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Old 11-15-2015, 08:25 PM   #1
whale tale
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Spear Speed Test

Hi All,

For a school project I'm measuring spear speed and gun recoil to compare a couple speargun designs. I wanted to show you folks my spear speed measurement technique and ask if anyone has seen this done with spearguns before, if it looks legit, etc.

The idea is simple. Look at the video frame-by-frame and see how fast the spear passes along the ruler. (30 frames per second)*(28 inch per frame) = 840 in/sec = 21 meters/sec.

Sound good?

https://youtu.be/_Jb17gx99HA
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Old 11-15-2015, 08:30 PM   #2
phil herranen
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Re: Spear Speed Test

Do you have a pool up here in sc ?
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Old 11-15-2015, 09:44 PM   #3
whale tale
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Re: Spear Speed Test

I'm in San Luis Obispo borrowing the services of one of the apartment complex pools near the university. Water's colder than the ocean. If I was in Santa Cruz I'd probably ask SC High if they had a gap in the schedule when I could use their pool.
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Old 11-15-2015, 10:47 PM   #4
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Re: Spear Speed Test

I think sound is a better medium. You set up a target with precisely controlled distances and then with a sound editor you measure the time it took. You can clearly identify the sounds when the gun is fired and when there is an impact on target. Obviously the target cannot be a thin piece of paper as that would not give you enough sound.
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Old 11-16-2015, 12:32 AM   #5
whale tale
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Re: Spear Speed Test

Sound does seem like a more exact method. One argument for the ruler/footage method is that it gets the spear speed right as it leaves the gun muzzle. Reduces the effect of spear design/drag. But that's probably splitting hairs.
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Old 11-16-2015, 03:02 AM   #6
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Re: Spear Speed Test

The thing with spearguns is that the shaft starts off very fast and then sort of hits a wall and velocity drops dramatically. So you could get a velocity drop from 5 to 6 meters that is more than the velocity drop from 0 to 5 meters. Velocity drop is directly related to of course the hydrodynamic characteristics of the shaft, how stable the shaft is (how much oscillation), the line the shaft is pulling and of course how much mass is in the shaft. The reason I say that because you can have a shaft that is going at 35 m/s at muzzle having less penetrating power at 5 meters than the same shaft that exits the muzzle at say 28 m/s. I know it sounds strange but it is true. That is why I think you need more than just data at muzzle, you need to see how that shaft is flying throughout its journey.

I do a lot of testing and spent a lot of time on optical means of figuring out velocity. I figured sound was just easier than trying to configure a chronograph to work underwater. In my case I am more interested in velocity as a means of tuning a speargun for spearfishing. Thus I use penetration testing on styroboard now rather than figuring out velocity. But penetration testing at distance is also not 100% as a shaft tip that has razor facets will penetrate more than a pencil tip even though it has less velocity. But I try to use a test that uses similar tips for testing and so far it has worked well.
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