Posts by popgun pete

The four axle version (a standard pneumatic gun is shown at the bottom as a comparison).

Here is a drawing of a two axle version showing the inner works.

The "Dreamair" speargun uses long wishbone cables to rotate external drums or pulleys on each outer end of the axles located in the muzzle. There have been proposed versions of the "Dreamair" using 1, 2, 3 and 4 axles, each of which has its own set of wishbone cables. These cables spiral wind on the outer drums so that when the gun is fired each side of the cable has its full length rapidly coiled around its respective drum leaving only the vee-shaped ends which you pull on to load the gun sitting above the spear guide track. The muzzle interior and the barrel is essentially a long pressure tank with a sliding piston sitting at the rear end of the barrel tube. This large cross-section piston is dragged forwards in the barrel tube when the cable wishbones are pulled back to cock the gun as the rotating axles in the muzzle co-axially mount an inner drum which winds in an inner cable connected to the front end of the moving piston just as the external cables are unwinding on each side from their outer drums. Dragging the piston forwards in the gun reduces the volume and increases the pressure in the forward section of the barrel while the barrel space located behind the departing piston develops a vacuum. The idea is that when the gun shoots the higher pressure in the front section of the gun can push the piston back with nothing behind the piston to impede its return to the rear position in the barrel. If the rear section of barrel was allowed to flood then water being pushed out of the gun by the returning piston would absorb power from the shot, hence the pressure tank is sealed off from the environment at both ends, front and rear, creating a completely closed system.

The axles and inner drums are located in the high pressure area, being inside the tank, which means that the axles have to protrude through dynamic pressure seals in order to have the outer drums mounted on their exposed ends. Given that the axles rotate at high speeds during the shot the sealing here is critical or the gun will lose high pressure air from around the spinning axles. If air leaks past the sliding piston then the vacuum developed at the rear will be lost, yet because the tank is completely sealed there would be no external indication of this happing until you pulled the trigger. For the piston to fully return to the rear of the barrel after the shot any pressurized air inside the gun has to remain in front of the piston, otherwise the piston will only move back until it reaches a position where the pressure on the front and rear piston faces equalizes.

Another critical aspect is that the cables accurately wind on and off their drums keeping to the spiral cable tracks formed in the inner and outer drums; during loading that may not be a problem, but they also have to do it at high speed during the shot. If the cables don't stay in the tracks then there will be problems like uneven cable pull from side to side and possibly tangles and jamming of the cables. The "Dreamair" has many moving parts when you consider the sets of inner and outer cables, the spiral track inner and outer winding drums and their axles and a much larger sliding piston than is usual for a pneumatic speargun (more friction on the larger periphery seals). The stored energy from loading effort has to move all these extra parts as well as propel the spear which will adversely affect the efficiency of the gun. Then all the internal and external cable connections need to have reliable anchor points on their respective drums so that forces transmit reliably from the cables through the drums and axles with the cable lengths all being properly adjusted, especially on the multi-axle versions.

A working gun may be possible, but its efficiency and long term reliability is open to question when so many parts need to move and function exactly as intended, particularly the cables moving laterally as they track across the grooves in the winding drums because the groove flanges are all that is directing them and for that to happen the cables need to remain taut on the drums.

A single wishbone cable/axle version would be just as powerful as the multi-axle versions which only serve to break up the loading effort. This is because pulling the first wishbone on a dual axle gun advances the piston for half of its travel, then pulling the second wishbone advances the piston for the other half of its travel to the fully cocked position. On a four axle gun each wishbone draw only advances the piston by a quarter of its travel. On a three axle gun this would involve one wishbone draw performing a half piston travel and the other two wishbones performing a quarter piston travel each. A system of sub-pulleys on the inner cables arranged in a block and tackle fashion creates this incremental loading of the piston, but this adds to the task of setting up all the cables inside the gun and securing all the cable ends with their pulley connections.

The "Sea Archer" used a cam type system to duplicate the "let off" of a compound bow at full draw, it was a sort of Hawaiian sling with no trigger mechanism. It has been discussed on the Web, but the article may no longer be up as it has been a long time since I looked at it on the "rocknfish" web-site.

Here is something to think about. If we take a single rubber band and wishbone assembly, pull it back on a gun to the rear tab or wishbone notch position and then let it go without engaging it on the shaft then the band zips forwards on its own at the maximum velocity it can achieve. Once we add a shaft for the band to pull along behind it then the band is slowed down and the heavier the shaft is then the slower the band can contract. Logic would suggest that if we add more bands of the same type to the spear then the bands individually have less mass to pull along as their share of the load and the band contraction velocity of the band group will increase, but at some point if sufficient bands have been added then the velocity limit will be when the bands once more reach their zero load contraction velocity. The contraction velocity will not really be at "zero load" as there will be hydrodynamic drag on the bands and the wishbones, but it will be what they experience when used in a speargun underwater. However the question would be whether bands can ever reach that limit on a practical speargun before the axial drive on the shaft started to buckle the spear.

If there is an advantage to some rollerguns then it must be related to the hydrodynamic drag effects as there are less wishbones and band surfaces, particularly the cut-off ends of the bands being pulled through the water for the same amount of energy stored in the bands, but at what magnitude of wishbone velocity do these effects become significant? One interesting feature of the sub-pulley cable rollergun drive system is the wishbone engaging the shaft travels at twice the velocity of the cut-off band ends, but the bands are slowed by having to pull the spear via the folded cable drive, so the system is not a simple band contraction speed multiplier. However drag effects related to velocity would be different for the wishbone than they would be for the band ends travelling through the water. Those band ends mount a sub-pulley, so that will have some drag effect as well.

I only mention this aspect as on the stored energy analysis the rollergun seems to store less energy and has more parts to move than the two band conventional gun. Hydrodynamic drag during the shot is not included in that analysis, but whether it makes a difference I don't know.

Quote from seal

Yes first thing is righ,but its also about the Efficiency.The gradual and longer transfer of energy is more efficient not only becouse of the lack of shaft bending but mainly becouse of lack of recoil.
This is so simple that I hope more people will understand now: Everybody admit that rollers has no recoil-well if you can measure this lack of lost energy,then add it to the energy of spear,becouse it goes there.
Another benefit is that lack of recoil improves accuracy and aiming.
And believe me the recoil of spearguns is signifficant value becouse guns are signifficantly light compared to the heavy spear-bullet. Especialy most pipeguns,their spears are only double lighter than the gun itself.Even the wood guns where this relation could be 3 to 5 : 1 ,its still not enough.
So not only the gun shoots the spear,but the spear shoots the gun back with quite big value of energy.
In compareson the relation of a riffle mass to its bullet is arrownd 200:1

Well efficiency is going to depend on how you define it as in a two-stage loading rollergun the preload never gets used for shooting, so the energy stored there is thrown away when the gun is reloaded each time for the next shot. Thus there is a difference between energy used for shooting and the energy stored in a gun. You could say a more efficient speargun uses all its energy for the shot, which is what you have with a conventional band gun (not counting the energy lost as the band tension drops as the bands slightly relax over time when cocked). Your own cable rollergun has the preload built-in when you assemble it as you use single-stage loading and therefore you use all the energy provided by cocking the gun for the next shot, hence it is a different type of weapon compared to the usual rollergun. Add to that the sub-pulley drive which allows only the upper end of the band energy to be delivered over the full length of the top deck, however that extra hardware of moving cables and pulleys has to take energy to move them and you lose efficiency there, not gain it.

What we should be talking about is effectiveness rather than efficiency as mechanically rollerguns have to be less efficient than a conventional band gun, but the rollergun has a better power distribution. Power is defined as the rate of doing work or using energy, in the conventional band gun all that energy is imparted to the spear right at the beginning of travel, whereas in a rollergun it is delivered all the way along the top deck until the spear reaches the muzzle, like in a pneumatic speargun. Hence if the gun is not ballasted sufficiently then the nature of the recoil can make a conventional band gun less manageable to hold steadily for aiming compared with the rollergun which provides less of a jerk when it shoots as the power is applied more gradually to the spear and consequently the gun. High grip, rear handle euroguns are usually not ballasted, but the bands are matched to the spear to make them more manageable for shooting with a straight arm position. If you are pushing the margins of energy storage and power delivery then the rollergun can handle it better, but at the necessity of a broader weapon which may or may not be a disadvantage depending on what you are using it on in terms of the maneuvering required to bring the gun onto the target or the likely track of the target as it swims towards the anticipated flight direction of the spear from the gun.

Energy released for the shot is divided between user and gun, as a combination, and the projectile with respect to their respective masses, regardless of whether we are talking firearms or spearguns and bullets or spears. The gun and user are not rigidly coupled together, so the user has to resist the gun moving with respect to himself and the rollergun's recoil is easier to manage as it loads up the diver's arm (or arms) less rapidly than with a conventional band gun. When a speargun is fired the diver and gun are both pushed back in the water and the recoil we perceive is how well we and the gun are coupled as a single unit when that movement occurs. We don't see ourselves being pushed back in the water, but that is what happens. Incidentally if a gun is braced against the wall of pool in a test shot then the results are distorted as now the "user" is the pool and the weight of the Earth if it is an in-ground concrete pool! Rollerguns certainly have some recoil as I was watching some sea experiments on video with a mid-handle rollergun, the only way the gun would shoot accurately was with an arm brace to better couple the user and gun together as a unit.

Interestingly when that same rollergun was fitted with thicker bands in the pursuit of increasing the range for a kill shot the spear had less performance when striking a foam target than it had before with weaker bands and my guess is the bands were now so strong that they could eject the spear from the gun with just the band stretch in the top deck, i.e. before the roller system got working and the bands had released all their stored energy for shooting. When a heavier spear (10 mm!) was substituted in the gun the performance improved as evidenced by greater spear penetration at the foam target, so like all spearguns the bands and the spear had to be matched with a combination that did not over power nor under power both the gun and the spear for stable flight of the latter to the target.

Running the comparison calculations using a diagrammatic approach.

Just looking back on something seal said about rollerguns way back in post #145.

"The idea of the pulleys is to break the speed limit of normal bands. Every shaft has a limit that can be accelerated to with normal bands. Even if you store five time more energy in multi bands configuration, you will give to the shaft only a small part of it, the rest will go for recoil and drag."

On reflection I think what seal is saying is that the rollergun's more gradual acceleration allows shafts of reduced diameter to be shot that would not be possible with a conventional gun as the latter gun's shorter and more brutal acceleration would load up the thinner shaft too quickly causing the shaft to bend and thus adversely affect the shot. Both gun types can store and deliver the same energy, but the rollergun can deliver it with less likelihood of overpowering the spear, although the extra margin gained will probably have a limit. If you shoot spears of a diameter that are able to take the acceleration then a rollergun and a conventional gun should deliver the same results, all other things being equal, as it is not about energy storage or gun efficiency but what the spear can absorb and how fast and how far you need to send it to disable what lies at the other end.

At the risk of stating the obvious this is all a rollergun is, a shortened version of the same gun with the front end lopped off and the slack band wound under the barrel. The conventional gun has one band, the diagram just shows it before and after cocking. A rollergun of the same length as the conventional gun is using longer bands than would fit on the conventional gun when cocked and any comparisons need to take this aspect into account.

In these comparisons you really need to look closely at the "energy storage battery" for any explanation of differences in how the guns perform. That involves knowing the band diameter or thickness, the slack band length, the stretched band length just prior to the shot and if there is any stretch left in the bands after the shot. I know that one of seal's cable rollerguns has a band release gadget to slacken off the bands when his gun is not being used (to save the bands from stress induced ozone cracking), thus his cable rollergun operates further up on the energy triangle for the bands. The attached schematic shows how it works as the side pulley system never uses the lower part of the energy triangle, but spreads the energy of the upper part over the full distance of the shot from the top deck, so the force is halved, but the stroke is doubled. You can see that a thicker band (B) battery can be used in this gun which only ever band wraps a large diameter pulley at the rear, however you need to load the gun in one pull as far as I can tell. Two stage loading starts cocking a rollergun from the slack band condition in many rollerguns, but here the bands are already at stage 2. I don't know how much preload seal actually uses, but here I am showing it at an equivalent draw to that used to cock the gun and you can see that the band battery can then store a lot of energy for shooting provided you don't over stretch the bands to the point where they begin to yield.

It is tempting to think compound bows have higher performance due to the pulleys or rollers which are usually cams, although on a crossbow they will be pulleys as on a crossbow there is no need to reduce the pull of the bow at full draw so you can hold the arrow while waiting to shoot as the trigger mechanism does that job. The cams create this effect by changing leverage as they rotate which changes the pull experienced on the bowstring of the bow to provide a let off where the force required to hold the arrow reduces significantly from that required to draw the arrow back. Unlike a recurve bow the pull transfers from the opposite bow limb which can be much shorter and stiffer as the pulleys have a Buss Cable that connects to the opposite side of the bow. Besides the advantage of higher energy storage available in deflecting stiff bow limbs the limbs can be disposed in a more horizontal configuration so that bow recoil is reduced by the limb movement cancelling out top and bottom. Recoil in a bow is forwards, not back as the vertical limbs flip forwards in a conventional bow.

Where there is a similarity with rollerguns is that the "energy storage battery" can be separated from the transmission element whereas in a conventional speargun they are one and the same thing. You have a lot more freedom to vary the "band battery" and that is what seal does with his guns using cables and rollers.

To compare "apples with apples" I used the same travel length for the barrels and the same band stretch in terms of the force to pull the band onto the wishbone notches or shaft tabs using the same type of rubber. The double band conventional gun receives twice the initial force (F max x 2), but the bands are shorter and only push for part of the barrel length while the rollergun pushes all the way with one band using only F max at the start. However the rollergun equipped with two bands (twin axles or doubled pulleys) does store more energy than the two band conventional gun as if you do the same manipulation on the energy diagram you get a longer rectangle extended by the length of the extra powered travel distance. Of course the energy diagram is not a rectangle as it should be two triangles stacked on top of each other giving a much taller triangle, but what we are interested in is the area for comparison purposes and it is easier to see as a rectangle. Basically a rollergun uses longer bands on a gun than it would normally be able to accommodate and you can do more with them in terms of the stretch and their routing on the gun because there is more linear space available to locate them. The term "rollergun" embraces a number of configurations that are different guns in terms of the energy that they can store, hence comparisons can be complicated when trying to work out what their equivalent is as a conventional gun. Some time ago I did a bunch of rollergun diagrams here to explore this variation as there certainly is no magic ingredient involved and rollerguns usually shoot a shorter shaft for the bands that they use as the rollergun is basically a longer gun folded up on itself.

A conventional band gun is more of an impulse weapon than a rollergun is as the shaft acceleration is short compared to the more progressive push the spear receives from a rollergun (as the energy triangle is stretched out and the "toe" of the triangle is often not used). The rollergun will be a wider gun if it uses enclosed bands as the pulleys push out the width of the stock and that will change other characteristics of the gun that help determine its shooting performance in terms of muzzle lift. Although rollerguns seem new they were around decades ago, but modern materials have made more complicated guns much lighter and stronger than they were in the distant past, particularly with the cable rollergun where bands don't have to travel around rollers at the muzzle as in a sense elongated wishbones do that job and they are no longer steel cables.

Quote from Dan

My premise is that personal anecdotes are just that, but physics don't lie. If you take a roller speargun powered as described above and turn it on its side, you'll see the same exact configuration as the conventional speargun, with one band stacked on top of the other. More so the bottom anchoring point doesn't always reach quite as far back as the last tab on the shaft, so band stretch is actually less. The pulley being the only differentiating factor, are divers supposed to believe that it somehow magically amplifies the stored energy of the bands?!

For the comparison that you are citing here the rollergun does not store more energy as shown in the diagrams below. So you are right, there is no magic in adding a pulley (single axle) to the gun. While the stored energy may always be more than a single band conventional gun it will always be less than a double band conventional gun for the same band stretch being used and thus the same F max on all guns.

The SMG spearguns were made by Tapmatic Corporation in the late sixties. They used Ramset .22 type cartridges that were inserted into a plastic sleeve that was sized to fit into the gun's pivoting breech. There were single barrel models, twin barrel models and quad barrel models; however the SMG gun only fires one shaft at a time as you revolve the breech to bring each barrel into line with the gun's firing pin. There was also a "Magnum" version of the twin barrel gun with longer barrels and a buoyancy element fitted between the barrels. Of course none of the guns float. Tapmatic sold the SMG parts and tooling to a New Zealand company once they finished producing them. A few guns have shown up from the New Zealand company over the years, but I don't know how many they made other than just assembling SMG guns from their stock of parts.

It is more than a rubber washer. The actual dynamic sealing lip is internally moulded at the entrance to what is basically a short rubber tube which in turn is terminated in the broad rubber flange at the rear end. This rubber vacuum cuff seals on the thin inner metal rim of the muzzle, hence it is basically a cap in terms of how it seals to the muzzle by being a press fit on that inner rim. The front metal part of the muzzle holds the cuff in place during the shot, otherwise it would pull off. I have a Russian "Taimen" that is very similar, but there the clamping of the rear flange of the vacuum cuff creates the seal in the muzzle body as there is no inner rim in the "Taimen" muzzle. While the sealing lip holds up the inner barrel develops a vacuum when you cock the gun. It is not truly dry as a small amount of water is trapped inside when you first insert the spear tail and the sealing edge takes up. That small amount of water is necessary to lubricate the inner barrel as otherwise friction would burn the piston seals during the shot.

The system works as the rubber tube part of the cuff allows the sealing lip to stretch circumferentially, whereas an "O" ring doing this job has no ability to stretch. Vacuum barrel guns that use "O" rings for the sealing job in the muzzle use bare tail spears as they have no stop and hence no radial step on the spear tail. An exception is the vacuum barrel systems that place the "O" ring on the spear instead and not in the muzzle, they can use spears with tail stops. A well known system is the "Tomba" that is an aftermarket muzzle/adaptor, there being a number of versions of it over the years.

The Salvimar "Vuoto Air" has a rubber seal or vacuum cuff shaped like a hat installed in the muzzle, the "brim" of this hat being clamped between the front and rear sections of the muzzle. The top of this hat carries an opening with a sealing lip that runs on the spear's cylindrical surface. Main problem is the sealing lip has to avoid the shaft stop diameter at the tail end catching on it as the tail exits the muzzle. As there is some water inside the muzzle before you insert the spear, the idea is that this water helps push the sealing lip open just as the step at the shaft diameter change hits the lip.

To differentiate between the types the terms "rollergun" and "cable rollergun" should be used, or "cable gun" for the latter. Cable guns have been used in the distant past, but they were unreliable and probably not well enough made.

Quote from LunkerBuster

Very cool info Pete. Thanks for sharing

My pleasure to do it, but thanks for the appreciation! Unfortunately these guns must be rare as hen's teeth as even after 2,000 plus viewings of this thread not a peep out of anyone about locating an example of this quirky gun.

I chanced on another diagram for the "Sagittario" gun which shows the notched spears.

The original handles are from Sporasub and Cavalero Champion; the "Stealth" and the "Apex" models respectively. Hence they were borrowed designs from the start, but equipped with stronger innards such as stainless steel levers. The "ARC2000" from Cavalero Champion was fitted with brass trigger mechanism levers when it came out in 1986, then Rabitech modified it for their product line.