Single and double band size comparisons.

[SinglerM]

Hi,

I've seen the chart from Daryl Wong that gives a comparison of different lengths of his Hybrid models comapred to double band Euro guns of different lengths.
This got me to thinking, what would be the comparisons of different length Euro guns configured with either single or double bands?
For example, I just bought a Rob Allen Caranx 1300. It has a 7mm X 170cm long spear, it has a single 20mm X 38" band.
What length Rob Allen Tuna (7.5mm shaft, double 16mm bands) would be the rough equivalent of my gun?
Is there any overlap power or effective range-wise?
Just curious and wanting to talk spearguns.
Thanks,
Mitch

 

[foxfish]

Mitch, a lot of us would like to know that kind of information as well! You can find many such questions on db & other spearfishing sites but very few answers. It would be great if manufacturer's could produce performance figures for their individual guns, virtually all fire arms & air riffles have muzzle velocity & ft lb ratings!

 

[Jonny250]

OK, here's my theory, bear in mind that i know NOTHING about ballistics, etc, but am trying to apply a couple of physics lessons... perhaps someone else can check/correct it as necessary...

From Hooke's Law:
F = -K.x

Elastic energy:
E = (K.x.x/[2.L])
K = elastic modulus
x = extension of band
L = original length

k should be the same for bands of the same material, not related to thickness of the band.
x and L can be measured.

This will give the amount of energy for each system, dont forget to apply this to each band used.

You then need to look at the momentum of the shaft and equate the energy in the system = 1/2 .m.v.v
This should tell you the velocity of the spear leaving the bands.
m is the mass of the spear and will be a function of the length and diameter.
You could use this to get an idea of equivalence between bands and guns, however i dont know if the speed of the shaft leaving the band is directly related to the distance it can travel, but i think so.

I hope someone else can add to or correct this...

 

[Old Man Dave]

This topic has always fasinated me. I am very much an engineer/maths type geek.

Trouble with using most physics formula are that they are not predictive. There are too many variables. You can apply formula to known data discovered through testing but you can't predict outcomes by imputing known data.

So if someone tests a gun and measures certain perammeters then you could for example calculate the energy generated using a simple formula. I did this in a recent thread. However, calculating the speed (or range) of a gun/spear by equating the energy input (force to load it) doesnt work. You need to know factors related to friction and hydrodynamic resistance. You just can't calculate these values without impirical data.

There are lots of annocdotal stories about speargun performance and most as Mart says are far too general. In its 2008 catalogue Omer gives some figures for performance with different bands which I found interesting but although they were conducted by a firearms testing laboratory they were for "dry" firing. Presumably "out of water", which to me makes them worthless. There are some sites on the net with some tests or data from tests but not nearly enough to make accurate conclusions.

I could write a load more but lets see if this goes anywhere first.

Dave

 

[Jonny250]

OMD I understand what you mean about empirical data and application of basic equations to reality, but if we are to compare bands and spears rather than shooting distance, then up to the point that the spear leaves the gun, perhaps the hydrodynamics could be neglected? also, as the question is to do with relative measures, or comparison, then if you apply the same case to both band/spear scenarios, do you think they wouod give a reasonable relative measure?

 

[Old Man Dave]

Once you start messing with this jonny wou'll realise how complex it is. Until DB came into my life 3 years ago I regarded my gun as a tool to catch fish. I messed with it a bit but not much. Since then I've done a fair bit of messing. I even built a test gun solely to satisfy the need to know.

I see where you're going with the one step at a time and comparison idea but the trouble is that gun performance is right on the edge. A good example is that spears bend when you reach a certain loading. So, say a 6.5mm diameter, X cm long, spear powered by a 16mm band, Y mm long travels at Z speed smoothly through the water. Put a 20mm band on the same set up and the spears ass end tries to overtake the pointy end resulting in non stable travel. Changing any parameter alters the outcome. Trying to analyse the factors is fun but doesn't relate to reality. You need a test tank, high speed camera, loads of spare time and lots of money. Enter stage left the speargun manufacturers. Except the speargun manufacturers have a slightly different agenda to you and me, like ease of manufacture, quantity of sales and profit.

All this aside I love to try to work out what would be the ultimate gun. There again I own an Omer XXV Gold. The 25mm barrel is too thin and light. The spear is way too thin for the length. The band is too strong for the set up. So all round in theory it's crap. Trouble is it shoots miles, with deadly accuracy. Something about an engineer proving a bumble bee can't fly.

Dave

 

[foxfish]

We just need to waterproof Magpies cronagraph & weigh the spear?

 

[spaghetti]

I will try to translate this some of this days. I'll help you understand what it means in english, so tha you will help me understand what it means in paractice...
Maorisub - Video

 

[strangelove]

These days when I think about double banding, I always have in mind decrease in reload speed and more difficult fish tracking. But having several bands is great in clear water since I seldom get that, I normally stay with a single band.
Just another side on the same argument.

 

[SinglerM]

Pretty interesting stuff,
I haven't really thought about all of the factors that would effect things. Different diameter shafts would add to the difficulty of comparison. I know in archery, using hollow aluminum or carbon shafts, the diameter and length correlate to determine the "spine" of the arrow. I'm guessing that shooting a heavier solid shaft through water makes this less of a factor.
Does anyone have any "seat of the pants" opinions. I'm guessing that some spearos that own or have shot multiple guns and configurations may have notices some overlap in performance.
The chart I got from Mr. Daryl Wong is:
90 cm = 42in hybrid
105cm = 50 in hybrid
115cm=53in
120cm = 55
130cm = 60in hybrid

I don't know if this list is the result of actual testing or if it's a rough guide based on experience. Due to all of the possible variables, a list based on the opinions of experienced spear hunters might be the most realistic and accurate way of having a comparison. I believe that the above list assumes the Euro guns listed on the left are setup with double 16mm bands and given an equivalent Wong Hybrid size.
I also saw some pictures of the Hybrid with carbon barrel in blue and green camo:inlove

In regards to my Rob Allen Caranx1300, as it's all I have to compare to. I noticed that all of the double band Tuna models are configured with a 7.5mm shaft and double 16mm bands.......the Caranx models are setup with a 7mm shaft and a single 20mm band.
Would this be because the 7mm shaft is too thin for a double band setup? so they use a 7.5mm shaft instead?
here's another thought that makes me wonder.....If placing a shorter set of bands on a gun makes it a little stronger, and each size of gun has a slightly longer set of bands and a slightly longer shaft. Then wouldn't the shorter gun actually be more powerful?
a 900 would be stronger than a 1200 for example. Is my thinking on the right track? within a range of guns, which one would be right on the "sweet spot" of performance.
For example of all of the different length Rob Allen Tuna models (7.5mm with double 16mm bands) which length would actually be the best performing? would the "spine" of the shaft start to become a factor?
And there is the original comparison, what would be most like my 1300 caranx. If I wanted to cut down a caranx and make it a shorter, double 16mm setup...at what point would I end up with the same performance, if I did this. just for discussion sake.
Sorry about all of the rambling, I'm a nerd. I'm a machinist so I'm always thinking about what makes things tick.

Take care,
Mitch

 

[armandozx]

Dude the caranx and the tuna are basically the same thing just the "package" comes in differently.

Like you stated before, change the bands on your Caranx and the shaft with that of a tuna version RA, and you pretty much have a RA TUNA 1300.

Now a 7mm is not consider to be a thin shaft by many spearos. I use the twin band set up in my caranx with out any problems, however I take 1 band off when vis is bad. As far as power goes you will get a faster shot with the 7mm spear & twin band set up.

 

[SinglerM]

Armando,

Did I read somewhere on here that you are using double 20mm bands on your Caranx? Or was that someone else?
If so, how do you like the double 20's with a 7mm shaft?
The single 20mm band on my gun is 38" long in the upper hole, I'm guessing that if I did the same thing, I'd put a 40" band in the lower hole. Double 20's seems like a lot of power though.
You're right about the 7mm shaft. It seem plenty big to me also.

Regards,
Mitch

 

[armandozx]

Yes I was, but when the RA open muzzle came out i switch to 16mm bands. hmm the 2x20 setup was very powerful, honestly I kind of miss it! but in reality I haven't really shot my gun with 2x16mm yet, only 1x16mm because I haven't been in the right place to use them.

The 2x20mm setup man is good but i would not recommend it for many reasons, 1 single 20mm is more than enough believe me just have the right size on it. you should be using if 20mm 27.1 inches in length or 69cm. This is the recommended length for you gun size 1300?

PS: leave ur RA alone they come more than ready to kill some times you end up wasting money for unnecessary things if something put double wrap of mono,you may consider the mono to short thus, hindering your shooting distance but, that's about it. GL! & dive safe!

 

[Mr. X]

From Hooke's Law:
F = -K.x

Elastic energy:
E = (K.x.x/[2.L])
K = elastic modulus
x = extension of band
L = original length

k should be the same for bands of the same material, not related to thickness of the band.
x and L can be measured.

This will give the amount of energy for each system, dont forget to apply this to each band used.

You then need to look at the momentum of the shaft and equate the energy in the system = 1/2 .m.v.v
This should tell you the velocity of the spear leaving the bands.
m is the mass of the spear and will be a function of the length and diameter.
You could use this to get an idea of equivalence between bands and guns, however i dont know if the speed of the shaft leaving the band is directly related to the distance it can travel, but i think so.

I hope someone else can add to or correct this...

The problem with this is that it does not take account of the number & diameter of the bands used -- which I think is what is being asked. It doesn't answer the question.

From earlier threads, it was proposed that the power of bands, for the purpose of comparison, is proportional to their cross-sectional area. Whether this is strictly true or not, I don't know. Also some like to subract the area of the hollow inner. An additional factor to take account of is band length. If you look at the band chart from Rob Allen or Rabitech, you will see that they recommend longer lengths for fatter bands - because they would be too difficult to load otherwise. So their extension will be less. To further confuse matters, spearguns using more powerful rubbers usually come equipped with thicker, heavier spears (I think spears that are too thin can flex when released(?)).

As best I can recall the "power" comparison of the various configs looked something like this:

14mm < 16mm < 18mm < 2x14mm < 20mm < 2x16mm < 2x20mm

So maybe you could use something like this: F = k x AreaRubbers where k is some constant

Assuming the spears are cylinders of the same material: Mass = density x volume = density x AreaSpear x length = density x length x 3.142 x (diameter/2)(diameter/2)
Density is roughly the same for both spears.

The focus on power sometimes overlooks the fact that we often already have enough power. I'm currently using a single 20mm band (my only option at the time) on my 90cm railgun but will switch back to the original 16mm next time if I can get some - as it was plenty powerful enough with that and is much easier to load. My new 75cm gun comes equipped with 18mm eurobands - haven't tried it yet though.

 

[Pastor]

The problem with this is that it does not take account of the number & diameter of the bands used -- which I think is what is being asked. It doesn't answer the question.

Also the graph of "F = -kX" is not linear for rubber, the further you stretch it the less the proportional force needed to cause that extention

 

[spaghetti]

Also the graph of "F = -kX" is not linear for rubber, the further you stretch it the less the proportional force needed to cause that extention

Spot on! That's exactly what the Maorisub engineer says in the article I linked above.:king

 

[Pastor]

I learned that 30 odd years ago in a Physics lesson at school rofl

 

[Mr. X]

In other words, we don't know. Better ask forum members Miles or Rob Allen as they have probably shot both configurations and tell you the difference in reality. [My guess would be that although the Tuna gun will be more powerful, there is little practical difference between the two configurations. Maybe a shorter 120cm Tuna gun with 2x16mm bands & thicker spear would have similar power/impact to the longer Caranx but it is a shorter gun, with a shorter spear and - depending on your rigging - shorter spear-line.]

Maybe we are missing out on something in the UK (it happens) but you don't hear of folk using twin band 80cm guns here instead of a 90cm gun, for example. I guess if I went to the med, I could put double bands on my 90cm to make it more like the ubiquitous 1m guns or a 1.1 even, but I probably wouldn't - it is powerful enough when it reaches the end of the spear-line already. ( I guess using an extra wrap or half wrap, or adding a reel, might allow for an increased range with the more powerful configuration - but not sure that's enough to justify having to load 2 bands).

 

[Jonny250]

I learned that 30 odd years ago in a Physics lesson at school

Ah you must have paid more attentian in school than i did - learn something new every day !!

I've done a bit of google-reading on rubber properties now and it's really amazing stuff - strain varies with temperature, but also strain varies with strain-rate! so the faster you stretch it, the easier... apparently it's the heat change in the rubber that makes it non-linear (you can even feel the heat created when you stretch a rubber band!)

So an alternative way to TEST the best solution would be to hang weights on the bands, or use a strain guage, to see which can carry the most load. Of course you'd have to do this in the water so that any heat would be dissipated... ;-)

Do you have a translation for that article Spaghetti? Looks good.

OMD are you interested in rigging up a test - just for interest - we could hang some weights off a wishbone and measure/plot the extension? (i tried it but dont have enough weight)
I have 16mm and 18mm black and clear bands we could use..

 

[SinglerM]

I sort of figured the same would be the case Mr.X, it's more likely that some with a lot of experience would have a rough idea. It's probably too difficult to set up an actual test. I'm thinking that there must be a point where a double or single overlap in performance for given lengths.
On my gun the shaft still carries a lot of speed when it reaches the end of the shooting line. I like the quicker loading of a single band also.
it's possible that the reason the 1000 or 1100 double band models get recommended so often, is because they are at the sweet spot of performance for 16mm doubles. I certainly don't know. I'm curious where my 1300 single band fits within the scheme of things.

take care,
Mitch