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All About Guns What's your weapon of choice, and why? Discuss the beloved speargun here!

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Old 04-02-2017, 10:23 PM   #1
Josec84
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demultiplied speargun

I stumbled across this "demultiplied" gun on youtube.

https://www.youtube.com/watch?v=XEwV0oynets

I am not quite sure what to make of it, but it does seem like it would be simpler than a standard roller.

Has anyone tried something like this before?
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Old 04-02-2017, 11:46 PM   #2
Diving Gecko
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Re: demultiplied speargun

It's not the first time I have heard demultiplied guns being said to have "progressive power" as Lamberto does in this presentation video. I think Pete and I talked about it before I and really hope he will chip in. The thing is, the bands are stretched to their fullest - like on a traditional gun - so I am still not sure the pulleys change the delivery as in making it more progressive? I would think the pulleys just divide the force, they can't make it more or less progressive. I could be mistaken but would love to know.
I think as, has been stated on the recent roller gun thread, that the advantages of this setup is the "cancellation of recoil" and very possibly less muzzle flip and lifting of the spear tail as there are no "stiff" rubbers being turned around the front roller.
I did bring one big carbon gun like this from the seller in the video to a friend in Thailand and he has been very happy with the system and the feel of the shots.

Last edited by Diving Gecko; 04-04-2017 at 06:14 AM.
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Old 04-03-2017, 12:28 AM   #3
popgun pete
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Re: demultiplied speargun

You can read about them here: http://www.spearboard.com/showthread.php?t=191747
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Old 04-03-2017, 09:44 PM   #4
Josec84
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Re: demultiplied speargun

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Thanks for the info Pete. You must have tons of hours put into guns, it's amazing how knowledgeable you are on the topic.
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Old 04-03-2017, 11:54 PM   #5
phil herranen
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Re: demultiplied speargun

The real advantage of a pullys gun is the wishbone speed can be double the band retraction speed , the current limiting factor for guns is the retraction speed of bands , so if you add a pullys , and enuff bands ( double what you would normaly need since the pullys subtract 50% of the band power) you can potentially have faster shaft speeds
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Old 04-04-2017, 01:59 AM   #6
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Re: demultiplied speargun

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Originally Posted by phil herranen View Post
The real advantage of a pullys gun is the wishbone speed can be double the band retraction speed , the current limiting factor for guns is the retraction speed of bands , so if you add a pullys , and enuff bands ( double what you would normaly need since the pullys subtract 50% of the band power) you can potentially have faster shaft speeds
That is the theory, and it is sound ... however other factors are involved in making a shaft shoot fast with a flat trajectory. I still don't quite understand why these type of guns also shoot low. By low I don't mean that just the aiming is different ... I mean the shaft is diving without having a flat trajectory. By having longer wishbones with a knot stopper, you can avoid having the wishbone push the tail up (or at least limit that), so that seems like a solution to prevent the shaft from diving. I do like the idea of multiplying the contraction speed by using pulleys and physics wise this is something that has been taken advantage of for centuries ... so far with banded guns, contraction speed is the limiting factor of shaft speed ... we may make things more efficient by taking steps to stabilize shaft flight, but the limiting factor with a classic setup will always be maximum speed of band contraction. It is just that I have yet to see a gun work really well with this system ... or at least as good as the top setup by a 3 banded classic gun. The Alemmani Vela 135 would seem like the best setup to test and two people I know that use that gun tell me it is the real deal ... but I need to test it myself. I will say however, that even if the Vela 135 did outperform the top classic gun, I don't think I would switch ... it really is a very complicated and tedious and time consuming process to load that gun ... not so sure I would want that in a hunting situation. I mean it takes almost no time to load a 3 banded 14.5mm gun in comparison.

I do think there would be a great breakthrough in invert roller multiplied setups if someone could come up with a linear actuator with a lithium battery pack in the handle. You load two wishbones on top ... then press a button in the handle and the linear actuator would load the bands to their full loading on the bottom. Idea being that a linear actuator would have no problems dealing with double the effort needed to pull the bands with the pulleys. This way the gun would be loaded single action on top and have full power of double action. I actually did look around for such an actuator and I do think it is something that can be done.
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Old 04-04-2017, 02:50 AM   #7
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Re: demultiplied speargun

I am sure it can be done with battery power. But I think a lot of people would find it lacking in sportsmanship. And if you are going that route - perhaps a CO2/compressed gas gun is as good an option and simpler, too.


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Old 04-04-2017, 06:16 AM   #8
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Re: demultiplied speargun

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Originally Posted by phil herranen View Post
The real advantage of a pullys gun is the wishbone speed can be double the band retraction speed , the current limiting factor for guns is the retraction speed of bands , so if you add a pullys , and enuff bands ( double what you would normaly need since the pullys subtract 50% of the band power) you can potentially have faster shaft speeds
Ah, that too, definitely sounds plausible.
Roisub actually had a Xmas sale on muzzles, but I forgot to buy one:-( The anchor point for the rubbers, bands and dyneema can be made/rigged pretty easily so it would have been a cheap enough option to try out. Well, maybe next time.

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Old 04-17-2017, 02:04 PM   #9
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Re: demultiplied speargun

We make that gun.. its awesome.. best setup for accuracy and distance.. it shoots like a canon
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Old 04-17-2017, 05:48 PM   #10
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Re: demultiplied speargun

Check out their catalog: http://www.roisub.net/uploads/3/8/9/...b_2016__2_.pdf
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Old 04-17-2017, 06:41 PM   #11
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Re: demultiplied speargun

Electricity and saltwater are not a good mix unless the electrically powered system is completely sealed off, something difficult to do in a device to be used in a speargun, unless we are talking about underwater lights, which is not the case here.

Also the energy demands to cock a rubber power band may not be able to be met by any compact battery system as the long stroke creates a problem for any geared electric drive train that could haul the band(s) back. It is not like doing up a nut or bolt with an impact driver hand tool which operates over a very short travel distance when under maximum force or torque loading.

The standard speargun can withstand a certain degree of abuse and careless handling, it is quickly fixed by inserting new bands in a very simple muzzle anchoring system, able to be carried out even in the water. The more pulleys, bands and cables in a rollergun then the more likely it will need attention on a worktable rather than in the water. Rollerguns have a place, but also incur an extra "price" which is more than the dollars spent in buying the gun.

You make your choice of gun for the task required of it and your own capabilities and where you will be diving, so it is not surprising that there are so many different guns on the market these days. The ability to pre-stretch the band, and use longer bands on a given length of gun, is what gives the rollergun an advantage as there is more energy stored to shoot the spear. Demultiplication systems allow those longer bands to be halved in length, but they then need to be doubled up in number to restore the energy able to be stored as with the power stroke of the band battery halved you need to double the force that it can apply. Basically F x d is replaced by 2F x d/2 which gives the same result, neglecting any extra losses in the system which are overcome during loading by the operator, but need to be supplied by the bands' stored energy supply when the gun shoots.

Another gun that has a shorter "power" battery travel than the length of the spear drive, and is soon to be released, is the pneumatic powered "Dreamair". However the "Dreamair" is not a demultiplied gun using "block and tackle" cable and sub-pulley arrangements, instead it uses a CVT winching system to transfer the pneumatic powered piston drive into a long cable drive to the wishbone that engages the spear tail. By manipulating the inner and outer drum diameters on the high speed winch the inner and outer cables can have different lengths and velocities. We know it works, but there is no service experience beyond that gained during the prototype testing.

Last edited by popgun pete; 04-17-2017 at 08:37 PM. Reason: further comment
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Old 04-17-2017, 09:09 PM   #12
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Re: demultiplied speargun

A few points: I agree with Pete that battery capacity could be an issue. But could be over come by putting a bigger batt in the stock and not the handle. Making it waterproof is not that much of an issue, really. Will just be more costly. But again, cost of R&D may not be recouped in a market where such a gun would be seen as "cheating" or as a last resort for spearos with disabilities.

As for the "progressive" statement on the pulley guns, perhaps the only gun on the market that will actually offer that is indeed the DreamAir?




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Old 04-17-2017, 09:30 PM   #13
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Re: demultiplied speargun

It is interesting to look at a Roisub diagram for loading with a fourth diagram added showing after the shot. Although the stretch lengths are not meant to be a scaled representation here, you can see the stretch left in the bands after the shot. Another form of 2 stage loading. To make life easier for the next loading you would need to de-cock the red band which is still under stretch after the shot. That is where the energy comes from, using the upper end of the band stretch characteristics. The black band is probably always under some stretch to keep the side sub-pulleys from rattling around on the sides of the gun.
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Old 04-17-2017, 09:35 PM   #14
Josec84
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Re: demultiplied speargun

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Originally Posted by popgun Pete View Post
It is interesting to look at a Roisub diagram for loading with a fourth diagram added showing after the shot. Although the stretch lengths are not meant to be a scaled representation here, you can see the stretch left in the bands after the shot. Another form of 2 stage loading. To make life easier for the next loading you would need to de-cock the red band which is still under stretch after the shot. That is where the energy comes from, using the upper end of the band stretch characteristics. The black band is probably always under some stretch to keep the side sub-pulleys from rattling around on the sides of the gun.
As someone who uses a roller gun with multiple loading stages, the thought of only pulling two bands back sounds very appealing.

Pete, I am curious as to what you think the green band would actually eliminate the need for a kicker band?
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Old 04-18-2017, 12:42 AM   #15
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Re: demultiplied speargun

Although their loading diagram does not show it, that green band will be stretched when the gun is cocked to shoot. Hence it adds a bit of drive rather than lose all of the drive from its loop length if it was just a longer solid cord that you could get both hands on. A longer wishbone loop projecting backwards from the wishbone arrester loses drive length, but by using a band they don't lose all of it as the loop stretches from slack length with a one hand pull in order to get your other hand onto it. The reason for it being there at all is that pulling right back from the muzzle on an already pre-stretched band can be a big ask, so this is a trade off sacrificing some drive length in order to load the gun, especially after many shots and some lengthy period in the water.

A kicker band is only necessary if a demultiplied gun has lost power by permanent elongation of the stacked band battery, so you add another "normal" band to put some power (actually energy storage capacity) back. Another approach would be to thicken up all the bands in the demultiplied band battery and don't use a kicker band, which I think is a bit of a misnomer anyway. The problem is can you then still load it? Not much use having a gun that you cannot load without turning yourself inside out to cock all the bands. At least with a pneumatic you can let some air out during the dive.
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