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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 09-04-2019, 05:12 AM   #16
kavachi
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Re: Doggies - can an inverted roller really cut the mustard??

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Originally Posted by popgun pete View Post
Shaft energy is ˝ mass of projectile x velocity squared when the shaft hits the fish. Longer shaft has a greater surface area and more drag from “skin friction”, but not a lot in it. Mass increase is proportional to shaft length.
So given identical material and same tip, 150 x 9.5mm should provide a tad more penetration at the fish than a 170 x 8.5mm?
Albeit with some additional recoil in the bargain
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Old 09-04-2019, 05:36 AM   #17
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Re: Doggies - can an inverted roller really cut the mustard??

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Originally Posted by kavachi View Post
So given identical material and same tip, 150 x 9.5mm should provide a tad more penetration at the fish than a 170 x 8.5mm?
Albeit with some additional recoil in the bargain
Yeah, in your example I think the shorter shaft actually weighs just short of 10% more than the longer, thinner one. So, if we don't look at drag and if we make an allowance to say the speed would be the same, your shorter shaft would punch 10% harder.
Then again, I don't know how to calculate the difference in speed if the energy input is the same (same rubbers) while the shaft mass changes.

But I think what is also important in the formula Pete quoted is that speed matters more than mass (if I understand it correctly). If you can increase the speed by, say 20%, you gain more energy (impact punch) than if you increase the mass of the shaft by 20%. Maybe this can help explain why airguns and reverse guns still carry ooomp (if they indeed do shoot faster).

(I could be totally mistaken in all this...)
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Old 09-04-2019, 05:43 AM   #18
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Re: Doggies - can an inverted roller really cut the mustard??

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Originally Posted by Diving Gecko View Post
Yeah, in your example I think the shorter shaft actually weighs just short of 10% more than the longer, thinner one. So, if we don't look at drag and if we make an allowance to say the speed would be the same, your shorter shaft would punch 10% harder.
Then again, I don't know how to calculate the difference in speed if the energy input is the same (same rubbers) while the shaft mass changes.
But I think what is also important in the formula Pete quoted is that speed matters more than mass (if I understand it correctly). If you can increase the speed by, say 20%, you gain more energy (impact punch) than if you increase the mass of the shaft by 20%. Maybe this can help explain why airguns and reverse guns still carry ooomp (if they indeed do shoot faster).
(I could be totally mistaken in all this...)
Thanks DG!
I need to get hold of some of that thick foam Majd uses, and compare the two shafts
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Old 09-04-2019, 08:28 AM   #19
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Re: Doggies - can an inverted roller really cut the mustard??

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Originally Posted by kavachi View Post
So given identical material and same tip, 150 x 9.5mm should provide a tad more penetration at the fish than a 170 x 8.5mm?
Albeit with some additional recoil in the bargain
That calculation was easy for a longer shaft versus a shorter shaft of the same diameter. You can work it out for different diameters, but then you calculate the shaft volumes which we did not have to do before as the length was the only variable between them. The ratio of the diameters squared tells you how much heavier shafts of the same lengths are at different diameters. Hence a 9.5 mm is (9.5/8.5)^2 = 1.249 or 25% heavier than an 8.5 mm shaft.

150/170 is 0.882 so we multiply 1.249 x 0.882 =1.102 and that means there is about 10% difference in the shaft masses. Using the equation we had before v2 = v1 x sqrt 1.10 = v1 x 1.049 or 5% extra speed on the longer, but smaller diameter shaft for the same impact. Remember v is the downrange velocity, not the launch velocity. Tests have shown that heavy shafts hold speed better at longer distances and maintain more destructive capability while lighter shafts fly faster initially until they are overhauled by heavier shafts, provided the guns have the power to get them up to velocity initially. That is why many spearoes have used 3/8” shafts for decades, but it was either that or 5/16” as there was not much else to choose from.
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Old 09-04-2019, 11:10 PM   #20
kavachi
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Re: Doggies - can an inverted roller really cut the mustard??

Quote:
Originally Posted by popgun pete View Post
That calculation was easy for a longer shaft versus a shorter shaft of the same diameter. You can work it out for different diameters, but then you calculate the shaft volumes which we did not have to do before as the length was the only variable between them. The ratio of the diameters squared tells you how much heavier shafts of the same lengths are at different diameters. Hence a 9.5 mm is (9.5/8.5)^2 = 1.249 or 25% heavier than an 8.5 mm shaft.
150/170 is 0.882 so we multiply 1.249 x 0.882 =1.102 and that means there is about 10% difference in the shaft masses. Using the equation we had before v2 = v1 x sqrt 1.10 = v1 x 1.049 or 5% extra speed on the longer, but smaller diameter shaft for the same impact. Remember v is the downrange velocity, not the launch velocity. Tests have shown that heavy shafts hold speed better at longer distances and maintain more destructive capability while lighter shafts fly faster initially until they are overhauled by heavier shafts, provided the guns have the power to get them up to velocity initially. That is why many spearoes have used 3/8” shafts for decades, but it was either that or 5/16” as there was not much else to choose from.
Thanks Pete!
Look forward to testing them out side by side, see if can retain that punch with the much shorter shaft.
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