Roller spearguns

    This is one of the MOST interesting threads I've read. I'm able to follow most of the calculations but it takes me to a saying that I use often when I teach International Aviation courses, it is:


    Confucius


    I hear and I forget
    I see and I remember
    I do and I understand


    So all extremely interesting and before "I forget" what I heard, and Based on what I "remember" from the equations - a Roller guns advantage is same power and range of a longer non roller speargun but to really "understand" - will wait for Makoa's demonstration. :confused1:

    Hey Xan, I don't think that you can use the Work = Force x Distance calc in any case, as the force is a function of band elongation. The energy would be the area under the graph of force vs elongation,
    so you would use Grossetti's function of 1/2kx^2
    But Grossetti's calculation is flawed because stiffness is inversely proportional to length.
    As k=EA/L, and the theoretical roller gun in Dan's example has a rubber length of twice the normal gun, its stiffness is half of the single band. As there are two parallel rubbers for the double band gun, its stiffness is twice a single band.
    Stiffness
    Single band k = 1
    Double band k = 2
    Roller band k = 0.5
    Elongation
    Single band x = 1
    Double band x = 1
    Roller band x = 2 (as it is half as stiff, you can stretch it twice as far for the same loading force)
    Energy
    So applying E=1/2kx^2
    Single band E = 0.5
    Double band E = 1
    Roller band E = 1


    So Dan was right all along.

    Xan, congrats on graduation.

    Quote from kmo

    So Dan was right all along.

    I like the sound of that. But I'm not going to pretend I understand the formulas fully. This is high school math and for me was 30 years ago. After being done with it I was happy to allocate that space in my brain to other more pertinent stuff. I could sit down and reason it out all over again, but that would take time which I'd rather spend diving or making new gear. More so because I see the calculations as unnecessary in this case, using simple logic it's evident there's no difference between the roller and conventional set ups.


    Stock length and consequently band stretch are almost the same, bands are the same. The only difference is that with the conventional speargun each of the two band's two half segments are contracting simultaneously towards the muzzle YES THAT'S 4 HALF BANDS CONTRACTING AT THE SAME TIME, and with the roller each of the two bands is contracting completely from end to end. I see no way for that in and of itself to increase the band's pulling power. Reduce recoil.. maybe slightly, but in reality a more significant portion of the recoil is reduced because the roller bands are stretched less, which makes the roller appear more manageable.


    Makoa, I didn't really expect you to go through with all that building. I recognize the good intentions but most of us have busy schedules and when we have free time we'd just rather be diving. So spending time and effort just to prove something quickly loses its appeal. But something does need to be pointed out again: in this case power is synonymous with range. So when I focused on proving that a roller is not more powerful than a conventional speargun, this also encompassed the myth of the roller having more range than an identical length conventional speargun.


    Also your 110cm roller's band length can't be 48cm. That would make it %450 stretch.

    What is the distance along the bottom of the gun, from the roller muzzle pulley to the point where the band ends when gun is loaded to maximum power please?


    And the distance along the top of the gun from the pulley to the furthest tab on the shaft?


    Could you please clarify what you mean by "shortened 110 roller". I assumed that you had a conventional 110cm speargun and just switched out the muzzle without cutting/shortening the barrel.


    So what you are saying is that a conventional gun twice the length of Dan's gun's and that is using the using the roller's bands (turning into a euro gun) would be just as powerful as Dan's half as long double band gun?


    I know that force is a function of distance. The forces I used are the max forces applied. Yes you are correct in saying that the work would be the area under the curve. Notice however that the bands are linear and the conventional band although starting slightly higher quickly drops to 0 while the roller band gradually decreases to a value slightly above 0. I did not show all this this because I had no way to to measure the exact outputs of the bands at different elongations. But because they are linear you could use average force applied over the distance. I have not done the calculations but I don't think this will change much. Maybe the final values but not the overall results.


    In Grossetti's calculations yes the stiffness are different. The K value of the conventional bands is exactly double. However it is also being stretched nearly 2 times as far.


    You are forgetting that a roller gun has 2 bands just like a double band conventional gun. So yes if you compared a roller with only one of its 2 normal bands then yes a double banded conventional gun would be more powerful. That is why as Grossetti pointed out if you added a a 3rd band to a conventional gun it would have more power.


    K
    Conventional gun K = 2
    Roller gun K = 1


    Elongation
    Conventional Gun x = 1
    Roller gun x = 2 (its actually closer to 1.6)


    Potential Energy (1/2*k*(x^2))
    Conventional gun = 1
    Roller gun = 2 (or if you use 1.6 for x ) = 1.28


    It is the X^2 (x squared) term in the Potential Energy equation that makes the distance SO important. It is also key to point out that in the energy equations this X is total distance not elongation relative to the length of the band. Also notice 115cm /1.28 = 89.8 cm. Which is very close to the 87 cm my initial calculations for the length of a roller gun with the same energy as a 115 cm conventional gun.


    P.S. Thanks Dan. I am both excited and nervous moving on to the next part of my life.

    Edited 5 times, last by Wishihadgills: spelling/grammar/clarity ().

    I'll measure when I get home. I say shortened because I cut my barrel down from the original Aimrite Super Venom 140 cm to make a 110 cm Super Venom with a roller muzzle.

    HUI KOA KAI O HAMAKUA
    MAHALO KE AKUA
    E MALAMA I KE KAI

    Ok, the measurement from the roller to the band end along the bottom of the barrel is 41 cm when loaded. The measurement along the shaft from the rollers to the aft slot of my Rob Allen shaft (slot closest to the trigger mech) when loaded is 100 cm.

    HUI KOA KAI O HAMAKUA
    MAHALO KE AKUA
    E MALAMA I KE KAI

    Quote from makoa

    For example, I found I could put a 17.5-18.0mm band on my roller at 320-350% stretch and have a gun powerful enough to make open water shots on bigger fish while using a gun that is shorter and more desirable on the reef, in caves, ledges etc than my longer guns rigged with 2-3 16mm bands.


    One thing I'm gathering from this thread is that one man's idea of "power" in a gun is different from another's.


    When I got my DWong 55 hybrid, Daryl had put two bands, 24" and 26", normal 14 mm (not small ID Dan Bands) bands on it. I guess he thinks I'm a feeble old fart. I brought it back and one of the first five or so fish I shot with it was a 43 lb cubera.
    It was a bit of a long shot, (two wraps of line extended) and it still penetrated through the body and out the gill plate, opening the flopper. You can see the entry and exit points in the pictures. I was lying on a ledge above the fish who was facing at about a 30 degree angle away from me.
    I suppose you could use a bit more power if a wahoo was swimming away from you in open water, but a cubera has thick skin and it penetrated the gill on the way out. How much power does one need?
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    Edited once, last by hank ().

    Quote from hank

    One thing I'm gathering from this thread is that one man's idea of "power" in a gun is different from another's.


    When I got my DWong 55 hybrid, Daryl had put two bands, 24" and 26", normal 14 mm (not small ID Dan Bands) bands on it. I guess he thinks I'm a feeble old fart. I brought it back and one of the first five or so fish I shot with it was a 43 lb cubera.
    It was a bit of a long shot, (two wraps of line extended) and it still penetrated through the body and out the gill plate, opening the flopper. You can see the entry and exit points in the pictures. I was lying on a ledge above the fish who was facing at about a 30 degree angle away from me.
    I suppose you could use a bit more power if a wahoo was swimming away from you in open water, but a cubera has thick skin and it penetrated the gill on the way out. How much power does one need?
    http://i1337.photobucket.com/albums/o675/Hank_Bauman/DSC00030_zpso7forxor.jpg</a><a href="http://s1337.photobucket.com/user/Hank_Bauman/media/DSC00030_zpso7forxor.jpg.html">
    http://i1337.photobucket.com/albums/o675/Hank_Bauman/DSC00031_zpsj5huyrsn.jpg</a><a href="http://s1337.photobucket.com/user/Hank_Bauman/media/DSC00030_zpso7forxor.jpg.html"></a><a href="http://s1337.photobucket.com/user/Hank_Bauman/media/DSC00031_zpsj5huyrsn.jpg.html">http://s1337.photobucket.com/user/Hank_Bauman/media/DSC00031_zpsj5huyrsn.jpg.html</a>


    Indeed

    Scupper Pro Gives You Wings!

    How much power does one need? As much as one can get away with in the blue [emoji4] ...On a reef? Not as much, so different Hank.

    A bad day at sea is better than a good day in the boatyard
    George Steele

    Thanks Makoa.


    When talking about euro speargun lengths it's hard to know what everyone means, because there are different ways of measuring. For example often the stated length of a gun is simply the length of the barrel, be it aluminum or carbon. But if the objective is to have a clear idea of the power and range of a gun that number can be misleading. For example the handle of the Speardiver Phantom speargun because of its reverse trigger mechanism adds another 10cm of band stretch, so together with a 110cm tube produces true 120cm of band stretch.


    So the only way to provide a correct idea of a gun's power/range potential is by stating the true band stretch, which is from the muzzle hole or pulley, to the furthermost tab/notch on the shaft. But this can also be misleading because different shafts have the tabs/notches in different places.


    This is the reason I decided to state the length of all Speardiver guns based on the distance from the foremost muzzle band hole to the mouth of the trigger mechanism. Because theoretically you can have a shaft that has the tab/notch right at the mouth of the mech. This measurement will be consistent regardless of the shaft used.


    That said, you referred to your gun as a 110 but it appears to have only 100cm of maximum band stretch potential. Probably a little more if you got a shaft with a tab closer to the mech, in all likelihood I'd call your gun a 105. If I were to build a conventional speargun to test against your roller I'd make sure it had 105cm of true band stretch potential.


    Back to your measurements. You have an actual total band stretch of 141cm. Divide 141 by 48 (the band length) and you get 2.94 which means you're stretching each one of your two bands to %294, not %330. Neither Xan nor Gabriel can argue with this number. It can be considered that the tied wishbones further cuts into this number, but we'll use Xan's extra 5cm of pulley circumference to offset it :) even though I think it's too much. The band is nestled in the pulley's track and there's no way half the circumference there is 5cm.


    What does all this add up to? You have 105cm long speargun with two 17.5mm bands stretched to a fairly conservative ratio of %294. Take that same 105cm speargun and rig it with two 16mm bands stretched to %350, and I think you will at least match if not surpass the roller in power and range, without overpowering the gun. If you'd stretch the 17.5mm roller bands to %350 you'll encounter recoil issues, the gun is now overpowered. Also with a roller you're stuck loading both bands together in one pull, this seriously cuts into how much you're able to stretch the band.


    That's quite a hole in the cubera Hank.


    A speargun thread with no drama and almost 2000 views, not bad :)

    Quote from Dude george

    How much power does one need? As much as one can get away with in the blue [emoji4] ...On a reef? Not as much, so different Hank.


    Ok, I can see the need for longer shots. I remember Daryl telling me, don't shoot at an ono until you can see its eyes. Meaning many shoot shots that are too far because it's deceiving in blue water with no reference points,
    This is why I use my 2x4 lunker of a Super Magnum for my limited wahoo attempts. I can put four 16 mm bands on it but it has the weight to absorb lot of recoil. Because accuracy seems like a huge factor on the long, skinny wahoo who never sit still. And although I don't use that gun of mine much, I have rarely missed with it. Smooth as butter but still has the power.


    I would much prefer using that rather than trying to power up one of my 55 hybrids.

    Quote from makoa

    Hank, where did you get the ci era?


    ci era? Is that an auto correct for Cubera? Not sure what you mean, Makoa. But if you DID mean cubera.....I got it at my ultra double secure secret spot. If you come here, I'll take you there. Just don't tell El Marco or George where it was.:laughing:

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