"Dry Barrel" Spearguns, not Vacuum!

[popgun pete]

Some of the earliest spearguns used a dry barrel system, including one of the earliest pneumatic spearguns. They did this in order to maximise the power from the guns which were not as powerful as what we have available today, but back then they probably did not need a great deal of power as shooting ranges were shorter with inquisitive reef fish quite ready to approach the novelty of a diver in their midst.

The simplest form of dry barrel speargun is one where you load it and tip the water out of the barrel, after which you must swim with the gun always pointed downwards to retain the bubble of air inside the barrel. This was not a problem with guns being loaded on shore and then taken into the water, you just needed faith (usually misplaced) in the security of the trigger mechanism and its safety device, if the gun had one! Safety concerns soon got rid of this practice as spearguns discharging on shore had the potential to send shafts travelling great distances with severe consequences if anyone was struck by the spear.

As you dive down increasing ambient pressure will cause water to invade the "dry" barrel, same principle as an open mouth diving bell, but with a lot of shooting taking place not far below the surface then the barrel could be considered to be mainly free of water. No water inside the barrel allowed the most to be made of the gun's limited power supply, especially if it was a spring gun. A further refinement was to cork the spear in the barrel and stop water trickling in with the gun horizontal or the muzzle elevated, but if the muzzle seal leaked then water would penetrate as the air bubbled out, or if the gun was taken to greater depths as the air in the barrel was only the full barrel volume at one atmosphere, i.e. at the surface.

This was only a short step away from a completely sealed barrel, but to do so needed quality "O" rings or a rubber packing system that was reliable and could withstand repeated shaft insertions without developing leaks. An example of a rubber packing system creating a muzzle seal is the one used in the RPS-3 speargun, only there it holds water in rather than keeps water out, as that gun uses hydrostatic pressure to shoot the spear. The RPS-3 muzzle seals last between 10 and 30 shots as the spear progressively wears the rubber packing ring out (or busts small pieces out of it), so the manufacturer supplied 40 spare rubber muzzle packing rings with each gun.

The use of line slides, which usually require a stop diameter or enlargement on the spear tail, made the dry barrel gun idea less attractive as if you have a line slide then you need an inner seal against the shaft and an outer seal for the muzzle to line slide connection in order to stop leaks, which is what a "Mamba" type system has, it being the most well known version of that arrangement. Loading the gun in the water creates the vacuum in the barrel and saves on having to tip water out, an ideal solution provided all the seals at the muzzle do not leak.

 

[Don Paul]

Great post Peter, Thanks.
Cheers, Don

 

[foxfish]

Thanks Pete another detailed masterpiece for our community.

 

[popgun pete]

If anyone is wondering why a pneumatic speargun needed a dry barrel when all one has to do is pump more air into it in order to increase the power of the "spring" created by the air stored in the inner barrel and reservoir then you first need to appreciate the unique design of this 1943 pneumatic speargun. The gun had a relatively small reservoir and a long barrel, being constructed as a virtual monotube, although it had a join where the tube was cranked downwards at an angle to form the wrist section for the rear shoulder stock. The rear air reservoir, essentially in the wrist section, was around the same internal diameter as the barrel, which was large by today's standards. This gave the single seal (a leather cuff type seal) piston a big cross-section frontal area, which maximised the force for the available pressure in the gun. The gun needed it because the initial charge pressure in the reservoir was low, the reservoir being virtually the entire length of the gun when unloaded, however the compression ratio was very high, so that when the piston was rammed back by the spear tail the cocked air pressure in the gun was reasonably high. Hence the gun had a rapidly decreasing pressure gradient as the piston during the shot travelled towards the muzzle, but the maximum was made of this pressure by using the big diameter piston as pressure times area equals force. The downside of this big diameter piston travelling in a big inner diameter barrel is that a water filled barrel would have completely killed the gun's performance, it would have just been one great big water pump. The inventor decided to circumvent this water pumping problem by keeping the barrel dry, which meant using the "air bubble" system described earlier. However there was still the problem of pushing air out through the small muzzle opening, the muzzle being an end plate fixed in the end of the barrel with a centralising hole only big enough for the shaft to pass through. To allow more pathways for the barrel trapped air to exit the gun, but keep water out, a blow-off valve system was created in the muzzle which let air out, but which shut against water trying to get in when the gun was armed and ready to shoot. The speargun was loaded on shore, then taken into the water with muzzle pointed downwards. The sear tooth of the release mechanism was located externally at the muzzle, the gun being a forward latching type with an annular groove in the spear just behind the speartip. The trigger in this "underwater rifle" operated a long pull rod running to the muzzle.

This may seem an odd speargun layout to us today, but it was designed when the mechanical speargun world was ruled by the then mighty spring gun which included 2 metre long monsters. Unfortunately the spring gun, some of which took considerable strength to reload, was an inefficient weapon unless maintained in first-class condition and the band gun was limited by the poor quality of rubber used initially, however once World War II was over things began to change very rapidly.

The gun type most usually associated with "dry barrels" was the "air bubble", compression type spring gun, but it was killed off by the perfection of pneumatic spearguns which held compressed air for very long periods. Early pneumatic spearguns tended to have slow leaks, so the guns required pumping up more often, but they only had small capacity reservoirs which required a lower number of pump strokes, being high compression ratio weapons. Today we use relatively high pressure, low compression ratio, pneumatic underwater weapons.

 

[Old Man Dave]

Great thread Pete. I love this stuff. Design is big with me and I love to theorise and draw ideas.

While working on some recent ideas I worked on the idea of a forward latch trigger using a slot in the spear just behind the tip. This would connest via a pushrod to the rear trigger. This gets around the problem of having the trigger mechanism inside the pressure chamber. Might have guessed it had been done before although I had never seen or heard of it. Just goes to show that even when you invent something then there's nothing new.

All the pnemo guns I've used were wet barrel until I bought my Mamba and I'd never thought of just emptying out the water above the surface. So simple a concept although not quite as simple as it seems when you get into the seals etc.

Again, great thread. Thanks.

Dave.

 

[Old Man Dave]

Today we use relatively high pressure, low compression ratio, pneumatic underwater weapons.

Pete is there an ideal ratio between the size of resevoir and the barrel internal volume displaced between the loaded and unloaded state?

For a fixed piston travel (gun length) and a set internal barrel diameter there would be a fixed volume. The compression ratio would be therefore dependant on the resevoir volume (volume with gun loaded) compared to this fixed barrel volume. Yes?

A large resevoir volume would mean the gun was hard to load in that the initial force to compress the air using the spear would be nearly as much as the final force needed to fully load the gun. Conversely a small resevoir would start to load easy and then become extremely difficult.

Presumably a large resevoir provides more force to the spear during all of the piston travel, even when the spear has nearly left the gun the air pressure behind the piston is still high and therefore pushing hard.

Is the only drawback of a large resevoir the physical size. Large resevoir guns must be quite bouyant although maybe heavier gauge material is needed for a bigger container?

Your thoughts/answers would be appreciated.

Dave.

 

[popgun pete]

The cabinet of speargun ideas was fully ransacked by the end of the fifties, start of the sixties, no concept was left unexplored, only many never made it into production because they either did not work properly or were not worth the complexity and cost of construction. Every now and then an idea gets reinvented, or is incorporated with some other old idea, the historic speargun literature is full of them. What inventors forget is the gun needs to be maintained, the sea is a tough environment, and guns need to be brought to bear on the target unless the fish is the size of a billboard and obliges by swimming in front of your cannon! A heavy gun not only has to be lugged around by the diver, it has to be handled under not always ideal sea conditions for reloading and aiming, so the KISS principle has usually triumphed.

The exposed or external trigger mechanism was one of the first tried as the pneumatic gun only requires one dynamic seal, the one on the sliding piston. Another seal at the screw threaded join to get the piston into the barrel, welded end tubes for everything else and you have a pneumatic speargun. More conventional production spearguns using the external mechanism arrangement are the Nemrod "Torpedero", the Russian RPB-1M (sold as the "Prizm") and the Pirelli "Aries", although the latter has other complexities just to keep things interesting, like a built in air pump using the inner barrel!

The inner barrel diameter to reservoir diameter ratio has been pretty well optimised. If the inner barrel diameter is too big then it poses water pumping problems for wet barrel guns and more frictional contact area for the dynamic seals on the piston, so more drag effects as the piston slides along the inner barrel. If you go for a small inner barrel diameter then the spear diameter has to be small and the gun requires more internal pressure to provide an adequate level of propulsive force on the piston or spear tail. The reservoir diameter determines the air capacity of the air reservoir, as does the length of the reservoir. Higher air capacity reservoirs produce the most power as the pressure gradient inside the gun as the piston travels towards the muzzle is not so steep, that means more energy stored in the gun for driving the spear out. If the reservoir is too big then the speargun becomes unwieldy and heavy unless it has a slim forward barrel, an example of a gun with a big diameter rear reservoir is the Nemrod "Mariner" at 50 mm!

What these two diameters do is set the compression ratio of the gun, as well as its buoyancy and its manoeuvrability. The purpose of the gun determines what is the best set-up, but general practice has settled around 13 mm diameter inner barrels and 40 mm diameter reservoirs or tanks. Better material in spears has allowed them to get thinner without bending during muzzle loading, so that is why we have guns like the "Cyrano" with its 11 mm diameter inner barrel. If the spear gets too small in diameter then it will be easily bent during use, particularly if the fish wants to turn it into a metal sculpture during its struggles. Short spears are harder for fish to bend, so you can have very slim shafts to improve shaft speed, but momentum is down due to the low mass of the spear. Like everything whatever you choose is a compromise biased towards the purpose for which the gun is built, shooting small fish hiding under sunken logs in murky lakes or maybe bouncing shafts off monsters in deep water.

 

[popgun pete]

Hi Dave, here is a more direct set of answers, I did the previous post when off-line.

Pete is there an ideal ratio between the size of resevoir and the barrel internal volume displaced between the loaded and unloaded state? Well that is the compression ratio once you add the length of the air reservoir, it really depends on the gun's use. A short gun is easy to manipulate during loading of the shaft, so you can push against relatively high pressure right from the start. That gun design is best with a low compression ratio. On a long gun, generally much longer than what we have today, you want the spear to go in easier at first, so a lower pressure is better to push against initially, but to give the gun some grunt you want a high compression ratio design which raises the pressure at the point of latching the release mechanism. The ideal situation is that you have the loading effort such that your strength is just sufficient to latch the gun, that gives you the maximum power per shot.

For a fixed piston travel (gun length) and a set internal barrel diameter there would be a fixed volume. The compression ratio would be therefore dependant on the resevoir volume (volume with gun loaded) compared to this fixed barrel volume. Yes? Yes, that is right. If you partition the reservoir then you can effectively change the compression ratio. On a hydraulic trigger hydropneumatic gun the compression ratio is variable. These guns should be called hydraulic guns to distinguish them from the mechanical trigger versions.

A large resevoir volume would mean the gun was hard to load in that the initial force to compress the air using the spear would be nearly as much as the final force needed to fully load the gun. Conversely a small resevoir would start to load easy and then become extremely difficult. That is exactly right. The former arrangement yields the most power, if you set the pressure correctly in the gun then it gives the best results for your level of strength.

Presumably a large resevoir provides more force to the spear during all of the piston travel, even when the spear has nearly left the gun the air pressure behind the piston is still high and therefore pushing hard. That is correct, a flatter pressure gradient is another way of expressing it.

Is the only drawback of a large resevoir the physical size. Large resevoir guns must be quite bouyant although maybe heavier gauge material is needed for a bigger container? I think the physical size becomes a limitation rather than heavier gauge tanks, you want ballast in a powerful gun, but at the end of the day you have to carry the thing and swing it around, possibly in a current or while being slapped by big waves while returning to shore, then you wish for a small gun!

 

[popgun pete]

Here is an example of a dry barrel (tip the water out of the barrel after first muzzle loading it) pneumatic speargun that is held in the Frederic Dumas Museum in France. All these early pneumatic spearguns are “home-built” in small metalworking workshops and only after the end of World War II in 1945 does spearfishing in general take a huge leap forwards and with it the pneumatic speargun.

Note that this monotube pneumatic speargun shoots with a line pulling directly off the forwards facing drum with the air reservoir in the body section located directly behind the trigger. Although difficult to see in the photo the gun is of the forward latching type with a pull rod running rearwards to the trigger.

Probably one of the first pneumatic spearguns ever! http://museedumas.fr/o/g.php

 

[popgun pete]

I have informed the Frederic Dumas museum exactly what they have on their “remarks” page which is set up to precisely do that, even sent the information in French, but I have received zero replies. I would guess that no-one is checking the incoming messages and like a lot of websites they think it can run all by itself!