Shaft weight vs. trajectory

[AJB8]

Hey

Just wondering if there are any neutrally buoyant shaft set ups on the market? I've never seen anything other than steel shafts. If there really isn't an alternative material, then there'd have to some kind of added buoyancy to make a steel shaft buoyant, which would likely add a fair amount of drag.

The advantage would be a flat trajectory. Are there any situations where a drop reduction would be advantageous?

It had never crossed my mind until I saw a side on shot of 6 - 8 metres (can't quite remember, long though) on a spearfishing dvd. The shot missed underneath with a clear drop in trajectory.

Dunno

 

[foxfish]

You used to be able to buy fiberglass spears many years ago, they were much lighter although the spear would still sink but, you need the weight to get any range!

 

[AJB8]

Okay. That was the only material I could think of using too.

If you need mass for range and density to lower drag (i.e. can't really use a more buoyant shaft without compromising one of these two parameters), then you're back at bulky additions to float it (which won't travel so well).

Oh well. Think I got caught up imagining this laser-like speargun and overlooked the basic physics.

 

[foxfish]

Okay. That was the only material I could think of using too.


Oh well. Think I got caught up imagining this laser-like speargun .

Thats called a 120 airgun fitted with a dry barrel kit & 7mm spear

 

[spaghetti]

AJ, spear drop is facts of life and we must live with it.
A fish shot from 8 meters will have enough time to duck the shaft, because a fish is not a fixed target, but a moving one. Unless we're talking of very large fish (i.e. larger target), those 8 meters shots are pointless in my humble opinion (provided there is really any speargun capable of straight 8 meters shots, which I've never experienced and frankly don't believe much till proven I'm wrong).

As long as we are shooting "normal" sized fish (say a bass, a bluefish a dentex, or maybe a sea trout if you're in Australia?), there are two things we can do to avoid the problems of spear drop:

1) get closer to the fish.
2) Aim a bit higher than the target, so to get a parabolic effect (arch trajectory). Question in this case is how much of a bit is the right bit?

PS: Cialis, or Viagra: I'm told they work wonders against spear drop! :thankyou

 

[AJB8]

Agreed. The moment before the tv spearman guy took the shot there was a collective, "Nuh," from 3 sides of the room...and a bit of hope too I suppose. That hope was what got me thinking.

Thought I'd try run the question by some of the experts before I whipped up some kind of death trap :martial needlessly.

Thanks

 

[foxfish]

Well perhaps you could design a sort of flight like the one fitted to darts, perhaps it could be designed into a slide ring that locked into place at the rear of the spear when the gun was fired & set the spear spinning like an arrow?
Most likely not work but who knows?

 

[spaghetti]

Most likely not work but who knows?

That's where great inventions came from!

 

[AJB8]

Well perhaps you could design a sort of flight

Or a torpedo style flight, twisted around...no, twisted flights slow arrows...give it its own fuel source maybe...i'll ask my archer brother.

 

[Jonny250]

Assuming that the shaft is negatively buoyant then gravity will act the same on a light shaft as it will on a heavy one. However a lighter shaft will accelerate faster and therefore reach the target sooner, so will experience less drop than a heavy one. but as Mart said, the lack of weight will reduce the range as there is less momentum in the shaft.
However if you could build a thinner shaft and maintain sufficient stiffness, then you could overcome the light shaft speed loss.
So maybe a super-thin shaft [maybe even a flexible shaft] is the way ahead? find a way of firing it from the tip instead of from the tail... not beyond the wit of man...

 

[AJB8]

I have't information about the both.
So i am not interested to give the comment about the both to provide you the fake information.

on second thoughts...pure poetry.

 

[OregonSpearo]

A neutrally buoyant shaft may have a perfectly flat trajectory but it will slow to a grinding halt within a few inches of the gun. It won't retain the same velocity as a heavier shaft which, granted, will sink.
An analogy would be a train and a bus with the same surface, both going at the same speed and shifting in neutral at the same time. The same air resistance slows them down because they have the same surface, but the train will travel further because its sectional density is higher (more weight behind each square centimeter of surface).
Underwater, fluid frictions are much more intense than in the air and this is amplified.

 

[Jonny250]

Hey orgeonspearo, nice analogy, just take that one step further; what if you have to use the same amount of power to accelerate the train and bus in the first place? I.e. Same bands fitted... The bus would be long-gone...

 

[AJB8]

I imagine that bus would probably damage a fair amount of everything on its short way, shoulder to shoulder...then end up on the tracks. The engineer would see it up ahead, upside down and crumpled. Most likely he'd decry, "God damn my superior cross-sectional density."

Probably he'd still hit the brakes.

 

[AJB8]

Really, if I had have taken the time the think about perhaps the most important statistic for any projectile, I'd have realised that being underwater spear shafts need amazingly high ballistic coefficients, i.e. mass of projectile per cross-sectional area. I thoroughly concur with Jonny250.

 

[OregonSpearo]

Hey orgeonspearo, nice analogy, just take that one step further; what if you have to use the same amount of power to accelerate the train and bus in the first place? I.e. Same bands fitted... The bus would be long-gone...

Correct, the initial speed of the bus will be much higher. However fluids exert a friction force that is proportional to speed. Let's say the bus starts off twice as fast. The wind resistance's force will be twice as strong on the bus as on the train and it will slow down twice as fast. For a time the bus will lead but there is a break even distance after which the train will pass the bus. At that very point both the bus and the train will be going at the same speed and will thus be subject to the same wind resistance. However the train is heavier and will be not as affected by it whereas the bus will slow down faster, meaning the train will pass it.
Same with the denser / lighter shaft under water. For a time the lighter shaft will be ahead but there's a break even distance at which the heavier shaft will catch up.
Another thing to keep in mind is "knockdown power". It's a ballistic measure that quantifies how much damage can be caused by a projectile and is basically the projectile's kinetic energy. Here, mass comes into play: E = 1/2 m * v^2 where m is the mass and v the speed.
When the bus and the train have slowed down at the same speed the train is still heavier and so its knockdown power is greater (has more energy). Same thing with the heavier shaft.

 

[Jonny250]

yes i'm right with you OS, but i just think that a thinner lighter shaft will still 'win' over the sorts of distances that we usually shoot at. In my case thats a few metres at most. Thats probably the key eh?
Another advantage of a lighter shaft is that if i incorrectly estimate the distance to the target, then a lighter shaft would be slightly more forgiving - less drop and shorter time to target [imo] so perhaps another benefit.
As i mentioned in my first post, the key would be a light shaft with a smaller diameter, but i suppose the critical payback of diameter and Cd Versus weight is really hard to calculate. I'm familiar with the KE and m.V calcs, but not sure about the efficiency of the bands for the KE calc; Archery info shows that a heavier arrow is released more efficiently...
Anyways, maybe it is all a load of poetry and we should just thrash out such daft ideas over a beer!! maybe a virtual beer lol.

 

[foxfish]

This sort of discussion is great.....
I think Tromic is the only member who actually carries out real life test of this nature..
http://forums.deeperblue.com/pneumatic-spearguns/91007-measuring-speed-shaft.html

 

[Jonny250]

The speed testing is clever stuff Mart!

I have just ordered a couple of carbon fibre arrows...

 

[OregonSpearo]

Johnny,
Yes, there is definitely a distance within which the lighter shaft is hands down more effective. Increasing that distance as much as possible is the holy grail of speargun building for kelp/reef hunting since we don't usually take a shot past a few meters due to visibility or accuracy/size of fish. In bluewater hunting things are different but it's almost another sport altogether: lots of power, thick shaft, maneuverability is not a concern, sizable fish.

I think that underwater a lot of concepts pertaining to aerial projectiles don't apply and ought to be revisited.
Take "knockdown power" for example. Bigger bullets are used to hunt bigger animals. The reason being that you don't want to animal to start running after it's hit. Well with a speargun the fish becomes tied to a line after it's hit and the shaft's weight has little to do with it. In fact, if instead of a shaft a speargun was able to propel a fishing hook weighing a few grams onto the fish, the fish would still be caught (concept behind slip tips). But a fishing hook has little stopping power as a projectile.
The main issue with a thinner/lighter shaft is that they flex a lot during the propelling phase on an overpowered gun and accuracy is real bad. Lately manufacturers have been addressing this with the following products:
- For band guns, 6mm steel shafts wrapped with carbon fiber, making a light shaft that flexes little such as this one from Coralign:

- In top of the line pneumatic guns, shafts are getting thinner and so is the piston to achieve faster launch speed. The shaft is still made of steel as it is enclosed in the barrel and does not flex as much as with a band gun (like an enclosed track). Such systems include the Omer Airbalette, Sporasub One Air, and the Seac Sub Caccia. And the original Tomba and Mamba kits these are "inspired" from.