Speargun Build Simulator

[Jesse_Spiller]

Hi John, thanks for your interest.

My problem, like I think a lot of other gun builders experimenting with new designs, is that it's really tough to make apples to apples comparisons between band types. Generally, with a 58" long, 11/32 diameter spear, I know that four 16 mm (5/8") bands stretched to 350% about maxes out the power where adding another band won't accelerate the spear any faster because the band retraction speed is the limiting factor. With traditional band guns, it's easy to add and remove bands until you find the sweet spot where you max out the power for whatever purpose you are seeking without overdoing it and needlessly increasing recoil and loading effort for no gain.

Different people have different opinion based on their experience... it's just not scientific to judge things based on feel or perception. There are only two ways to determine if adding another band adds a commensurate amount of shaft energy: measuring shaft muzzle velocity (which is difficult) or a penetration test through a uniform medium. Don't forget: energy is proportional to velocity squared, so adding 25% more energy (a 5th band) could not add more than 12% velocity - but it'll still penetrate 25% more into a fish. It's really hard to perceive a less-than-12% velocity increase.

But I don't know exactly where that point is with rollers or inverted rollers. And based on my limited understanding of physics and the guns I've built, I have some serious doubts whether inverted rollers with four or less bands have enough power to take advantage of the pullies (i.e. I don't think most of them are powerful enough to accelerate a thick spear beyond the band retraction speed, which is the only real advantage inverted rollers offer - besides narrower muzzles, recoil, and other ancillary concerns).

Good point - since inverted rollers retract at half the speed of their counterparts then they *should* be less affected by rate of contraction limitations. However, packaging an inverted roller as a system is difficult. You need twice as many bands for the same energy - that why a lot of them are less powerful. There's always a compromise with physics.

To that end, I built a rig using a winch, a crane scale, some lumber, and some hooks to take static measurements in 2" increments of each type of band from 0" to 50". While my methodology is not perfect, I think it is good enough for comparative purposes. I am wondering if you, or anyone else on this site, could take a shot at calculating the total force/energy created by each of the bands based on the measurements in my chart. Presumably there is a way to simplify the data to make it easier (i.e. assuming a simple linear plot vs. the more complex one implicated by the raw data).

Mate - this is similar to what I did, except I left them stretched at 350% for 30 mins to account for dissipation over time. Assuming a linear spring is pretty close up until 310-320% and good enough for Rob Allen and Jeremy from Dive Factory. I finished the thesis last year and Annex A has the raw rubber data. Assuming your data is in pounds then it's pretty close to what I got when I was stretching the bands, although my recorded data is measuring relaxation after 30 mins.

Sorry mate, don't have the time atm to unwind that spreadsheet - maybe someone else here would be able to help. I'm adding a rubber section to the website when I get time, it might be able to help with things like this in future.

Good luck!

 

[johnakerblom]

Hi John, thanks for your interest.

Different people have different opinion based on their experience... it's just not scientific to judge things based on feel or perception. There are only two ways to determine if adding another band adds a commensurate amount of shaft energy: measuring shaft muzzle velocity (which is difficult) or a penetration test through a uniform medium. Don't forget: energy is proportional to velocity squared, so adding 25% more energy (a 5th band) could not add more than 12% velocity - but it'll still penetrate 25% more into a fish. It's really hard to perceive a less-than-12% velocity increase.

Good point - since inverted rollers retract at half the speed of their counterparts then they *should* be less affected by rate of contraction limitations. However, packaging an inverted roller as a system is difficult. You need twice as many bands for the same energy - that why a lot of them are less powerful. There's always a compromise with physics.

Mate - this is similar to what I did, except I left them stretched at 350% for 30 mins to account for dissipation over time. Assuming a linear spring is pretty close up until 310-320% and good enough for Rob Allen and Jeremy from Dive Factory. I finished the thesis last year and Annex A has the raw rubber data. Assuming your data is in pounds then it's pretty close to what I got when I was stretching the bands, although my recorded data is measuring relaxation after 30 mins.

Sorry mate, don't have the time atm to unwind that spreadsheet - maybe someone else here would be able to help. I'm adding a rubber section to the website when I get time, it might be able to help with things like this in future.

Good luck!

Appreciate the response. I think I'll be able to play around with the tool you created to compare my bands in a general sense.

Agree with what you are saying about the need to measure power based on science instead of experiential perception. My point is only that it is pretty easy with a traditional band gun to set up a target with heavy resistance and figure out what the "max" power is by adding bands until you get to the point where adding another band does not increase penetration. Obviously methodology and other factors (i.e. spear whip/bend) may come into play, but being able to experiment with different numbers of bands relatively quickly and easily definitely offers an advantage when you are trying to understand these things.

That same type of testing is much more difficult with a roller gun. With the two-roller gun I built, I can't just throw on a third roller to run a penetration test in a pool, I have to build a separate three-roller gun.

Maybe one of these days I'll strike it rich and buy the equipment to calculate actual velocities. Would be a ton of fun and take a lot of the guesswork out and get real-world numbers answering my questions.

 

[xristos]

Do you think bands loose power at depth?

Also i cant tell the color of the two different graphs, which is which etc. If you could make it different color instead of two similar blues it would be great. Like it a lot in general.

I think the important thing you can improve as you said is shaft dynamics and drag coefficient. For example it overestimates the speed of an overpowered shaft. I put 3 18mm rubbers 3.5 stretch and it thinks the poor 8mm 140cm shaft will go 40 m/s when in practice it will not hit target for sure and it will not achieve faster speed than 2 16mm bands at 2 meters.

 

[Sorcerer]

@Jesse_Spiller

Good point - since inverted rollers retract at half the speed of their counterparts then they *should* be less affected by rate of contraction limitations. However, packaging an inverted roller as a system is difficult. You need twice as many bands for the same energy - that why a lot of them are less powerful. There's always a compromise with physics.

First of all I am gratefull for your research. Amazing work being done. Finally something that we can work on that is not perception based.

I have one question, while working on setup for my new guns and working around inverted idea.
I hope I am wrong but I think the inverted calculations are wrong. Somewhere in the calcs instead DIVIDE by 2 there is MULTIPLY by 2 for inverted.

I have read the paper and science behind it which I love but I BELIEVE the simulator is doing the inverted rollers math wrong but I hope its me still not understanding it properly.

If you compare gun
X = Double 2x14mm 350%
Y = Inverted 1x14mm 350%

X=Y with very small difference. There is no WAY I can believe that considering that with inverted everything is halfed.

Same story if you double the bands in X and Y by 2. All results end up pretty much equal with minor differences.
It is possible that the inverted calculations are multiplied by 2 instead diveded by 2?

EDIT: DID FURTHER TESTING
Inverted 1 window - X=Y
Inverted 2 window - X = 1/2Y

EDIT2. I managed to contact the author and everything works fine, it was me missing that there is the "doubled" window - YES in inverted 1 window that would double the amount of bands basicly.