It is currently 29 Mar 2024, 12:18

All times are UTC+11:00




Post new topic  Reply to topic  [ 10 posts ] 
Author Message
 Post subject: Rubbers / Spear Speeds
PostPosted: 19 Jun 2012, 17:04 
Offline
Expert Member (SFD)
User avatar

Joined: 30 Jun 2011, 23:27
Posts: 127
Location: Diamond Creek
Seeing that pic of Brett's new gun got me thinking about barrel designs and forces. Thought I'd be lazy and avoid doing an experiment by asking the brains trust if anyone (hopefully legendary boys) has some info/spec on:

- Gun Rubbers E (youngs modulus/modulus of elasticity) or k (Hookean spring constant) values
or
- Spear velocities that their guns produce

Hopefully I'll turn it into a wacky barrel design to try out next summer.

Thanks


Top
   
PostPosted: 18 Aug 2012, 00:46 
Offline
Novice Member

Joined: 16 May 2012, 16:35
Posts: 40
I think this one's Hookes law, F = kx

Assuming we pull the rubber back as far as we can and there is a notch on the spear at this location then F (force) will be the same for all different types of rubbers, because it is governed by the spearos strength and not the rubber band.

If fluid was frictionless (water or air or whatever) then the speed of the shaft leaving both guns (gun 1 - thick rubber stretched a little, gun 2 - thin rubber stretched a lot) would be the same speed, like if they were shot in a vacuum.

I'm thinking that in real life the thicker rubber stretched over a shorter distance would have a higher exit velocity because the spear gains full velocity over a shorter distance during which it experiences friction. But it is more complicated so I'm not entirely sure.

Also, I think shorter rubbers will have a higher exit velocity because the point at which the rubber ceases to propel the spear is closer to the end of the gun, because the rubber is shorter. A longer rubber would stop propelling the spear at a point further down the gun and the spear would begin to experience drag between itself and the water earlier, resulting in a lower velocity once it has completely left the gun.

Practically speaking you could only reduce the size of the rubber to a point. eg. you cant have a rubber in the shape of a donut - extremely thick and extremely short.

Mmm, something else. If you grab the rubber and pull it back and the entire band stretches, except for the portion in your hand which remains unstretched, then when you attach it to the spear and let go of the unstretched rubber the force in the rubber would drop (because k decreases and x remains constant). This would be more significant for shorter rubbers because the % of unstretched rubber being held is larger.

So those are a few pros and cons for different rubbers, but as long as the rubbers (same material) are of reasonable length then I reckon I'm swaying towards the shorter of the two resulting in a higher exit velocity, but I think the difference would be quite small.

And obviously the 2 best way to increase the force on the spear would be to:

1 - add extra rubbers. eg. 2 rubbers = 2 times your max pulling force loaded into a rubber, or
2 - go to the gym


Top
   
PostPosted: 18 Aug 2012, 06:14 
Offline
Fanatical Senior Member

Joined: 26 Nov 2005, 18:10
Posts: 1044
You think too much. :mrgreen:


Top
   
PostPosted: 18 Aug 2012, 08:49 
Offline
Senior Member

Joined: 29 May 2011, 14:09
Posts: 170
Location: Chelsea, Victoria
The following thesis describes the way I have methodically analysed the current issue of gun rubber elasticity and shaft velocities:

1. Buy a speargun.
2. Shoot fish.

:lol:


Top
   
PostPosted: 18 Aug 2012, 11:18 
Offline
Senior Member

Joined: 19 Aug 2011, 21:36
Posts: 293
Heres my amazing insight:

The max range on my 95cm gun is approximately 4 meters. I've hit a fish before at max range (it was a big dusky kinda just sitting there - so It wasn't a particularly amazing shot).

It took LESS than a second for the spear to travel from my gun to the dusky.

Ergo we can conclude that the muzzle velocity of this spear gun is much greater than 4 meters per second.

Case closed.

Though seriously, Im sure there are manufacturing specs on these. Even if theres not, you could set up like a high speed camera (GoPros can shoot at 60 fps - might be enough) and get like a little grid pattern at known intervals... maybe one marker every 30 cm? Chuck thisin your swimming pool - fire the speargun underwater next to the grid pattern and review the footage to work out muzzle velocity.... Mythbusters style!


Top
   
PostPosted: 18 Aug 2012, 12:26 
Offline
General Committee Member
User avatar

Joined: 18 Mar 2008, 10:01
Posts: 761
Location: Shepparton Vic
Maybe a proposal for Myth busters.

_________________
Why is salt water so far away and the weather so crappy at this time of the year ?


Top
   
PostPosted: 18 Aug 2012, 12:47 
Offline
Expert Member (SFD)
User avatar

Joined: 30 Jun 2011, 23:27
Posts: 127
Location: Diamond Creek
I'll get in the shed this arvo do some experiments and let you know


Top
   
PostPosted: 18 Aug 2012, 13:27 
Offline
Fanatical Senior Member

Joined: 26 Nov 2005, 18:10
Posts: 1044
Why? Dan....get out of shed and go for a dive. Diving is for mental reaxation, is an escape from gravity and an escape from the bloody mobile phone. A recreational activity. A joy for the senses. Simply.....an uncomplicated slice of heaven. No need to question Nirvana.


Top
   
PostPosted: 18 Aug 2012, 13:41 
Offline
Novice Member

Joined: 16 May 2012, 16:35
Posts: 40
Brett Illingworth wrote:
You think too much. :mrgreen:


So if I angle the rubber at 4.75 degrees and load the rubber to max capacity then... Fuck... Where'd that fish go. Haha
I love spearfishing but I cant help thinking about this kinda stuff.


Top
   
PostPosted: 19 Aug 2012, 23:43 
Offline
Novice Member

Joined: 09 May 2012, 00:49
Posts: 1
dilwad wrote:
Seeing that pic of Brett's new gun got me thinking about barrel designs and forces. Thought I'd be lazy and avoid doing an experiment by asking the brains trust if anyone (hopefully legendary boys) has some info/spec on:

- Gun Rubbers E (youngs modulus/modulus of elasticity) or k (Hookean spring constant) values
or
- Spear velocities that their guns produce

Hopefully I'll turn it into a wacky barrel design to try out next summer.

Thanks


Was wondering about this myself earlier today so I just went through some video.
1.5m spear crossed a point on my screen in 0.1 seconds (go pro footage at 60fps, took 6 frames, possibly less)
So with pretty loose figures to work with, I get 15m/s and 54km/h

The gun is a 1m picasso asegai with 1 x 16mm rubber, fairly well worn in I think - would like to see what a new rubber would do


Top
   
Display posts from previous:  Sort by  
Post new topic  Reply to topic  [ 10 posts ] 

All times are UTC+11:00


Who is online

Users browsing this forum: No registered users and 31 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Limited
[ GZIP: Off ]