I have just built a large, what we call bluewater speargun which is to be used for hunting pelagic fish like tuna marlin and wahoo. For this kind of hunting you often shoot quite far (up to 25 feet+) and complete penetration of the fish is vital in order to deploy the special kind of tips we use to hold them to the line.
The shaft is powered by rubber bands, in this case 5 of them so the propulsion energy is constant. So is the Drag coefficient, and we can assume that a thicker shaft will have a slightly lower ballistic coefficient.

Most people use a 3/8 inch shaft for this kind of gun, where I have built mine to take a 5/16.
11/32 shaft are also available but from one company only, and in the US. (I live in Itlay)

The shaft weights:
5/16 (8mm) - 700grams
11/32 (8.7mm) - 840 grams
3/8 (9.5mm) - 1000grams

Here is the question: Given that the propulsion energy and drag coefficients remain the same, how much more kinetic energy will the heavier shaft take downrange?

I am asking this question because I do not have access to any of the 11/32 or 3/8 shafts, so I cannot test it before I go modifying the guns' barrel channel to take a thicker shaft.

I do however have a 9/32 (7mm shaft) that I could use to compare to the 8mm and hope that the differences might remain relatively constant between an 8mm and the 8.7mm. Should it hold true??

Anyone got an idea?

Hello,
I have not the foggiest idea of your sport, but would think you would want to know the velocity of the shaft and knowing the mass of the shaft, drag factor, etc., you could arrive at the energy number?? Have you access to a chronograph?? Furthermore, the various archery firms, of which there are many in the U.S., do have "numbers" on their shafts performance and the load factors involved. As to how the shaft reacts underwater is total mystery to me, but the archery firms may be able to point you in the right direction/calculations to arrive an answer to your original question. You can do a search on archery mfg.'s and some of their engineering folks may like the challenge of solving your problem?? Not much help, just a suggestion. Would think it amazing to do what you are doing and good luck.

PS Could you not custom machine the desired shafts providing not punishing cost wise?? How many shafts would you need, handfull?? Again, just a suggestion.

Well, I have been thinking about this, and the number I really need is the ballistic coefficient of each shaft, in water.

See, the kinetic energy will be identical regardless of the weight since the propelling energy is the same.
Since water is so dense it has a very dramatic effect on flight of objects, here BC becomes the most important part of the ballistic equasion, whereas with rifle bullets, it is a relatively(comparatively) inneffectual factor for most circumstances.

This will be determined by it's hydrodynamics, which to a point will be also effected by the way you attatch the line and what kind of point you use.

I have a chronograph, but I doubt it will work underwater since it works on light being reflected back to it and would be thrown off by the water water effects light travelling through it. Also it would not be possible to waterproof it in any way that would allow it to still function.

There comes a point at which the rubbers are labouring to transfer their energy to the shaft and the launch becomes inneficient. Likewise, a too light a shaft will be initally propelled much faster than it's underwater BC should allow, therefore it would slow down very quickly.

Other problems involved are also recoil and shaft whip(rigidity), which both effect accuracy, but with this gun those factors should be minimal due to the enclosed track and the large mass and stability of the gun.

It is a given that the heavier shaft will carry more punch downrange, however I need to find out how thick a shaft I can shoot before it starts to loose effeciency, or how long the lighter shaft will continue to fly with enough speed.

Unless I can chrono the shafts underwater at the muzzle and at the target in order to extract their BC the best bet is going to have to be testing the 7mm and comparing that difference to the 8mm then drawing some conclusions from those results, judjing them simply by appearance of speed in the water.

The most effective way to do this would probably be to set up a high speed camera with a backdrop that allows to you visualize and time flight, as well as drop. But there is no way I am going to be able to get that kind of equipment...

I was thinking that maybe, just maybe, if I could chrono these shafts in the air both at the muzzle and the target then I could get a BC of sorts.
Then I could multiply this BC by the difference in density of seawater compared to air and get an answer.

Something tells me that this wouldn't work, since fluid dynamics are not that linear, nor is the BC of an object weighing a kilogram initally moving at say 100fps going to be effected enough by drag as much as simply running out of energy and gravity.

It would be so simple to test but I don't have the shafts I need and I live an hour away from the sea, so I am trying to nut it out in paper before I commit to anything concrete.

Thanks.

Express, it's been a long time since I did any spear fishing but I remember my surprise with the short range of all the guns I tried. Basically, it seemed the EFFECTIVE range was limited to the length of the shaft!

I mean, the fish had better not be much further away from the spearhead than it was long. Water slowed the shaft so quickly there was insufficent force to drive the point through even with a good hit if we shot longer distances, it would slow and just push the fish aside. Thus, we went to longer guns and shafts, but never longer than 4 ft., it's to hard to handle longer ones even on the boat.

If you are getting hits with penetration at distances of 25 feet I'd say you are already in the range of magic!

But, there ain't NO way to do this by shooting in air a equating that to water at some 900 times the density of air.

I have done a little spear fishing.

Given the equipment you have, one test you could do is to take both shafts to the water and shoot them through something, plywood, a foam archery target [might be hard to hold that under water ], but into something that would give you an indication of the shafts "power".

You could do the test at different distances to see the effect of the different mass of the 2 shafts you have access to.

You could also do the test with different a amout of power, [number of "rubber bands], and different points as well, to see how that effected performance.

quote:

Here is the question: Given that the propulsion energy and drag coefficients remain the same, how much more kinetic energy will the heavier shaft take downrange?

As long as the energy you are putting into the system is the same the energy you get out of th system will be the same regardless of the weight (mass) of the projectile.

A lighter shaft will exit the system faster and will get to the target in less time hence a better trajectory.....but the remaining energy will be the same as a heavier shaft.

Express,
I certainly understand the problems w/ chronograph under water and was not suggesting that;however, would be great to do so, but doubt anyone set up to do that other than Navy Seal teams!! My thought was that if you had a number of velocity out of the water and then determine the drag, resistance, etc. in the water, one could I think correlate the two environments and make some reasonable assumptions from that point forward?? Again, pretend to know nothing about spear fishing nor the flight of shafts. Apparently as pointed out by others, the drag is obviously considerable.

quote:

Originally posted by EXPRESS:

It would be so simple to test but I don't have the shafts I need and I live an hour away from the sea, so I am trying to nut it out in paper before I commit to anything concrete.

Thanks.

Speaking of concrete, how far do you live from a swimming pool?

May I suggest you drop the whole theoretical approach and make some practical tests on targets of varying thicknesses and densities in a nearby wimmning pool?

Sounds like your guns are a modernized, more complex update of the old surgical tubing powered "Hawaiian slings" we used when I lived in Hawaii and did a lot of skin diving and spear fishing off Oahu and Maui.

We found testing and practice in about 12 feet depth of water to be a very insightful education as to their effectiveness, especially for newbies to the sport.

If you want to stick with the theoretical aspects, then I'd try establishing e-mail contact with some guys at the U.S. Naval Academy. I'm sure some of those budding hydraulic engineers could help you out.

The BC's of your spear shafts may not be as important as their Reynolds numbers.

Kinetic energy being equal, the heavier projectile will have more MOMENTUM, therefore greater penetration. If penetration is your most important factor, then go for the largest shaft. The difference in sectional density (and closely related ballistic coefficient) will not be significant.

Trajectory is much different in water than in air. The density of the shaft is so relatively close to the density of water that the effects of gravity are greatly offset. In other words, the tendancy of the shaft to float (or sink more slowly in the fluid medium) makes calculating trajectory entirely different than in air. In air, the boyancy of the projectile is negligible, meaning that its tendancy to "float" is not a significant factor in slowing the rate at which it sinks. On the other hand, if you were using helium balloons for projectiles, you would find that they have a positive rather than negative trajectory in air (as well as a really crummy BC). Similarly, wooden shafts would also have a positive trajectory in water.

If you used a weighted wooden shaft, could you have a perfectly flat trajectory? Penetration would be sacrificed however.

You could resolve the issue of smaller diameter and light spear, by going to a smaller diameter that weighs the same as the bigger one. You could change materials or weight the spear, most probably at the point. Friction is also a factor of contact between the surface of the spear and the water. I read that our swimmers wear special suits that reduce the resistance in the water. Could this technology be applied to your spears?? Kudude

Tyler you have a very valid point there, if I could find a shaft that was neautrally bouyant but still heavy and stiff enough that would be ideal, but I highly doubt that there is such a material. Possibly a sealed titanium tube with an air volume inside it would work, but it would be a nightmate sourcing the material and experimenting with it to find the right length/internal air volume. Very costly too, and these shafts often get bent up by fish like tuna.
A wooden shaft just wouln't be strong enough to withstand the forces involved once a a heavy fish has been speared.
Tomorrow I am going to go try it in a friends' pool, I have made two trips to the beach here but it's a nightmate in August with to many people, so few parking spaces and recently, 2 meter visibility.
Only problem with the pool is that it might not be long enough to shoot in there withought risking breaking either a shaft or the pool.
I haven't gone to a public pool, which would probably be reluctant to let me in there with my gun.