Speargun Dynamics Research - Help Sought | DeeperBlue.com Forums
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Thread Status: Hello , There was no answer in this thread for more than 60 days.
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Jesse_Spiller

Active Member
Feb 14, 2020
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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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Mr. X

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Cool, it is just the sort of thing we spearos have loved to discuss over the years. So this is just theoretical currently?

I think I see a couple of dubious assumptions. In the rubber comparison you say something about if you can stretch a 16mm rubber to double it's length but it is quite normal to stretch a rubber to about 3.5x it's original length. Also, it is normal to stretch thinner bands more than thicker bands e.g 14mm > 16mm > 18mm-20mm. Rubber diameter can vary as well e.g. 3/4" rubbers could be 18-20mm. The inside hole size varies too (e.g. US bulk rubber tend to have much smaller holes than traditional European bands).

I am sceptical that an inverted roller would be more efficient in practice. That extra roller is a moving part. Also the shorter rubbers (assuming 2x the number used on the regular roller) would , I suspect, not work as well. But prove me wrong in the pool!

I'm surprised that you did not standardize on one length of speargun, e.g. 100cm, to simplify calculations and side-by-side comparisons. Does that reflect your personal speargun arsenal? :)

Good luck. This is information that should prove useful to spearos and manufacturers, and could form the basis for future research. ;)

BTW What is"Free Recoil Energy"? Do you mean recoil-free energy? Is it the energy that propels the spear forward?
 

popgun pete

Well-Known Member
Jul 30, 2008
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Recently someone else was working on a speargun modelling project, but I cannot remember where that was, however a search would find it and it may have been on Spearboard. I recall the model only allowed for constant cross section barrels, but in many timber band guns they are anything but.

An area that would be worthy of in-pool testing is the standard gun, rollergun and inverted rollergun as to their performance and jerk reactions when shot. There are some tube guns such as MVD spearguns that come in standard, rollergun and inverted rollergun forms, so they would lend themselves to instrumented testing in a pool. The comparison only makes sense if they are all tested with the same projectile, although different diameter shafts could be used in all three types.

Standard: https://www.ebay.com.au/itm/MVD-Spe...hash=item1a7e413ba9:m:my9xScEXUHUVnHhWKJV2QUg

Rollergun: https://www.ebay.com.au/itm/MVD-Pre...669236?hash=item1a615d87b4:g:t-oAAOSwt5FbvzWy

Inverted Rollergun: https://www.ebay.com.au/itm/MVD-Pre...150030?hash=item1a917eb58e:g:C1kAAOSwPHReQzhK or
The above are just the various types, you would require guns of the same length barrel, however handles will be identical.

MVD guns tend to be expensive, but there are other guns using the same works https://www.huntmaster.com/collecti...DrwK0rvz3-J1dvUwhYmQGfyYiTABdZlbu8i7kE88W16eQ
Huntmaster gun.jpg

For pool tests you will not need the reels, although they do add to ballast and hydrodynamic drag.
 
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sharkey

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Nov 22, 2013
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Another member “Jworthy” linked his similar project to the recent thread “Which speargun is better”.
Interestingly he is also based in the UK. With all this interest, are we going to see a UK built speargun brand going forward?
 
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Mr. X

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Interesting info. Re. Power of spearguns being same and inverted roller.

Re. Recoil does this assume that the spearo had a firm, straight arm grip? I am thinking about shotgun recoil, if the shooter holds their gun firmly in their shoulder and lean forward recoil is absorbed by their body and the gun moves back very little. But if, on other hand, the shooter holds the butt slightly off their shoulder, the gun will kick backwards hard and fast bruising the shoulder painfully as it knocks it back.

Re. Band stretch v. diameter this has been around for some time. I tend to just cut my bands to the lengths suggested by Rob Allen's chart, so hadn't really thought much about this until recently. Yes, I think it is probably just a matter of loading difficulty. I found fresh 20mm American bulk rubber bands quite painful and bruising to load but, after two years of use they soften considerably and are not an issue.
 

Mr. X

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We once estimated that band power increased as follows, it considered a lot of factors around rubber dimensions but I don't recall if it factored in increased band stretch for smaller diameters. We did consider big hole euro bands and small hole US bands though.

14mm < 16mm < 18-20mm < 2 x 16mm

I think we estimated 2x14mm to be similar to 1x 19/20mm. There was a chap using 2x20mm bands but it is not a popular or common configuration, not very sensible IMHO.

We did not consider configurations with more than 2 bands although such configurations are not uncommon on Woodies, especially the USA. More recently, 3 and occasionally 4 rubber nuzzles have started to become more common on SA-style railguns and euro railguns (which are, I suppose, based on the traditional SA-style). I think to accommodate the currently popular 2x14mm configuration while offering the highly desirable - but often unnecessary - ability to upgrade power, to e.g. 3 x 14mm, or, perhaps more importantly, to be able to change configuration later. Of course, it takes more time and effort to load more bands.
 
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