Speargun Dynamics

    I think this will be useful for gun builders. Seeing the whole picture of what's going on will help.


    This might not be %100 of what's going on but it is pretty close. Any mistakes seen will be gladly appreciated.


    I will put a couple of hand drawings up trying to make sense of everything going on.



    I'll add descriptions later on.



    Point P is the point around which a gun can pivot. Somewhere in the wrist depending on how you hold the gun. The W's are the weights of the gun and spear and the By is the buoyancy force. Bouyancy force is the weight of the water displaced minus the weight of the wood gun.


    b is the band lenght and d is the distance from band hole to notch and B is a point on the notch.


    Also the mass of the person is much greater than the mass of the gun and spear which I have asssumed makes point P stationary. That is, a force exerted by the gun cannot move the person due to their mass and surface area.

    Davie Peguero

    Edited 2 times, last by seaweed ().

    2



    This explains how the vertical force exerted by the bands causes no muzzle flip.


    This came out kind of slopy but Point B is the notch drawing 1 is the applied forces and drawing 2 is the effective forces.


    Drawing 1


    The force of the band can be split into 2 components. The one that drives the spear FBx and the one that pushes the spear into the track due to band not being horizontal, FBy[/SIZE].



    Drawing 2


    The force driving the band causes reaction force which is what we feel, FBx=msas. It is equal and oppistite in direction.


    The vertical force is counteracted by the wood. Therefore it does nothing except cause friction between the wood and spear. It's like squeezing the wood and spear together. The friction force is equal to ud*N. N is small, around 5 lbs,and u[size=8]d is even smaller, around .02-.1 so the friction force is negligible, .1-.5 lb, compared to the driving force 200lb.

    Davie Peguero

    Edited 3 times, last by seaweed ().

    3



    This is the forces acting at point H.


    The normal force, vertical force, is negligible because the band angle is so small also it's counteracted by the wood.

    Davie Peguero

    Edited 2 times, last by seaweed ().

    4



    Now the muzzle flip is a rotation of the gun about point P. This rotation is equal to I* (alpha), I being the moment of inertia and alpha being the rotational acceleration. This is equal to the sum of moments (torques) caused by all of the forces acting at their respective distance such as the driving force at a distance o| from point P, the moment caused by the mass of the spear traveling forward, moment of mass of gun, moment of buoyancy force. Surface area also comes into play which causes drag. Some of these forces are negligible but actual calculations have to be made to gauge their impact.


    I think I is the most important since we have much control over it in balancing the gun. If no balancing is needed the surface area dominates.

    Davie Peguero

    Edited 3 times, last by seaweed ().

    Your handwriting says your a Doctor not a Engineer. :D:D
    My gun is as heavy as a crow bar, it helps me get down fast and muzzle flip is not an issue.:D:D
    Hey I hope you know I just joking about the handwriting.:D
    Cheers, Don Paul

    "Great mother ocean brought forth all life, it is my eternal home'' Don Berry from Blue Water Hunters.


    Spearfishing Store the freediving and spearfishing equipment specialists.

    hahaha, see, this is what happens when a real engineer sees a non-engineer's scematics...he has to draw better ones...nice.


    in comparing the force loads from parallel bands to those mounted a little lower in the muzzle, what is the difference in the additional force pulling up on the muzzle?

    i like to spear fish

    As Davie noted in his calcs, it's pretty small.


    If we had a band slot 1" below the shaft, and the bands were stretched 40", then the angle would be about 1.4°. If we had a 200 lb band load, then the vertical component would be 200 x Sin(1.4°) = 5 lbs.


    But here's the part that has always confused me. That small vertical force only serves to pull the shaft and stock together. Whatever force is pulling the stock up, is exactly the same as the force pulling the shaft down. They are already touching, so one cannot be pulled closer to the other. So I don't understand how the vertical component of the band force can generate muzzle jump, even if this component were large.


    I can see it if this were an old style gun where the spear were unsupported over much of its length, because then there can be relative motion between the shaft and stock. But that's not the case for most guns.


    In my head, it has always seemed that muzzle jump had more to do with the way the recoil force acted about 1) the grip, and 2) the gun's center of mass. Davie points out that the mass of the gun is small compared to the mass of the diver, so it would seem that the height of the grip would dominate. If you follow Davie's analysis a little further, it may even reach the theoretical conclusion that you could reduce muzzle jump by LOWERING the band slot, to put it inline with the grip?


    It seems to me that if the Band Slot, grip, and Center of Mass of the gun were all in line, then you should have no muzzle jump. I have wondered if the way the bands jump around at the end of their contraction could somehow cause the muzzle to move. But I don't know how this would be different for inline versus normal bands.

    Edited once, last by Guest ().

    Quote from Tin Man


    If you follow Davie's analysis a little further, it may even reach the theoretical conclusion that you could reduce muzzle jump by LOWERING the band slot, to put it inline with the grip?


    It seems to me that if the Band Slot, grip, and Center of Mass of the gun were all in line, then you should have no muzzle jump. I have wondered if the way the bands jump around at the end of their contraction could somehow cause the muzzle to move. But I don't know how this would be different for inline versus normal bands.



    Wow you nailed it!!! I thought my writing was pretty sloppy since it was done quick but you are right.


    Pretty much it all surmises to distance between handle and band hole.


    Also something I didn't talk about is how surface area comes in for reducing muzzle jump.

    Davie Peguero


    Jeff,
    I follow what you're saying but the fact that the bands are stationary on one end confuses me a bit. If the bands were being released from both ends then the force would be equal, but it isn't. When the bands are released the center point of the bands(contraction) continues to change(forward) until the shaft is released. Even after this occurs the bands will contract further than their resting state and then come to a rest. Maybe this is the point that you are talking about. When the bands ends actually push away from their center instead of toward one another.
    One more thing ,the holes in comparison to handle does make sense. Interesting to say the least.

    Edited once, last by Toledo ().

    the hand on the gun has to act like a fulcrum to some extent. the muzzle is a lot easier to move than your hand and arm. I always felt the gun wants to come up because of you arm moving the force that was going back, up

    i like to spear fish

    Maybe this is a good place to make a distinction. I think that muzzle flip is a very real phenomenon. I'm just not sure that it's due to parallel (or non-parallel) band pull. I haven't used band elevators, but I have read where others swear by them. Maybe there is another interaction at work. Maybe the band elevators (or parallel bands in a euro) do something else to reduce muzzle flip.


    For example, the motion of the bands at the end of the firing sequence. Or the way they disengage from the fins / notches, etc.

    I wonder if anyone has ever setup a high speed (super slow motion) camera underwater. It sure would be nice to be able to see what is happening during the firing of a speargun. When watching an arrow shot from a bow is super slow motion, I was stunned at how much the arrow was bent. I wouldn't be surprised to see that there are things going on that we didn't fully expect - especially with "high power" guns.


    Jeff, if you want to come over sometime soon, we could take some video that will at least show whether or not the front of our guns are kicking up during the shot. One of us can film the side of the gun while the other shoots at the target. I have a new HD underwater video camera which might capture some decent data. You need to pull the trigger on the hybrid don't you?

    Quote from griswold

    I wonder if anyone has ever setup a high speed (super slow motion) camera underwater. It sure would be nice to be able to see what is happening during the firing of a speargun. When watching an arrow shot from a bow is super slow motion, I was stunned at how much the arrow was bent. I wouldn't be surprised to see that there are things going on that we didn't fully expect - especially with "high power" guns.


    Jeff, if you want to come over sometime soon, we could take some video that will at least show whether or not the front of our guns are kicking up during the shot. One of us can film the side of the gun while the other shoots at the target. I have a new HD underwater video camera which might capture some decent data. You need to pull the trigger on the hybrid don't you?


    I have lots of pool tests shot on Hi 8 and 8mm film but I would need to get permission to post on the
    web. I used to use a spring scale and blue grease to measure shaft energy at the end of the wire rope.
    later on some guys were using mono with precision black marks and a bar code reader feeding a lap top.This
    gave good data on shaft velocity. Shite :0:0 this is sounding too much like work.... now where is that bikini
    thread?:D:D FWIW my bands are on the same axis of symmetry as the thumb and index finger of my
    gun handle.
    Cheers, Don

    "Great mother ocean brought forth all life, it is my eternal home'' Don Berry from Blue Water Hunters.


    Spearfishing Store the freediving and spearfishing equipment specialists.

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