод, чем выше. Интересное чтение.

Jégwan

I have sent you a message about how to buy one, but you need Vladimir to be willing to do it.

 

[Jegwan]

Пистолет 20 сантиметров...
Щука 4 килограмма...
Дистанция стрельбы 1.5 метра...
-------
The Gun 20 centimetres...
The Pike 4 kilograms...
The Distance of the shooting 1.5 metre...

Impressive "gun to fish ratio". Very nice
Впечатляющий "пистолет соотношение рыбы". Очень хороший
Jégwan

 

[popgun pete]

Here is an idea to provide somewhere for the water in the tail cap to go and prevent hydraulic lock. If the spear shaft is a hollow tube it can then have a valve built into the rear end of it, like a pneumatic gun's inlet valve, but with a stronger biasing spring. The front of the hollow spear is sealed off with a female thread accepting the male thread of the spear tip, such as the type of tip used in the old spring gun spears which were also tubular. A rubber gasket is used here at the front end to seal the spear interior once the spear tip is screwed in. The concept is that when loading the gun the excess water trapped in the tail cap is pushed through the valve into the interior of the spear each time. Each successive shot will add more water to the spear, but the quantity transferred will be small. At the end of the hunting session (or each day) the spear tip is removed by unscrewing it and the water emptied out of the spear ready for next time. To prevent corrosion the spear is washed and stored with everything opened up so that it can fully dry out internally between dive trips. The spring in the spear tail valve, a ball type valve, must be strong enough to resist ambient pressure at depth flooding the bare ended spear after the shot, but weak enough so that the force of loading the gun causes it to open before the tail cap moves into the gun from its place in the muzzle where it acts as a plug. Then the air pressure in the gun should hold the tail cap firmly on the spear as for it to fall off it will need to create a slight vacuum, the tail cap also preventing air from the compressed air reservoir getting into the hollow interior of the spear. Thus the hollow spear gradually accumulates water, but not enough to change its mass if the usual number of shots per dive are say around twenty shots. The longer the hollow spear is then the more capacity it has to contain water, hence it would take a greater number of shots to fill it up, but the idea is to empty it out well before that happens. If the hollow shaft half filled with water coming through the rear valve then the pressure inside the spear would double to 1 atm. (gauge pressure) having been at ambient pressure initially (i.e. gauge pressure = zero).

The diagram which I finally got around to doing. Also now with extension for shaft notch at front end behind the spear tip.

 

[tromic]

Interesting idea Pete. That would work. It is little complicated and the shaft would be lighter and faster but also having smaller momentum. Only problem might be that you could not be sure how much shots still left.

I would make something like this:
(look only at detail in elipse)

 

[popgun pete]

The rubber bladder's recovery to its initial shape will push the water out after each shot so that it can accommodate the volume to be squeezed in during reloading when the tail cap next presses into position on the shaft tail, but how to make that rubber bladder stay in its original position? Also it will be stretched more by ambient pressure at increasing depth, so the space in front of it (the dry space) will need to be large enough to allow for that. I selected a ball valve as the pressure that it opens at can be adjusted. The ball valve spring would need to be like the one in the "Mirage" over-pressure ball valve, not the inlet ball valve. I know that if your gun loses all pressure underwater the gun starts to fill up with water coming in through the inlet valve, I have had it happen twice (when no inlet valve cover cap was fitted, a bayonet type, I had lost it) as the gun became heavy and I could feel the water sloshing around inside it. A total dismantling was required each time to remove all traces of saltwater and fix the leak in the nose cone's inner seat.

Note that "pistonless" spearguns gradually take on water, but it is a tiny amount each time that is carried through by the joint lines where the spear tail cap meets the shaft. With nothing much inside the tank in terms of parts and threaded connections there is less to clean out. The hollow spear that I am suggesting has an extra joint line at the rear spear tail bulkhead in front of the tail cap, whereas the joint line at the front of the hollow shaft should not penetrate the muzzle seal, but stay in front of it. Small precision bore stainless steel tubing is expensive stuff, I found that out when I bought a length of it for a speargun project that never really worked out many years ago.

 

[tromic]

...
Note that "pistonless" spearguns gradually take on water, but it is a tiny amount each time that is carried through by the joint lines where the spear tail cap meets the shaft.
...

Agree.
Just to add. Same happens to air, in oposite direction, reducing the pressure inside the gun.

 

[popgun pete]

The fundamental difference between a "pistonless" (also in a sense "inner-barrel-less") speargun and a standard pneumatic speargun is that for the "pistonless" speargun the radius of the spear "r" determines nearly everything about the speargun except the speargun's compression ratio (as you can always increase the size of the outer tank or air reservoir independently). The standard pneumatic speargun is more versatile in that you can vary the spear's mass and its wetted surface area (which partly determines its hydrodynamic drag) by changing that radius "r" while an inner barrel sliding piston of a somewhat larger radius "R" can remain constant. This sliding piston provides the same force level at lower chamber pressures than those used in a "pistonless" speargun as its cross-sectional area facing the chamber pressure is that much greater. Water surrounding the spear in the inner barrel then becomes a problem when "r" is much smaller than "R", hence we need a vacuum barrel system to eliminate that water, but at the cost of providing an extra dynamic seal in the gun located at the muzzle in addition to the piston mounted seal moving along the inner barrel bore.

Summarizing in "pistonless" spearguns "r" = "R" (although there is no inner barrel as such, "R" can be thought of as representing the muzzle bore), in hydropneumatic spearguns "r" < "R" (here "R" relates to the enlarged diameter tail or "charge piston" on the spear that carries a seal) and in pneumatic spearguns "r" << "R". Note that "much less than" is a relative term here, but in the latter the sliding piston diameter has to be large enough to be stopped by the muzzle restriction or it may depart with the spear after demolishing the shock absorber, thus there needs to be a reasonable step in diameter at the muzzle.

The "RPS-3" is a "pistonless" speargun as is the "Kobra", but the multi-ported inner barrel that supports the rubber tube membrane separating pressurized air and water inside the guns acts as a variable throttle which is controlled by the spear body movement within it and introduces inefficiency.

 

[tromic]

The rubber bladder's recovery to its initial shape will push the water out after each shot so that it can accommodate the volume to be squeezed in during reloading when the tail cap next presses into position on the shaft tail, but how to make that rubber bladder stay in its original position?
...

Maybe something like this on image:

 

[Jegwan]

Tromic; could it be an idea to use a magnet in the tail cap? I think this might make everything a little simpler.

I don't think that water would be a problem, if the tail cap is long enough, so it reaches the muzzle seal.
A small channel can be drilled/milled from the end of the spear with exit on the other side off the muzzle sealing - if necessary. You might need two muzzle seals with a little space between them.

Jégwan

 

[tromic]

Jegwan, maybe strong magnet might be used, in some configuration? Shaft tail cap should be of some light but strong material, duralumin, titanium, ... I am not sure If understand your idea using two muzzle seals.

 

[popgun pete]

Drilling a long hole through the shaft would be a difficult thing to do, but allowing the water to have an exit outside the muzzle seal would be the ideal way to eliminate any hydraulic lock preventing the spear tail cap fully pushing onto the shaft. Maybe the tubular spear could be adapted to this idea. The water exit holes at the front end of the shaft would have the sear tooth pass over them, but maybe they could be drilled small enough so as not to cause any problems. During the shot you would not want water driving through the shaft and pushing the spear tail cap off before the spear pulls free of the tail cap.

 

[tromic]

Happy New year to everybody!

This last version seems to be more simple, more reliable than the previous. Only real problem is drilling such a long hole through the shaft.

If I would make this kind of desing I would probably try this one first (look only at the shaft end). I believe it would work fine and would be very simple, reliable and easy to make. However it would require to have air bubble in shaft tail boring before inserting the shaft into the gun. I think that would not be a problem.

http://img42.imageshack.us/img42/1855/vlanikt5.jpg

http://forums.deeperblue.com/threads/pneumatic-speargun-without-piston.94538/

 

[popgun pete]

Yes, a Happy New Year to everyone. Of course any failure of the tail cap to spear tail "O" ring seal will allow all the compressed air to escape from the cocked gun via the interior of the hollow spear in my diagram shown above, as it will if the threaded joins at the ends of the tubing section are not also airtight.

So gun reliability, which we often tend to forget about, may not be as good as that obtained with a simple friction fit spear tail cap and water will still be transported inside the gun with each loading of the shaft in any case, although potentially not as much in terms of the transferred volume. I think it is impossible to keep ambient water out of these guns, so maybe better to look at the construction materials used so that the interior is more resistive to corrosion, but still has the necessary strength for a pressure vessel. Some of the "Vlanik" guns look to be nearly all plastic, I am thinking of the tiny gun shown alongside a "Cricket" disposable cigarette lighter; I remember seeing its photo, but cannot find it.

 

[Vlanik]

Some of the "Vlanik" guns look to be nearly all plastic, I am thinking of the tiny gun shown alongside a "Cricket" disposable cigarette lighter;

 

[tromic]

I suppose this is the smallest spear gun in a world, yet usable on 1 - 1,5 m distance!

 

[Jegwan]

Jegwan, maybe strong magnet might be used, in some configuration? Shaft tail cap should be of some light but strong material, duralumin, titanium, ... I am not sure If understand your idea using two muzzle seals.

First a happy new year to all.

Well, my idea was to have a small separate chamber for the water to enter and exit – but both the complexity and the friction would increase, so it not a useful idea.

I have attached a principle drawing for a tail plug with a magnet. It's true that it will make the plug more heavy, but the new Neodymium magnets are quite powerful – so a small one can be used. I suppose that the tail plug could be made in a light weight material such as aluminum or POM.

My idea is to have a small relief channel on the spear from the tail to the other side of the muzzle seal, and a seal to close this channel when the spear is inserted in the tail plug. This will "lock" the remaining water in the channel/spear tail when the gun is loaded.

The Neodymium magnet will keep the tail plug in place. It will also be useful to keep the plug in place (with a steel rod or similar) when the gun is depressurized.

Please let me know what you think…

Jégwan

 

[Jegwan]

I suppose this is the smallest spear gun in a world, yet usable on 1 - 1,5 m distance!

That's a really cute one.

Jégwan

 

[tromic]

I would rather try something like this:

 

[Jegwan]

Tromic: I'm not sure I understand.. please clarify - the blue and red...?
Jégwan

Edit: I think I got it. You're right. The seal inside the tail plug is not adding anything.
It's very simple. I like it