Shaft weight vs. trajectory

[foxfish]

There are other issues to be considered - A lot of folk loose speared fish, not something anyone wants to happen but it does!
I like to shoot my fish from a very close range if possible but, I still much prefer to use thin spears & high powered guns because the spear will very often pass straight through the fish.
If this happens there will be very little resistance for the fish to fight against!
Should the spear partly penetrate the fish then there is plenty of resistance for the fish to struggle against, open up the wound & allow the fish to escape.
I like dry barrel airguns with their short thin spears that easily passing though 1-4kg fish at upto two mts range.
I have also found that 6-6.5mm spears fly pretty true with single band guns it is only when you up the power that spear flex becomes an issue.
My latest gun build has been designed to fire a thin spear, the problem has been finding a 6mm spear with shark fins or tabs!

 

[OregonSpearo]

There are other issues to be considered - A lot of folk loose speared fish, not something anyone wants to happen but it does!
I like to shoot my fish from a very close range if possible but, I still much prefer to use thin spears & high powered guns because the spear will very often pass straight through the fish.
If this happens there will be very little resistance for the fish to fight against!
Should the spear partly penetrate the fish then there is plenty of resistance for the fish to struggle against, open up the wound & allow the fish to escape.
I like dry barrel airguns with their short thin spears that easily passing though 1-4kg fish at upto two mts range.
I have also found that 6-6.5mm spears fly pretty true with single band guns it is only when you up the power that spear flex becomes an issue.
My latest gun build has been designed to fire a thin spear, the problem has been finding a 6mm spear with shark fins or tabs!

Interesting. Can you describe your current setup (barrel length, bands width)?

 

[foxfish]

I have quite a few guns but, my favourite is an Omer 90 fitting with a Tomba dry barrel kit & 100cm free shaft (line attached at the front of spear).
I use around 22 bar so still pretty easy to load & the front fixed line system ensures any fish are toggled by the spear.
I also use an enclosed track 100cm gun powered by twin bands that fire 6.25 spears..
Here is a gun I have designed to be very light. use a single band & a 6mm spear.. http://forums.deeperblue.com/diy-homemade/90231-new-gun-build.html

My air gun http://forums.deeperblue.com/pneumatic-spearguns/84009-more-mamba-mods.html

 

[Tin Man]

I think that there is a critical piece missing from the discussion.

My dad and I did some tank testing while playing with roller guns, and we learned that smaller and smaller shafts do not go faster and faster without limit. In other words, there seems to be a limit to how fast the bands can contract. Once they are contracting as fast as the rubber will allow, then a lighter shaft will not go any faster.

 

[Bill McIntyre]

I think that there is a critical piece missing from the discussion.

My dad and I did some tank testing while playing with roller guns, and we learned that smaller and smaller shafts do not go faster and faster without limit. In other words, there seems to be a limit to how fast the bands can contract. Once they are contracting as fast as the rubber will allow, then a lighter shaft will not go any faster.

At what point do you think this occurs? For instance, I have two guns with tracks and ballast for 5/16" shafts. In one of them I'm using a 7.5 mm, and in the other I'm using 9/32". Do you think I'm fooling myself that I'm getting more speed?

 

[Tin Man]

We were playing with pretty short shafts Bill. I think that for any normal sized speargun, a 9/32" shaft will have more speed from the same band setup than a 5/16" shaft will. I just meant that a REALLY light shaft, of carbon fiber or such, would probably not go as much faster as the difference in weight might suggest.

 

[Bill McIntyre]

We were playing with pretty short shafts Bill. I think that for any normal sized speargun, a 9/32" shaft will have more speed from the same band setup than a 5/16" shaft will. I just meant that a REALLY light shaft, of carbon fiber or such, would probably not go as much faster as the difference in weight might suggest.

OK, thanks.

 

[OregonSpearo]

Interesting observation, Tin Man. It'd be tricky to measure speed underwater.
But band spearguns function on the same principle as slingshots whose speed is easier to measure. Some slingshot dude must have come across the same thing, right?
Well sure enough there's a paper on that: "Power - a small theoretical approach to understanding and improving slingshot power"
From the study's conclusion:
The acceleration – and therefore speed – of a very lightweight projectile is the same with strong and weak bands. In other words, using light projectiles is a waste of energy with powerful slingshots
The paper also contains formulas to calculate that maximum speed.

 

[chipinga]

Not much I can think of except timber or aluminium both have major problems. Think of an arrow shaft light and accurate...above water.

Both timber and alloy you would only ever get one shot before they bust up.

Having said that you could consider a very dense timber and make the diameter a little larger....all in all a real pain in the butt.

The best way to overcome fall in trajectory (to a point) is speed and depending on your target that can be done by either loading up in an enclosed track or reducing your shaft diameter.

I recently made a client a speargun 1600mm with 5 x 16mm rubbers with the option of 10mm or 7.9mm spears (he changes the enclosed track in a couple of seconds) and he claims he shot a spanish mackeral at just on 9 meters with the 7.9mm riffe shaft and 5 rubbers.

You just have to play around and practice your shots at various distances.

 

[Jonny250]

From the study's conclusion:
The acceleration – and therefore speed – of a very lightweight projectile is the same with strong and weak bands. In other words, using light projectiles is a waste of energy with powerful slingshots

Actually this study does not apply correctly to spearguns as the assumption stated 4 lines above your quote is:
"Let’s assume that projectile and pouch weight are or close to zero"

Thats how the a = F/M[of slings] is arrived at, but in our case the mass of the slings is small compared to the mass of the shaft.
I have just weighed a pair of 18mm slings for my 75cm gun and they were 130g. The 1m spear weight is 232g, so not 'close to zero'.

So in fact our case is more directly related to the mass of the shaft than the mass of the bands, and hence a change in start velocity is more affected by the shaft weight than in that paper.


I agree with Tin Man that the difference in speed for a really light shaft is unlikely to be as much as the weight difference would suggest. From my reading on Archery theory and tests, it is generally accepted that heavier arrows are released from the bow more efficiently [about 8%] between a carbon arrow and a wooden arrow [i'm not sure of the exact weights or diameters tho].

I have just received my 2 arrows in the post, i'm going to try an experiment [just because this subject has appealed to my interest ].
I have chosen a super-fat hollow carbon arrow [fatboy] and a slightly thinner hollow carbon Powerflight arrow. I can glue each of these onto a cut-down standard shaft as the i.d. is large enough to fit over the top of the steel shaft. I will use the standard notches at the tail end for loading. The larger diameter will exaggerate and drag reduction effect on speed, so should give clearer results. The Hybrid shaft should weigh 40% of the all-steel shaft weight...

Any suggestions for measuring the differences in speed [relative difference is all i can hope for!] I am thinking to use a camera at 90 degrees to the shot and video the shot. 30frames/sec should hopefully give enough info...

just need to some decent weather and clear water... may be a while lol

Here's the info [i cant seem to post the images properly but if you click the link at the top of the frame you can see the images]:
[ame="http://www.flickr.com/photos/34897908@N05/6498838359/"]arrows data | Flickr - Photo Sharing![/ame]
[ame="http://www.flickr.com/photos/34897908@N05/6498680187/"]12122011783Arrows1 | Flickr - Photo Sharing![/ame]
[ame="http://www.flickr.com/photos/34897908@N05/6498680303/"]12122011782arrows2 | Flickr - Photo Sharing![/ame]

 

[AJB8]

Cool idea Jonny. It'd be nice to get some experimental evidence.

I like your idea about simply filming at a perpendicular. I'd say make sure to have a high vis tape measure at the same latitude from the shaft as the camera, and to make a more interesting visual, maybe two identical guns/rigs if possible (shaft as dependent variable), one gun on top of the other. That way we could get a great trajectory vs. speed of shaft comparison.

Also, and I'm no expert here either, but make sure you film in a format that you can post. Obviously you'd want the fastest frame rate available, but that might prove prblematic...? Dunno.

Another question too: would the line drag effect different weight shafts differently?