Posts by kmo

Hey Xan, I don't think that you can use the Work = Force x Distance calc in any case, as the force is a function of band elongation. The energy would be the area under the graph of force vs elongation,
so you would use Grossetti's function of 1/2kx^2
But Grossetti's calculation is flawed because stiffness is inversely proportional to length.
As k=EA/L, and the theoretical roller gun in Dan's example has a rubber length of twice the normal gun, its stiffness is half of the single band. As there are two parallel rubbers for the double band gun, its stiffness is twice a single band.
Stiffness
Single band k = 1
Double band k = 2
Roller band k = 0.5
Elongation
Single band x = 1
Double band x = 1
Roller band x = 2 (as it is half as stiff, you can stretch it twice as far for the same loading force)
Energy
So applying E=1/2kx^2
Single band E = 0.5
Double band E = 1
Roller band E = 1

So Dan was right all along.

The basic difference is the same as applying two springs in series (roller) instead of two springs in parallel (two band gun).
Series and parallel springs - Wikipedia, the free encyclopedia


What Dan is basically saying is that the potential energy captured by the two bands is the same for two bands in series or parallel if the strain is the same, which checks out with physics if we assume rubber is a linear spring. If we convert potential energy into kinetic energy don't we then get the same speed?
As the rubbers on a roller are in series you need less force to get the same strain than the same two rubbers in parallel. If the limiting factor on loading is arm strength you could theoretically apply more strain to the rubbers, have more stored energy and get a higher speed on firing. But that assumes that you load the two rubbers in parallel together. Because we load the two bands one at a time I would have to do the maths which is all getting to hard so I'm going back to my single rubber pipe gun.

Wasn't there was a massive thread on the forum about rollers which was full of graphs and maths and tests? I can't find it unfortunately, but I remember seeing it and thinking I should spend some time trying to understand it at some point. It was about 12 months ago from memory.

Dive with a partner and you statistically halve your chances of shark attack You also get a set of eyes for the back of your head, more intimidation ability and an extra poking stick.

As a contrast, I was just reading this yesterday:
Freediving myth #1

Quote from Reefchief

Based on the freediving science stuff ive seen (people doing science of V02 min etc) there are definitely physiological adaptations and dive reflexes etc that get more pronounced as you train for diving and dive more. For instance, the slowing of your heartbeat.

I do think c02 tolerance does play a large role as well, which is scary because its exactly as you said, your body being able to tolerate being closer to blackout. This is exactly why statics and c02 tables are horrible. Its purely increasing your ability to hold your breath without at all increasing your ability to conserve oxygen.

I'm sure that it would somewhere in the middle (physical vs mental tolerance) but for someone who used to dive once a week (before kids) and now more like once a fortnight I do wonder if the physical improvements continue to apply. As an example, if I'm on a dive holiday and in the water a few days in a row my bottom times increase dramatically towards the end of the trip. I doubt that short term adaptation is any actual improvement in my ability to ward off blackout and I wonder if I'm just getting closer to putting myself in danger.
Being confident, relaxed and streamlined reduces oxygen consumption and I'm sure contributes to short and long term gains in bottom time and depth, and I think much more than the long term physical adaptations. If you are pushing at your limits regularly, like a competitive freediver, then the physical adaptations would play a larger role.
I agree with Marco, 1:30 is a long dive and plenty for what I would consider deep spearfishing.

Bump ... Any fish with the weapon yet? I saw in another thread that you are back in the water at least

I do often wonder if developing the ability to dive longer and deeper as the years pass is a result of a physical adaptation which improves my ability to suvive and function at low oxygen levels, or just a psychological adaptation that allows me to be comfortable pushing myself closer to blackout/death.

Mate's spear pierces diver's chest
I'm pretty sure the photo from the cliff top in the first post is Brett.

I'm so sorry for your loss Andrewk, the rest who have lost a friend, and of course Nate's family.
I know that it is hard not to replay the scenario over and over in your mind and think how you could have reacted better. It sounds to me like you made a really good decision in a terrible circumstance to dump the anchor immediately and get help on the way ASAP. Many would have delayed, hoping for the best.
Try not to be hard on yourself, grieving the loss of a friend is enough to deal with already.

I have one made by Rob Allen imported from South Africa. If you look on the Rob Allen website they are available in carbon steel, but I'm not sure how you'd order from there. Page 11 of his catalogue:
http://www.roballen.co.za/prom…ELECTRONIC_CATALOGUE_.pdf

Aussie Reels - Home
Barry's reels

I guess you need to weigh up which is more likely - getting trapped in a wreck or blacking out with no overhead obstacles. Given most blackouts are near the surface on ascent, I think that your overall risk profile would decrease wearing the FRV even considering the potential for your scenario.
Lots of safety devices are the same - you can die if left hanging in a fall arrest harness but it is less likely than dying from falling from a great height without one.

Here in West Oz we are one of the last states in Australia which still allow spearfing on scuba. It comes up for discussion fairly regularly when Fisheries are reviewing the state of our fishery and proposing new laws. Fisheries statistics, mainly from boat ramp surveys, show that less than 1% of our fish are taken by spearfishing, with a very small proportion of that from scuba. Based on that data, scuba spearfishing remains legal.

What they have done here though is introduced tight daily bag limits (2 prized demersals and 3 prized pelagics) and a 2 month closed season to prized demersals every year. With these measures it really doesn't matter how you are catching your feed of fish, when you come home with two fish there is a pretty good chance you have left some behind.

Like others have said, having the scuba community on side during these periods of debate has been a huge blessing for us freedivers. In other states of Australia the call for marine parks comes most loudly from scuba diving charter operators and clubs. When half of their members are extracting either crays or fish the scuba divers tend to leave us alone.

Enforcement here is pretty tight too, and penalties are severe.

Quote from LunkerBuster

Shame he is anti spearing.

I organised for Erez to do a free talk to spearos on spearfishing safety in West Oz, and he was in no way anti spearing. I know most if not all of his instructors and students are keen spearos. From what he was saying he has done quite a bit himself.
He has given heaps to the spearfishing community in Australia, including the talk in the original post to a spearfishing club.

I've done a bit of diving and drinking over the years with Andre from Terra Australis, it's been a while though. They are based out of West Oz and commercial divers with heaps of time on their hands. Good spearos.

Burleying up the beaches should do the trick.
This policy is causing a lot of heated discussion down here. More political huff and puff than anything though at the end of the day.
This is the Govt media release:
Pages - Statement Details

Quote

Premier Colin Barnett and Fisheries Minister Troy Buswell today announced new measures to address public safety and help mitigate the risk sharks pose to water users.

These include:

•Setting baited drum lines to catch large sharks one kilometre from shore, with vessels monitoring the drum lines. These drum lines will be set along heavily used beaches in the metropolitan area and the South-West, and will be deployed 24 hours a day initially from January 2014 through until April 2014.

•Boosting the response to shark attack by immediately setting drum lines, leaving them in place for longer and setting them in a wider area. More vessels will be available for faster response to an attack.

•The long term establishment of specific Coastal Shark Management Zones along the coast, to be determined by geographical and environmental features and water use profiles (for example, swimming, surfing, diving).

•Developing a ‘Tool Kit’ for communities in each zone in partnership with the State Government to mitigate the risks of a shark attack at local beaches. Measures could include education pamphlets, aerial and beach patrols, signage, providing beachside trauma packs and the deployment of drum lines. Each plan will be reviewed annually.

•A community recovery policy to ensure support for communities affected by a shark attack.
Mr Barnett said the new measures would improve public safety and build on the State Government’s strong approach to shark hazard management.

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We lost another young father in my home town this weekend out doing what he loves - it's bloody hard to stomach people chasing them around with go pros at the moment :crazy:
Man dead after Gracetown shark attack | Augusta-Margaret River Mail

G'day Stoj, welcome aboard.

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Greg's telling his story on TV this weekend. So stoked that he made it out of this in such good nick!