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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 08-30-2017, 11:02 AM   #1
Impaler Spearguns
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weight of SS spears

i need the weight per inch for 9mm and 10 mm spears in grams.
anybody know?
I know 7mm weigh 8.24grams per inch
7.5mm= 9.12 grams
and 8mm is just under 10 grams per inch
I'm not sure if it just scales up - or if it's exponential
easier just to weigh a spear and divide it by number of inches
if it just scales up - then it would be 1.78(ish) per gram increase per mm increase in diameter .
so 9 mm should be 11.8 g per inch
and 10 mm 13.6 g per inch - anyone know it for any of these larger diameters?
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Old 08-30-2017, 11:57 AM   #2
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Re: weight of SS spears

Do you want 9mm? or 11/32" and 3/8"

A 61" 11/32" Shaft weighs 720grams
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Old 08-30-2017, 12:17 PM   #3
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Re: weight of SS spears

you have to use area of circle to calculate...

7mm = 38.48 sq.mm @ 8.24g = 0.214g per sq.mm
7.5mm = 44.18 sq.mm @ 9.12g = 0.207g per sq.mm
8mm = 50.27 sq.mm @ 10g = 0.199g per sq.mm

assume average to be 0.21 grams per inch

9mm = 63.62 sq.mm x 0.21 = 13.36 g per inch
10mm = 78.54 sq.mm x 0.21 = 16.49g per inch
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Old 08-30-2017, 12:26 PM   #4
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Re: weight of SS spears

Jason, the best advice is just to go out and get a big Water trough from your local Agricultural supply store. Get some Salt. Get a Hydrometer. Phil H. gave me that advice a long time ago. There's a ballast tank next to my sanding table.
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Old 08-30-2017, 01:42 PM   #5
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Re: weight of SS spears

http://www.random-science-tools.com/.../beams/rod.htm
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Old 08-30-2017, 03:04 PM   #6
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Re: weight of SS spears

Yeah, just use an online calculator, I've done that ballasting my airguns and they get you super close. If you want the weight in fresh water, you subtract the volume in cm3 from the total mass.

Let's say you have a 8mm x 160cm spear. It will weigh close to 630g. But it will also have a volume of 80cm3, so the spear will "weigh" (630-80) 550g in fresh water. And I guess about 2.5% less in salt water.
(As always, there are smarter math guys than me out there, so if I am wrong please do correct me).
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Old 08-30-2017, 06:46 PM   #7
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Re: weight of SS spears

I Love it...But how do you compensate for the reduced area of the point, and the increase for loading fins, or the decrease for loading notches...OH..and how about the flopper and pin..or two..and then there is the trigger loading notch/'s, !?
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Old 08-30-2017, 07:33 PM   #8
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Re: weight of SS spears

Quote:
Originally Posted by SEATUX View Post
I Love it...But how do you compensate for the reduced area of the point, and the increase for loading fins, or the decrease for loading notches...OH..and how about the flopper and pin..or two..and then there is the trigger loading notch/'s, !?

Come on, Seatux. Jason asked a very specific, some would say overly so, question and some of us indulged him. Please allow us to do so I know 80cm3 of buoyancy makes zero real life difference but being nothing wrong with the exercise
The exercise is actually the same for building a whole gun. I have seen people try to do the math upfront but forget that all parts of the gun, even the negative ones, still displace water and thus increase buoyancy.

Last edited by Diving Gecko; 08-30-2017 at 08:04 PM.
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Old 08-30-2017, 08:25 PM   #9
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Re: weight of SS spears

I said I loved it!
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Old 08-31-2017, 01:04 AM   #10
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Re: weight of SS spears

I have it long time back get it from a forum ,hope it will help
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Old 08-31-2017, 09:16 AM   #11
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Re: weight of SS spears

Seatux the answer to your question is the Ballast Tank.
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Old 08-31-2017, 10:38 AM   #12
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Re: weight of SS spears

Quote:
Originally Posted by Behslayer View Post
Seatux the answer to your question is the Ballast Tank.
That's way too easy...and makes way too much sense!
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Old 08-31-2017, 02:16 PM   #13
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Re: weight of SS spears

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Originally Posted by SEATUX View Post
I said I loved it!

haha, sorry man:-0)
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Old 09-04-2017, 08:55 AM   #14
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Re: weight of SS spears

Quote:
Originally Posted by Tunaking View Post
you have to use area of circle to calculate...

7mm = 38.48 sq.mm @ 8.24g = 0.214g per sq.mm
7.5mm = 44.18 sq.mm @ 9.12g = 0.207g per sq.mm
8mm = 50.27 sq.mm @ 10g = 0.199g per sq.mm

assume average to be 0.21 grams per inch

9mm = 63.62 sq.mm x 0.21 = 13.36 g per inch
10mm = 78.54 sq.mm x 0.21 = 16.49g per inch
if a 9mm x 61"shaft weighs 720 grams - then it is 11.8 grams per in
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Old 09-04-2017, 08:59 AM   #15
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Re: weight of SS spears

Jason, this is all only part of the puzzle. Even if you know exactly how much weight to add.. you still don't know where to add it.
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