Spearfishers and freedivers train rigorously to attain those depths and bottom times. It takes years of experience to be able to know your bodies limits and to learn the proper techniques for diving like this. I highly recommend you take a freedive course, or at the very least, read some literature on safe freediving technique.
It is possible with
Education,
Mental training,
Physical training,
Safety training,
Good health,
Good gear,
Great Partners,
Planning.
Dive safe,
Josh
Exactly,
I doubt most rifle barrels are the "optimal" length for muzzle velocity, but rather cut shorter to improve maneuverability, as well as to keep the barrels relatively thin, but still stiff.
back to spearguns,
Ladvr is correct, however I'd use the equation to find a starting point, and begin to tune your speargun from there. When you find this sweet spot between accuracy and power you'll be golden. In most cases, you arent shooting fish from 20-30ft away (unless you're George Steele), so when your bands start to degrade it shouldnt affect your shot too much, and when it does, you know its time to replace your bands.
Dan,
using the rifle analogy;
the cartridge (powder/primer/case) is the rubber,
the bullet is the shaft
the barrel is the track
and the rifled is basically a REALLY well tuned speargun set up for the cartridge to be fired.
if there was not enough powder to overcome the frictional forces of the barrel, sure it would slow the bullet.
But these rounds have so much power behind them, when the expansion of gasses occurs upon firing, the bullet has no where to go but straight out the muzzle of the barrel. With so much pressure, the bullet will continue to accelerate until it leaves the barrel at its terminal velocity (muzzle velocity). After leaving the muzzle, the bullet will have no accelerating force upon it and will continue with the momentum it has achieved in the barrel, slowing down with the friction of the air until it reaches its target, or drops to the ground.
For the barrel to slow the bullet down it would have to be comically long, or the bullet would have to be chambered wrong (too tight). most likely resulting in an exploded receiver.
This is basically the principal behind the argument for enclosed track spearguns. When overpowered, the enclosed track acts like the barrel of a rifle, enclosing the shaft, and directing all forces forward. So even if the enclosed track presents more surface area to cause friction, the overpowering of the bands should overcome these forces.
When i started trying to calculate band lengths for different guns I use and build. I started a little simpler. I basically measured the barrel length (on pipes) or the length between the mech and the muzzle holes (on wood).
I then divided that number by 3
and multiplied that number by 2.
ex: 100cm/3=33.3 33.3cm X 2 =66.7
simpler equation with similar result
I find that this band stretch is really smooth for pipeguns utilizing a 7-7.5mm shaft and two 16mm rubbers or 14mm small id.
I have modified the equation for larger bodied wooden guns to
barrel length / 3.15 x 2
this gives the shot a little more power.
but this is all subjective, and based on your own opinion and feel. I know guys who run bands long, for a smoother shot, and i know guys who run bands short for more power.
I hope this makes a little more sense than the spearit guide.
The most efficient rubber powered speargun (or any speargun for that matter) would have a band stretch (% elongation) as long as the speargun itself. This is the reason roller spearguns, and pneumatic spearguns are so "powerful". They utilize nearly the entire length of the gun stock.
For a "conventional" circular rubber, screw in rubber, or tie in rubber speargun, the length of the band at 0% stretch is "wasted".
The spearit calculator calculates the "optimal" band length based on the distance between your muzzle and your loading tab.
The number you input is the distance between muzzle and loading tab (D)
The number in yellow in the black box is your optimal band length (total circular rubber)
This is based on elongation which can be inputted
Their formula is L=2D/(E+1)
ex: L=2[100cm]/(2.0+1)
L=66.7cm or 26.2" this E+1 is elongation or %band stretch so (200%+100%) the (1) is the untensioned rubber
They explain it as 0% being an untensioned rubber but in the formula an untensioned rubber is 100%, anything beyond that is tensioned.
Instead of using the spearit calculator as the answer to your band setup questions, use it more like a guide line. Sure the calculation may work well on one speargun, but there are too many factors to consider, mass of the gun, mass of the spear, mass of the bands, wishbone length, type of track, balance of gun, fit of the handle, etc...
If the question is which band setup will propel an identical shaft further, yes the longer power-stroke with the same stored energy will be faster off the sling.
But if the question is the amount of stored energy, they should be the same with the same % elongation.
Look at rifles, you can have two rifles chambered in .308. If shooting identical ammo, the ammo has the exact same stored energy. But if one rifle has a longer barrel, the bullet will have more time to reach max muzzle velocity................... wait what forum am I on?
Jon, Dan, this thread and these video examples are truly eye opening! This is a hazard that many freedivers and spearfishermen are totally unaware of, at least where I live (Hawaii).
I like the idea of knowing the bathymetry of the area you are diving to better understand where water is moving. I can also assume that areas where there is a large tidal shift are more prone to such phenomenon, as well as areas where two currents or even bodies of water converge (like Cameron Kirkconnel's example of the Pacific and Indian ocean emptying into one another. All of this is good information to prepare divers and prevent accidents from occurring.
I assume that where there are down currents this strong there may be up currents with similar strength, (harder to descend) or is the dynamics of these currents primarily based on gravity, and these up currents I am describing result in upwelling.
mosaad, not sure what youll be using the powder for. Mixing with epoxy?
in any case, copper does react with salt water so I wouldnt use it for a marine application.
I have a few scraps of abs, ill do some testing as you mention, uv/saltwater exposure over a few days
Ah uv resistance, ABS sucks at that apparently, but if coated with a UV inhibitor (ie paint, epoxy) it shouldnt have too much of a problem. Abs seems to be porous so salt water may have some effect but i haven't tested this.
One reason for this interest in abs as a track material candidate is that the Cressi Comanche line of speargun handles and muzzles were made from ABS (not sure if this is true today) but they seem to do ok, I have a Comanche that was originally purchased over 10 years ago and it still holds up to my abuse.
Hi all you experienced Gun builders out there,
Wondering if anyone has an opinion on using ABS plastic as a suitable material for enclosed tracks. Ive been toying around with the idea for a while but havent brought myself to test it out yet.
Reasons I believe it may be a good material for tracks are;
1) relatively cheap material to buy in sheets blocks or strips
2) relatively good adhesion to wood using epoxy (as compared to acetal, nylon, uhmw, hdpe, etc..
3) good machinability
4) very good impact strength as well as tensile strength
I realize many will think "Why not use epoxy, graphite, chopped fiberglass etc...?", or "just rout a wooden enclosed track"
While I do see these as the industry standard of enclosed track materials, I'd like to try something different.
Also I feel like I'm just pouring money down the drain whenever i pour an epoxy track that I will rout 60% of the material from it anyway.
Also, chopped fibers EAT jointer, planer and router bit cutters.
Id love to hear your opinions,
Thanks,
Josh
Just make sure your pilot holes are the right size.
I generally add a drop of thinned epoxy (epoxy with a bit of acetone) into the pilot hole before installing screws. This will penetrate into the wood and prevent swelling at the screw hole. It isn't all that necessary, you can just as easily drop some teak oil into the pilot holes with little swelling.
That is some scary stuff. I dont think I've ever experienced a down current to the degree of those videos, but there are areas where I dive that have what I describe as "heavy water" where ascending feels like it takes ages.
Thanks for this very important information.
condolences to his family and friends.
Definitely a true innovator and visionary of speargun design. always willing to share information of his one of a kind builds. A big role model for me. Sad day.
barnacles on turtle shells is not uncommon, though this looks like an extreme case. The turtle could have possibly been injured or its shell damaged or even just cleaned too well. For barnacles like that (look like goose necks) to settle and flourish on a turtles shell, the shell would need to be free of other other stuff growing. Probably why its best not to wipe the algae off turtle shells or touch them unnecessarily if possible. If the turtle was injured for a good amount of time it could possibly have not been able to get to the "Shell station" (bad joke) and get cleaned by fish or shrimp etc...
or maybe the turtle is screwed now cuz you took off his awesome pikotec camou.
Thanks for the vid.
band length should not matter, as long as %elongation is the same. They will both reach 0% elongation a the same time and have the same power stroke. ( Im assuming these bands are in the same muzzle hole but anchored on different loading tabs)
This is in a vacuum, however.
In practice. a longer and heavier band will throw more weight forward and probably have a bit more drag.
I like to stick with the least amount of band mass possible using small ID rubber achieves this.
In my opinion the most efficient set up on a traditional multi band gun is allowing your bands to have the longest power stroke possible. having your bands and your loading tabs as far apart as possible.
stick some masking tape on the trigger mechanism
locate the line release slot
trace or cut out the slot in the tape (exacto knife)
also outline dorsal and ventral edges of the trigger mech profile
then stick the tape to your stock
You seem to be a pretty handy guy, you could use the tape to make your own jig out of a piece of angle aluminum.
just make sure you take into account the thickness of the angle aluminum piece
Hope this helps.
Josh
Looks good Dan.
Amazing what people will do for the likes.
...also why take a selfie without first shooting a fish?
what s your recommendation for spear OH ? for both 7.5mm and 8mm
I like to keep shaft overhang to a minimum, especially using heavier gauge shafts (7.5 and up) this will keep your gun from being nose heavy.
You'll have to figure the exact length of overhang out on your own, find whats comfortable for you and the gun you are using.
In general pipe guns ( 26mm id carbon fiber tubes).
7mm shaft = Barrel length + 40cm
7.5mm shaft = BL + 30cm
8mm shaft = BL + 20-30cm ( I dont recommend going this thick on circular barrels, unless it is a polyplast, demultiplied or single roller)
380% may be a little too much. with multi band guns i try to stay around 315%-330% stretch...using 16mm rubber as well as small ID 14mm rubber.
Low shots and loss of energy could be caused by muzzle flip and shaft whip.
Try to find that sweet spot where the shaft can handle the force of the rubber without bending (that's what she said):D
I find railguns and wood guns will perform well with this band stretch, where i believe enclosed tracks can handle a bit more force and not whip....it is not needed. If youre using a wood gun or enclosed track, dont overpower it, utilize the mass of the gun to your advantage: reducing recoil.
also 33cm seems a bit long for over hang especially using a 7.5mm shaft. this may be causing the shaft drop
Im assuming 380% is counting the relaxed rubber as 100%.
