shafts - notched vs sharkfin, and diameter questions

[Andrew the fish]

I am thinking about rigging my 100cm Pathos with thicker shaft. I suspect there is a fair bit of whip going on, when I load 2 rubbers. There were a few misses, or rather hits but away from the aiming point. This was while shooting flat fish, easy to see what was used for aiming and where the shaft actually penetrated. I checked and double checked to make sure this is not my sight picture or lateral movement of the gun that causes it. Basically, I am thinking my 6.5mm shaft will have to be more like 7.5mm at this length ( it is about 120cm). The reason I am suspecting a shaft whip is beause I once touched the shaft while it was loaded, and it flexed to the side with no effort, despite being firmly in the track. Track didn't seem to hold the shaft at all. I would like to hear comments from you more experienced folks. Will thickness difference by 1mm help? How much thicker heavier shaft will be slower to accelerate, and how about hydro dynamics of it, will it suffer much?

Also, since I am shopping for new shaft, I can switch to notched type if this will help in accuracy at all. This is my main concern right now. This Pathos is my first 100cm gun, all others were 60-75 in that range. I never had such pronounced accuracy issues with shorter guns. I like Pathos, apart from accuracy it seem to be well-built otherwise.

 

[SimonCH]

To start with the simple part shaft type: I think its just up to preference and what youre using it for. A lot of people say sharkfins are sturdier since theyre just "added" to the shaft instead of removing material for the notches which makes the shaft weaker (in theory). "Upside" for notches, if you wanna call it that, is that the shaft is accelerated more from the center instead of the top, ive never noticed a difference though. If youre not going fo true monsters either option will be fine. just depends on what you like. I use sharkfins if thats any help to you.

Shaft diameter: Yes a thicker shaft is sturdier. It might solve your problem but might throw the balance of the gun off since youre adding more weight withought adding buoyancy. Maybe youre overpowering the gun. Or not wrapping it correctly so the shaft doesnt sit in the track properly? Are your "misses" consistent? As in always too high/too low/left/right or is it random? If theyre consistent you might want to check if the shaft is bent. I always check before buying shafts and youd wonder how many factory new shafts arent straight. just take 2 chairs and put them 20cm apart both their backs facing each other and place your shaft on them and roll it. If it rolls smoothly its not bent. Also if youre using a tri tip it might not be entirely symmetrical.

Id stick with the shaft (if its not bent) and try to solve the problems by tweaking other areas and not adding new factors to the whole problem.
Cheers

 

[Andrew the fish]

Siman, thank you for advice. I never thought about the balance, might have to think about adding bouyancy with thicker shaft. I hear you about consistency, I thought of that too. Misses are not consistent, and that is the thing that made me suspect a shaft whip. Frequent fliers though. And another thing I never thought is that notches are closer to shaft axis, should probably alleviate the shaft whip right there. Obvious, but only in retrospect. I am thinking, for notched shaft I will have to re-do rubbers for wire wishbones. And finally, I might have to keep an eye on how shaft sits in the track. I am sure wrap them properly, but swimming though kelp might dislodge it. Simon you gave a lot to think about.

 

[SimonCH]

You can keep dyneema bridles (wishbones) with notches. but i guess thats not the problem... what rubbers are you using and what % flex do they have and for how much % do u have them set up? No matter how alligned the wishbone becomes (using notches) shaft whip will still be present if youre overpowering the gun (similar to balancing a billard queue on your finger). Try shooting with one rubber and see how that goes. And definitely check if the shaft is bent using the method i told u.
Hope you find a solution!

 

[hteas]

How long is the 6.5 shaft you are using. It should be 140cm long. Anything shorter, based on personal experience, will lose accuracy. The altewrnative is to go to a 7mm shaft. It can be 10 cm shorter and still be accurate. The 40cm extension on a standard gun is used by almost everyone in the industry, and has been true for me.

 

[sharkey]

Open muzzle or closed? Could it be the line fouling on the muzzle or anything else?
How much shaft overhang? Saying it’s a100cm gun doesn’t mean much unless this is the actual measurement from the trigger mech to the muzzle. As has already been mentioned 350-400mm overhang is favoured by many for accurate aiming.
Why do you need two rubbers for a 6.5mm spear on such a small gun? I’m using a 6.75mm x 1.5m pathos spear in my little 1100mm gun with only a single rubber ( pictured). It will punch through a 20kg fish 4m from the muzzle. Tube guns are usually too light in mass to get the best accuracy from two rubbers. If you are shooting with your right hand & missing to the left then you can bet that you are using too much power for the guns weight for it to perform accurately. If you do need two rubbers it may be worth investing in a new gun which is much heavier.

 

[Andrew the fish]

As has already been mentioned 350-400mm overhang is favoured by many for accurate aiming.

I am ok aiming down a 115cm shaft (about 15cm overhang), this is plenty of sight radius. I shoot pistols with 10cm of a sight radius, to 25 meters. Here we are aiming a meter long shaft to about 4 m distance. I don't understand. Longer shaft will definitely be more prone to whip.

Why do you need two rubbers for a 6.5mm spear on such a small gun?

I needed two rubbers because it is a small gun. (wow)

I installed the second rubber last summer, specifically for flat fish. You know how far the barb is on the shaft, from the tip of spear? This is how far spear must penetrate the fish and bury itself into the sand to allow the barb to open. At point blank I can catch flounder even with one rubber, but give it 2meters and you need a helluva more power.

I’m using a 6.75mm x 1.5m pathos spear in my little 1100mm gun with only a single rubber ( pictured). It will punch through a 20kg fish 4m from the muzzle.

Longer shaft carries more momentum, I kind of agree, I have to agree under the pressure of laws of physics. But, the moment I put 1.5m spear onto my 1m gun, it becomes 1.5 m gun. I can't fish here with 1.5m gun. Not only that, but it becomes 1.5m gun with the power stroke of original 1m gun.

 

[sharkey]

If you are familiar with pistols & rifles you would understand that a longer sight radius is much more accurate than a shorter one. Your current shaft has a sectional density of 9.2 , by going to a 1.4m spear the SD is 11.2. That is a bit more than a 20% increase in penetration & range, not to mention the better sight radius. It is also going to take the punch or knockout factor of that straight spear from about 2.5 to about 3.2 ,which most likely isn't important for the fish you are shooting , but may be interesting to some.
One rubber on a 6.5mm spear that length should easily get it moving faster than 20m per sec, a second rubber is only going to increase that by a fraction, but it will increase the recoil at the initial "jerk" for very litle gain. A 1.4 m x 6.5 mm spear with a single 16mm rubber at 350% stretch on a 1m gun shouldn't be difficult to get to shoot well. The trick is ensuring that the spear is pushing against the track or rail every time it is fired. Even with two rubbers it is possible to tune that gun & spear ( just like tunning the "centre shot" on archery equipment) to shoot accurately & consistently, however you are going to have to find a way to manage the extra recoil ( the easiest way is to add ballast).
You are actually putting the cart before the horse. The spear & point should be chosen first for their particular task & species, then the gun which provides the best accuracy, reliability & utility for this should then be chosen.

 

[Jesse_Spiller]

Hi Andrew,

By my calcs, you shaft should be travelling approx 28-31 m/s if you stretch to 350% and your gun weighs approx 2kg.
Conventional speargun energy = 196 J, of which 153 J is spear kinetic.
The gun is 78 percent efficient.
Theoretical maximum spear velocity is 31.0 metres per second with 1 band(s).
Max pull force is 53 Kg with no pre-tension.
Gun free recoil energy is 31.92 J, free recoil velocity is 5.65 m/s.

Your peak band force is approx 53 Kg, which is well within the buckling range according to Euler's critical load.

Depending on how effective your rail is (doesn't sound very effective), you might need to go up to 7.5mm in a 1.2m to avoid buckling/whip with certainty.

If you add a 2nd 16mm band with a 7.5mm spear:
Conventional speargun energy = 392 J, of which 276 J is spear kinetic.
The gun is 70 percent efficient.
Theoretical maximum spear velocity is 36.1 metres per second with 2 band(s).
Max pull force is 53 Kg with no pre-tension.
Gun free recoil energy is 86.68 J, free recoil velocity is 9.31 m/s.
Aka you gain 80% more power and 5.1 m/s at the cost of 172% more recoil.

I'd be interested to hear the weight of your gun? It has a huge impact on efficiency, recoil and accuracy.

Jesse

 

[Andrew the fish]

Jesse, thank you for informative post. I can't weigh my gun right now, but it is light, aluminium tube gun, no ballast of any sort.