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Old 05-14-2019, 11:35 AM   #1
Rsullivan1015
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Band stretch/size for custom speargun

Im in the process of building a speargun that I intend to use for reef/midwater fishing for grouper, pargo, AJ, wahoo, etc down in mexico.

The gun is ~170 cm overall length. I will be rigging it with a 180 cm 7.5 mm shaft. It should be similar in size to a 140 cm euro gun.

The shaft I have currently have two tabs, so I'm looking at running only two bands

I intend to use 14.5 mm small ID rubber, with a wishbone length of about 4 inches.

The band stretch from the center of the band slot to the tabs is 52" and 55"

Using some formulas posted on the board, I've come up with band lengths of 29.5" and 31"

Just fishing for opinions/feedback concerning band thickness, length, number of bands, etc....
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Old 05-14-2019, 04:39 PM   #2
Marco
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Re: Band stretch/size for custom speargun

I would go with 33" and 32.5". That would be 330%. You can always cut if needed.

(Effective lenght * 2 / 3.3)

If you go too short, it will be a pain to load, you will have more recoil and you will have the possibility of shaft wip.

Again, you can always cut if you feel that you can load too easy or if you feel lack of power.
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Old 05-14-2019, 08:37 PM   #3
doyenofcastle
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Re: Band stretch/size for custom speargun

Quote:
Originally Posted by Marco View Post
I would go with 33" and 32.5". That would be 330%. You can always cut if needed.

(Effective lenght * 2 / 3.3)

If you go too short, it will be a pain to load, you will have more recoil and you will have the possibility of shaft wip.

Again, you can always cut if you feel that you can load too easy or if you feel lack of power.
Rubber 14.5 at 330 % with this heavy spear ? if it 16 mm I think 330% is fine
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Old 05-15-2019, 06:30 AM   #4
Marco
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Re: Band stretch/size for custom speargun

As I said, you can always cut some more. Not everybody has the same needs or spearing style.
I like to take close shots. Others like to shoot from the couch...
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Old 05-15-2019, 08:01 AM   #5
Rsullivan1015
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Re: Band stretch/size for custom speargun

Thanks guys.

I went with effective length *2 / 3.5 for the formula (350%) so thats how I came up with my numbers.

Also, I believe the small ID 14.5 mm is equivalent to normal 16mm rubber, no?

The gun also has some mass to it... Made from black walnut with a bit of heft in the rear half of the gun, so I'm not too concerned about the recoil.
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Old 05-15-2019, 03:32 PM   #6
doyenofcastle
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Re: Band stretch/size for custom speargun

[quote=Rsullivan1015;2186236]Thanks guys.


Also, I believe the small ID 14.5 mm is equivalent to normal 16mm rubber, no?


Using the formula in primeline
14.5 mm measured by caliber stretch at 360 % =16 mm measured by caliber stretch at 325 %
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Old 05-15-2019, 03:57 PM   #7
popgun pete
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Re: Band stretch/size for custom speargun

Here is a band calculator devised by those lovers of mathematics, the Russians.
Just plug in your numbers and you have the answer.
https://sportmarket.su/articles/calc...e-strands.html
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Old 05-16-2019, 06:50 AM   #8
Marco
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Re: Band stretch/size for custom speargun

The "math lovers" have the same recommendation as me: "Don't cut too short. You can always cut more"
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