Wednesday, April 01, 2020
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Wanted Speargun Dynamics Research - Help Sought

Jesse_Spiller

Member
Feb 14, 2020
5
4
8
29
United Kingdom
Good afternoon All,

I’m an avid Spearfisher and postgraduate Gun Systems Design student at Cranfield University in the United Kingdom. My thesis, which will be released in August, is based around speargun dynamics and building a better scientific consensus on their function. I have use of a dedicated pool facility, an underwater high-speed camera, machining workshop and 6 months labour.

I have built a mathematical program that predicts speargun performance. It considers rubber to be non-linear (more complex than a spring) and accounts for band/spear movement in recoil and efficiency:

Speargun Simulator Screenshot.png


Some interesting outputs so far:

Gun_Data.png


- Rollerguns offer about 55% less recoil than conventional guns, for the same shot energy.
- Inverted guns offer about 48% less recoil than conventional guns, for the same shot energy.
- Rollerguns are far more efficient (the energy you put in vs the energy in the spear), than conventional guns, and
- Inverted guns are significantly more efficient than rollerguns.
- The heavier the gun is, the more efficient, and the less recoil (conservation of momentum).

To accomplish this, there was some comparison of energy storage in rubber:

Comparison_Most Energy for a Fixed Pull Length.png


With this insight, you can make justified statements like: 'There's no point upgrading from 14mm to 16mm rubbers unless you can pull 28 Kg, or stretch the 16mm rubbers to at least 2 times their original length, else the spear will go slower. '

I'm also getting interested in shaft dynamics:

Shaft Whip_Bending.png


With these tables, you can make justified statements like: 'If you move from a 1.3m conventional gun to a 1m inverted gun (spear length 1.5-1.2m), you should use a 7.5mm shaft instead of an your old 8mm shaft; it will go faster, the gun will be more efficient, penetrate further and it's less likely to whip or bend in a fish despite being thinner '.

The info above is still just a theoretical model though. It's currently using data from others which doesn't include rubber energy dissipation over time. It's also missing friction losses, which can only be obtained with pool testing. The next stage is to obtain and test a variety of rubbers. In April/May, I will be conducting experiments to determine friction and hydrodynamic losses in a pool, using a high speed camera.

The end-state is to build a freeware simulation tool that spearfishing manufacturers and hobbyists can use to predict and compare speargun performance. This will be in the form of an Excel spreadsheet; but I'm hoping I can make it into a native website.

I’m need whatever material support can be offered - the things I still need are listed here (hyperlink). I'm in dire need of rubber (different diameters and from as many different manufacturers as possible). Rob Allen, Jeremy from DiveFactory, Marco from Ermes Sub and Carlos from Picasso have expressed interest in helping with this research. Radio silence from other manufacturers so far.
If you can help, or now anyone who could, could you please get in touch with me on this forum, else [email protected]?

Else, just keen to hear what the peanut gallery thinks of these ideas, and if there's any areas of interest that might be worth further research?

Regards,

Jesse.
 
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