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Shooting fish

A place to ask general spud cannon related questions.
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Unread postAuthor: Ragnarok » Sun Mar 08, 2009 11:41 am

Sure, if you're using a 5mm barrel, you could disregard the sabot. I was thinking in terms of using a higher bore barrel.

You wouldn't need to be underwater, but if firing into water, one must remember that both light and ballistic trajectories bend at water's surface. (i.e. in terms of the projectile, firing at a shallow angle relative to the water might cause it to skip off the water's surface rather than passing through it.)
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Unread postAuthor: john bunsenburner » Sun Mar 08, 2009 11:50 am

and if i sprayed a bit of soap onto the water before fireing? How long shoudl the darts be? And what should I do tot he tip, just sharpen it or soemthign else?
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Unread postAuthor: limbeh » Sun Mar 08, 2009 11:53 am

A lot of kinetic energy is usually lost in the transition from air to water, you can't help it anyway so with such massive loss no point with the sabots. I'm a swimmer, I know that well enough.

Now something you'd have to consider is a compromise:

What is good hydrodynamically may not be so good aerodynamically.

Although the same basic shape (thin ends & fatter middle) works best, the dimensional ratios to get that achieve differ greatly for both air and water. Or rather, just heck it, file down the ends of the 5mm rod so that the ends are like sharp points.

I was thinking though, why penetrate underwater targets when you can blow them up with a explosive round? A near miss could still be lethal from the pressure wave from the blast.

And someone like you having just about every lethal chemical in your cellar and a vast base of chemistry knowledge should have little problems manufacturing the stuff.
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Unread postAuthor: john bunsenburner » Sun Mar 08, 2009 12:09 pm

well seeing as how i prefer my fish in once piece over fish minced meat in a lake I think my "vast base of chemistry knowledge " is of little use to me(the above is ment as a joke).

Now seriously, I ont mind fireing under water using wate rinstead of air as a propelant if that helps. Otherwise I would be very grate ful if you could either find a picture or make a very simple diagram of a hydronomic dart fecause you discription is slightly confusing to me. Thanks for the help every one!
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Unread postAuthor: THUNDERLORD » Sun Mar 08, 2009 12:27 pm

I was thinking in the past about a long hollow bullet similar to a pipe only the back inside diameter would be slightly more narrow to help guide it, like an internal fin(s).

The problem is that water would enter the muzzle and the projectile would weigh much more due to the internal water so the round would be much heavier and slower.

My solution would be a light wieght foam internal "sabot" with wadding at the rear that breaks free on firing. ( The force at the front would set the foam loose and the round continues straight as water passes through it).

I think a long hollow, tubular shaped round would be most effective and accurate under-water.
Otherwise a long narrow spear others mentioned would work second best IMO. 8)
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Unread postAuthor: john bunsenburner » Sun Mar 08, 2009 12:33 pm

I think that if you fire underwater then the water inside the tube will not make a difference as there is water around it too!
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Unread postAuthor: THUNDERLORD » Sun Mar 08, 2009 12:45 pm

john bunsenburner wrote:I think that if you fire underwater then the water inside the tube will not make a difference as there is water around it too!


But think about it, The muzzle velocity of a projectile is highest when it exits the muzzle.
(underwater or not).

The projectile weight is one of the largest factors in it's velocity.

Therefor if the projectile is accelerated at a lighter weight (internal foam wadding) into water outside the muzzle, rather than the combined weight of the projectile AND the useless water inside it, It should move faster right? :roll:
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Unread postAuthor: john bunsenburner » Sun Mar 08, 2009 12:51 pm

no as the foam doesnt let water pass through it is not hydro dynamic, also the foam captures air it will make the projectile flaot upwards. The water inside the projectile will add NO WEIGHT under water, thats like having a bottel at 100psi in a room of 100psi, you could unscew the cap and no air would come out.
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Unread postAuthor: THUNDERLORD » Sun Mar 08, 2009 1:00 pm

john bunsenburner wrote:no as the foam doesnt let water pass through it is not hydro dynamic, also the foam captures air it will make the projectile flaot upwards....


You must have misunderstood, A "sabot" which is usually an external "suit" and wadding (behind the projectile) both break free just after exiting the muzzle.

... The water inside the projectile will add NO WEIGHT under water, thats like having a bottel at 100psi in a room of 100psi, you could unscew the cap and no air would come out.


I think you are mistaken.
Water has the same mass underwater as it does above water. And with water inside the muzzle, the projectile will be accelerated slower due to more mass.

I am running out of time on-line today though. :( 8)
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Unread postAuthor: psycix » Sun Mar 08, 2009 3:23 pm

Please note that when you think of triggering a cannon underwater, the shockwave is NOT damped by a compressible gas (air). The incompressible water will transport the shockwave very efficiently, making your swimming session, well, unpleasant.

--

I think that if you fire underwater then the water inside the tube will not make a difference as there is water around it too!

The water inside the projectile will add NO WEIGHT under water, thats like having a bottel at 100psi in a room of 100psi, you could unscew the cap and no air would come out.

There's a difference between weight and mass. Even though the buoyancy is neutral and thus there is no notable force of gravity, the mass is still the same. The water will do so goddamn heavy that you effectively are firing a way heavier projectile.
What you are saying is the same as stating that something in space doesnt weigh anything and thus has no mass.
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Unread postAuthor: john bunsenburner » Sun Mar 08, 2009 3:25 pm

ok, so it doesnt matter if I use compressed water or air as a propellant under water?
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Unread postAuthor: Eddbot » Sun Mar 08, 2009 3:45 pm

just as a side note, if you aim at something underwater, while you are above water, you will miss...

you have to account for the way light bends when it's medium changes from water to air, if you are looking straight down on the water it increases your chances of hitting something, but if you plan to shoot at something underwater from the shore of a lake for instance, you have to know how much the light is bending so you get an accurate shot, and that's assuming you manage to get your projectile to stay straight on the transfer from air to water...

just my $.02
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Unread postAuthor: john bunsenburner » Sun Mar 08, 2009 3:46 pm

Thats why i will be underwater, ill try to make a vid!
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Unread postAuthor: psycix » Sun Mar 08, 2009 5:29 pm

john bunsenburner wrote:ok, so it doesnt matter if I use compressed water or air as a propellant under water?

Gases are compressible.
Liquids are (almost) incompressible. For the pressures you are using, compressed water does not exist.

john bunsenburner wrote:Thats why i will be underwater, ill try to make a vid!

psycix wrote:Please note that when you think of triggering a cannon underwater, the shockwave is NOT damped by a compressible gas (air). The incompressible water will transport the shockwave very efficiently, making your swimming session, well, unpleasant.

Beware, you could blow your eardrums into your brain (or at least feel like that). We spudders do not know how big the described effect is, but it is a potential hazard. Please be careful.
Maybe first fire the cannon at a low pressure underwater, while also being underwater, 10 meters away. Gradually take it up.
Dont take the risk of effectively holding the gun point blank against your head.
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Unread postAuthor: D_Hall » Sun Mar 08, 2009 8:50 pm

Ragnarok wrote:My advice would be to replicate what the APS underwater assault rifle did, essentially firing steel rods -

Even the APS utilizes supercavitation. 'Tis a field the Ruskies have excelled at for a very long time (fluid dynamics).
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