Roller spearguns

    I went at it a bit diferently from Wishihadgills, but I end up with the same type of results:


    Using the chat taken here (Speargun Band Force Calculator - Spearboard.com - The World's Largest Spearfishing Diving Boating Social Media Forum), for a 16mm tube you get about 267 Newtons of force for 300% stretch


    Hookes law states that F=kX where:


    K = force in N
    X = streach distance in m
    k = a constante defined for each spring


    For parallel springs you have k = k1 + k2 + k3 + ...


    we are mising k so k=F/X


    The Engery is PEe = 1/2kX^2, PEe is in Joules


    I used Spearit Knowledge Base How to Calculate Speargun Band Length to calculate a band of length 51cm for a distance of 1m from the attachement to the 1st sharkfin, this gives us 25.5 for each side of the gun (either for a euro or once bent in two). This means X = 0.756 so k = 267/0.756 = 353.2; For a roller, the bands are twice as long, X = 153 so k = 267/1.53 = 174.5


    That gives us:


    dual band gun: PEe = 1/2*(4*353.2)*0.756^2 = 403.7 J
    single euro gun: PEe = 1/2*(2*353.2)*0.756^2 = 201.86 J
    roller gun: PEe = 1/2*(2*174.5)*1.53^2 = 408.5 J
    triple band gun: PEe = 1/2*(6*353.2)*0.756^2 = 605.6 J


    So technically, the roller is more powerful than the dual band gun but not more than the triple band gun


    Ref:
    PhysicsLAB: Springs: Hooke's Law

    I think I should have calculated k like k = 267/1 = 267, this supposes a 1m piece streached 100%, then k (Newtons/m) is a constant so that gives:


    dual band gun: PEe = 1/2*(4*267)*0.756^2 = 305.2 J
    single euro gun: PEe = 1/2*(2*267)*0.756^2 = 152.6 J
    roller gun: PEe = 1/2*(2*267)*1.53^2 = 625 J
    triple band gun: PEe = 1/2*(6*267)*0.756^2 = 457.8 J

    Really? I thought it was like load gun, point at fish and pull trigger:laughing:


    I'm glad there are guys out there with big brains and lots of brain cells. You all lost me at the word "calculation"! I had to go fill the bathtub, submerse myself holding my gun, and hold my breath till my brain disengaged.


    Ok, so I have two roller muzzles on the way. One for the teak gun, and one for my Pathos 90. So we will have three guns to compare performance with:
    110 roller vs 110 standard, 90 roller vs 90 standard, 55" wood roller vs 55" wood standard. Then I'll compare 110 standard to 90 roller. Like I said though this project will take me a couple months to put together. I'm not canceling my kids spring break spearing trip to Mexico and back home on BIg Island to conduct testing. I hope to have two cameras filming; point of view and side view in slow motion. I would like this to be a well produced and thought out presentation. Until This project is complete feel free to continue the mental gymnastics. It truly will be interesting to see who gets the closest to explaining the future results.


    A Hui Hou,
    Makoa


    PS I do have mad respect for analytical dudes. My dad is a PH.D, a Naval Academy grad with graduate work at Stanford, UC Berkley, and U of Penn, and the Navy's pioneer on the GPS that most of you use for navigating your flotilla to your secret spots! Me? I barely finished college in spite of always ditching class to be out on the water or hiking in the mountains! So yeah, props to you!:thumbsup2:

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

    Wishihadgills,
    I'm confuse...
    Don't You have to use the same % elongation in both guns?:confused1::crazy: If in the rollergun the stretched length is 200 cm and in the conventional it is 115 cm, they can't use the same size of bands:nono:
    In the roller gun the bands has to be bigger in your example doesn't it?
    Or it may get more power shorting the bands in the conventional gun...


    PS. 5 cm in the roller muzzle is too much:):)


    Regards,


    Kleberson

    The elongation was calculated using the assumption of the same size band (which was one of Dan's original points of contention). The the roller band is stretched differently because it is oriented length wise. Relative to the full length of 77cm the band is not stretched 3 times its length, which would require 231 cm of stretch instead of the 200 it actually sees.


    5 cm is the circumference around the roller. Ciricumference = 2*pi*radius = pi*Diameter. The band travels around half of the circumference so the full circumference would be 10cm. 10/pi = a diameter of about 3.18cm or 1.25 inches. Maybe a bit large but certainly not unreasonable and 5cm was a nice round number. You could make that value slightly smaller and it won't affect much even if it was zero. 195 cm vs 200 cm is only a 2.5% difference.

    Quote from grossetti

    I went at it a bit diferently from Wishihadgills, but I end up with the same type of results:


    You did the extra work of looking up real values of the bands which I applaud. I was working off the assumption the bands would be the same and was just doing a rough analysis to prove a point.


    I recently had an interesting realization. If band thickness is the same and elongation is the same the force should be equal. Most guns use the 300% elongation rule and therefore longer guns have longer bands. So why would a longer gun have more range/power? Excluding a slight increase in the mass of the shaft the answer is that they apply very similar forces over a longer distance. The only thing a roller gun has done is to change the location of where the relaxed band is located.


    Imagine for a second you had a 100 cm long gun. When the band is relaxed the wishbone is roughly 75 cm form the muzzle. Put an imaginary hinge right where the wishbone is and bend the tip under the gun. Put a roller at the new tip for the band to go around and now you have the idea of a roller. Same bands same everything but now the gun is only 75 cm from mech to tip. Not magic just a clever use of physics.

    Quote from Wishihadgills

    You did the extra work of looking up real values of the bands which I applaud. I was working off the assumption the bands would be the same and was just doing a rough analysis to prove a point.


    Thanks, I was surprised on how hard it is to find the data linked to the elasticity/force of the band, even from makers (which are not easy to find either). The chart mentioned above is one of the best I found.


    I think your analysis got idea through quite well I think. Now all we need is the real world test which Makoa said should happen in the up coming months.


    Quote from Wishihadgills

    The only thing a roller gun has done is to change the location of where the relaxed band is located.


    Imagine for a second you had a 100 cm long gun. When the band is relaxed the wishbone is roughly 75 cm form the muzzle. Put an imaginary hinge right where the wishbone is and bend the tip under the gun. Put a roller at the new tip for the band to go around and now you have the idea of a roller. Same bands same everything but now the gun is only 75 cm from mech to tip. Not magic just a clever use of physics.


    Yes, that is what I had tried to explain a couple of posts back, you probably explained it better as nobody understood me I think.

    38.5 is 77/2. This is the position on the track at which the conventional band is in its relaxed state. In its relaxed state it no longer applies any force to the shaft.


    Quote from marginatus

    I've read an article about testing rollers and non rollers and, according to the tests, to get the most out of rollers, the band should be pre tensioned...


    In the calculations I provided the band is pre-tensioned. Not much but slightly; the band is roughly at 110% elongation when it reaches the end of the gun. The pre tension is determined by where, on the underside of the gun, the band is anchored.

    Quote from Wishihadgills


    In the calculations I provided the band is pre-tensioned. Not much but slightly; the band is roughly at 110% elongation when it reaches the end of the gun. The pre tension is determined by where, on the underside of the gun, the band is anchored.


    Why not pre-tension the band(s) to 150%? Wouldn't that yield a higher velocity shaft leaving the gun?

    Edited once, last by parker ().

    Quote from Treebilly3

    Ok so who's doing the tests? Lots of interesting info but I don't see it winning any arguments. Seems like it could be a fairly simple test setup if some one has the time.


    Mokoa has agreed to do it with 3 diff setups he had but it will take a while cause he's busy he said.




    If anyone has two comparable railguns, one standard and one roller maybe it will happen sooner

    Scupper Pro Gives You Wings!

    Quote from Treebilly3

    One rail gun and a roller muzzle is all you'd need. It wouldn't take long to swap out the muzzles
    Well and bands for each muzzle type.


    Yep, took me like 2 hours total to switch out a RA 110 (with ET) to a roller.
    Problem was I did not have an UNDERWATER chronograph to measure REAL spear speed. IMO, all this number crunching and math is USELESS unless you have chronographed spear speeds from each gun to PROVE the difference btw roller and conventional spear speeds. To this date NO ONE has chronographed these spear speeds comparisons.

    SPEARFISHING and RECREATIONAL FISHING NEEDS THE NRA
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    Edited 2 times, last by hau ().

    NO ONE has yet to realize a HUGELY important band rigging method possible for roller guns. I cannot believe that NO ONE else has figured out this simple and easy method. I got this idea and tried it out on my RA roller conversion and could EASILY pull over 400%. In the end I had to lengthen my roller bands cause my hand kept getting bruised from the recoil.

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