The Italian Alcedo "Hydra" was the first hydraulic valve operation hydropneumatic speargun. It probably inspired a number of other hydropneumatic guns, but most of those used a sear tooth to directly hold onto the spear shaft rather than the patented hydraulic valve system in the "Hydra" which utilizes a "hydraulic locking chamber" to control water flow inside the gun, so these other hydropneumatic guns are not much different in operation to a standard pneumatic gun. You push the spear into the gun and the mechanism clicks as the sear tooth engages the spear shaft tail, there being no sliding piston inside the inner barrel of a hydropneumatic speargun. Some simpler construction guns engaged the spear shaft at the muzzle end, being forward latching spearguns, the shaft carrying a notch at the front end just behind the speartip, which weakens the spear.
Some years ago I pulled a long non-operational "Hydra" speargun apart to see what was inside and to discover how it worked, you can read the results here along with the many detail photos of the gun's innards. http://aquatech1.narod.ru/eng17.html
The accompanying article is a compilation of an original 1964 review of the "Hydra" gun and e-mail conversations with Sergiy Kravchenko, but some of it has been edited out, so the question has to be implied from the answer in some cases where the question is missing. The reason for the article being placed on the Aquatech web-site is that the "Hydra" speargun's operating principles were the direct inspiration for the subsequent "Black Sea" speargun developmental work (the "Black Sea" name came much later), although the layout is very different in the latter to produce a much more compact weapon for underwater hunting. The "Hydra" speargun's rubber bladder (located inside the blue anodized bulbous rear alloy tank) is an odd arrangement by today's standards, but it's use fitted in with the general appearance of the big carbon dioxide guns of the time (early to late sixties). It is interesting to note that if you reconfigure the heavy vented metal grille that separates the air filled and hence pressurized rubber bladder from the forward flooded sections of the gun by turning the grille into a pierced wall metal tube and wrapping the bladder around this tube in the form of a rubber hose you obtain the system used in the "RPS-3" hydropneumatic speargun. This space saving arrangement means that the pressurized air reservoir can now sit around the inner barrel instead of sitting directly behind it, but if you have the air reservoir forward of the gun's grip handle then you cannot use a hydraulic locking chamber design because water flows directly onto the spear shaft through the many holes in what is now the inner barrel of the gun. So the gun needs a mechanical sear tooth mechanism and a seal at the muzzle around the spear shaft to keep pressurized water inside the inner barrel, a very unlikely arrangement, or the rubber hose covered vented metal tube has to be restricted to a rear tank behind the gun's trigger mechanism (such hydropneumatic guns have been made, but they will not be discussed here).
Later hydropneumatic guns, such as the "Aquatech" and "Zelinka" models, replaced the rubber bladder/rubber hose flexible partition on the air reservoir with an annular piston sliding on the exterior of the inner barrel tube instead, but the "Aquatech" guns retained the "hydraulic locking chamber", although it was a much more compact arrangement than the large skyrocket shaped valve used in the "Hydra". Note that the bore of the hydraulic locking chamber must always be greater than the bore of the inner barrel or the system will not work, the lowest pressure has to be to the rear for the trigger or releasing valve to open rearwards and unplug the inner barrel once the rear section of the chamber is depressurized by opening the gun's trigger control valve.
As for the "Hydra" push-pull rear mounted hydropump, this is how it works. The hydropump's inner pump rod carries a cup type rubber seal, not unlike the one on the spear tail, that pushes water on the pumping stroke and temporarily collapses in on itself on the return stroke, thus allowing the pumping barrel to suck in water past the rubber cup seal for the next pumping stroke when the cup seal will flare outwards to again seal tightly on the pump's inner bore. The spear can be pumped in the front barrel in exactly the same way, but there the bore of the "pump" is larger, so more effort is required. A non-return valve, on this particular gun a thick rubber flap, seals the hydropump's small outlet hole inside the gun's flooded centre section thus preventing the building water pressure inside the gun escaping by pushing the pump handle backwards. Just as well, because the hydropump pump handle would hit you in the face otherwise!
There is a later version of the "Hydra" with some plastic parts and a more elongated appearance with respect to the rear tank which seems to be an even bigger gun than the original, but I have never located one in order to dismantle it. Collectors usually don't want to risk damage to their guns, but of all the speargun types you really need to open up hydropneumatic guns as saltwater can do nasty things inside the gun if you don't clean them out.
The "Hydra" shown here looks like it was washed out, but the original owner never flushed out the hydraulic locking chamber or the hydropump, the two most critical areas inside the gun and so it finally seized up. Aluminium oxide and fine sand particles spilled out of the hydraulic locking chamber once it was freed, but the hydropump is well and truly seized, so the gun is out of action. I suspect that the thick rubber bladder would split if recharged with pressurized air today, rubber product's lose their strength over time and any spare parts like a rubber bladder would be non-existent after all these years.
Some years ago I pulled a long non-operational "Hydra" speargun apart to see what was inside and to discover how it worked, you can read the results here along with the many detail photos of the gun's innards. http://aquatech1.narod.ru/eng17.html
The accompanying article is a compilation of an original 1964 review of the "Hydra" gun and e-mail conversations with Sergiy Kravchenko, but some of it has been edited out, so the question has to be implied from the answer in some cases where the question is missing. The reason for the article being placed on the Aquatech web-site is that the "Hydra" speargun's operating principles were the direct inspiration for the subsequent "Black Sea" speargun developmental work (the "Black Sea" name came much later), although the layout is very different in the latter to produce a much more compact weapon for underwater hunting. The "Hydra" speargun's rubber bladder (located inside the blue anodized bulbous rear alloy tank) is an odd arrangement by today's standards, but it's use fitted in with the general appearance of the big carbon dioxide guns of the time (early to late sixties). It is interesting to note that if you reconfigure the heavy vented metal grille that separates the air filled and hence pressurized rubber bladder from the forward flooded sections of the gun by turning the grille into a pierced wall metal tube and wrapping the bladder around this tube in the form of a rubber hose you obtain the system used in the "RPS-3" hydropneumatic speargun. This space saving arrangement means that the pressurized air reservoir can now sit around the inner barrel instead of sitting directly behind it, but if you have the air reservoir forward of the gun's grip handle then you cannot use a hydraulic locking chamber design because water flows directly onto the spear shaft through the many holes in what is now the inner barrel of the gun. So the gun needs a mechanical sear tooth mechanism and a seal at the muzzle around the spear shaft to keep pressurized water inside the inner barrel, a very unlikely arrangement, or the rubber hose covered vented metal tube has to be restricted to a rear tank behind the gun's trigger mechanism (such hydropneumatic guns have been made, but they will not be discussed here).
Later hydropneumatic guns, such as the "Aquatech" and "Zelinka" models, replaced the rubber bladder/rubber hose flexible partition on the air reservoir with an annular piston sliding on the exterior of the inner barrel tube instead, but the "Aquatech" guns retained the "hydraulic locking chamber", although it was a much more compact arrangement than the large skyrocket shaped valve used in the "Hydra". Note that the bore of the hydraulic locking chamber must always be greater than the bore of the inner barrel or the system will not work, the lowest pressure has to be to the rear for the trigger or releasing valve to open rearwards and unplug the inner barrel once the rear section of the chamber is depressurized by opening the gun's trigger control valve.
As for the "Hydra" push-pull rear mounted hydropump, this is how it works. The hydropump's inner pump rod carries a cup type rubber seal, not unlike the one on the spear tail, that pushes water on the pumping stroke and temporarily collapses in on itself on the return stroke, thus allowing the pumping barrel to suck in water past the rubber cup seal for the next pumping stroke when the cup seal will flare outwards to again seal tightly on the pump's inner bore. The spear can be pumped in the front barrel in exactly the same way, but there the bore of the "pump" is larger, so more effort is required. A non-return valve, on this particular gun a thick rubber flap, seals the hydropump's small outlet hole inside the gun's flooded centre section thus preventing the building water pressure inside the gun escaping by pushing the pump handle backwards. Just as well, because the hydropump pump handle would hit you in the face otherwise!
There is a later version of the "Hydra" with some plastic parts and a more elongated appearance with respect to the rear tank which seems to be an even bigger gun than the original, but I have never located one in order to dismantle it. Collectors usually don't want to risk damage to their guns, but of all the speargun types you really need to open up hydropneumatic guns as saltwater can do nasty things inside the gun if you don't clean them out.
The "Hydra" shown here looks like it was washed out, but the original owner never flushed out the hydraulic locking chamber or the hydropump, the two most critical areas inside the gun and so it finally seized up. Aluminium oxide and fine sand particles spilled out of the hydraulic locking chamber once it was freed, but the hydropump is well and truly seized, so the gun is out of action. I suspect that the thick rubber bladder would split if recharged with pressurized air today, rubber product's lose their strength over time and any spare parts like a rubber bladder would be non-existent after all these years.
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