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All About Guns What's your weapon of choice, and why? Discuss the beloved speargun here!

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Old 10-07-2019, 11:08 PM   #16
spearq8
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Re: Moving sharkfins further back on spear?

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Originally Posted by Andersp90 View Post
Its a 105 cm 7mm spear. Shouldent it be more resistent to whipping compared to a 150 cm shaft of the same thickness?



My muzzle looks like that. The spear is resting in a shallow track, milled into the carbon.
That is just way too much shaft over hang and that will cause the shaft to shoot lower … especially if you move the shark fins even further back. You want the absolute minimum shaft extension from the muzzle. The thicker diameter shaft you have the more you can get away with. Also the harder the metal your shaft is (stiffer) the more you can get away with. Another factor is how good your gun design is … the more inline your bands are with the CG of shaft the more you can get away with … and of course the higher the handle is and the closer the CG of the recoil is to the center of your palm the more you can get away with.

To get a perfect setup where you have a gun shooting groups inside 1 inch at 4 meters or even 5 meters, with minimal shaft drop is easy if you know the basics. I think if you look at the video I put up a few years back you will find a lot of useful information (Theory part). Basically you need to realize there are two forces working against keeping the shaft stable … one vertical and one horizontal. The more you take steps to reduce these 2 forces the more accurate your gun will be and the more retained velocity you will get. Velocity out of muzzle really is not important as if the shaft is not stable velocity will very quickly fizzle out by the time the shaft reaches the target. It is much better to first get the shaft stable and increase power where that stability does not change. If you do everything correctly … there is really no limit to how much power you can add without affecting stability.

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Old 10-07-2019, 11:13 PM   #17
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Re: Moving sharkfins further back on spear?

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Originally Posted by Diving Gecko View Post
Not sure I got this...? You mean, if the sharkfins were further at the rear, then 360% would overpower the shaft? But as the fins are now, 360 is ok? Not sure that makes sense to me, as 360% on a more rearwards fin should give the same amount of max power - but a longer pull, no? Prolly just me misunderstanding something

Yes exactly. I know that it doesn't make sense but after making 30 custom shafts @ 7mm with fins maxed out I had to find out the hard way! There is a lot of things with a speargun that just don't make sense as the way everything interacts and affects everything else is very hard to predict.
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Old 10-08-2019, 04:13 AM   #18
Andersp90
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Re: Moving sharkfins further back on spear?

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Originally Posted by Diving Gecko View Post
Personally, if I don't have a good TIG dude I could ask to do it, I would cut it down and loose the 3g...
I guess you are right. I still hope that I can get hunt to just make me one.

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Originally Posted by popgun pete View Post
If you put the shark fin tab back to the leading edge of the sear box then it will be all too easy to bounce the wishbone off the top of the sear box roof and miss the tab when using metal wishbones as the wishbone jerks forwards.
If I move it back, it will only be 3,5cm, leaving a 5mm gap between the fin and the sear box edge.

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Originally Posted by RichT View Post
Unfortunately you end up with a sharkfin that isn't nearly as strong and one which will trap saltwater under it. I suspect longevity may also be an issue...
I have already seen several of these types of sharkfins broken off btw...
Its just for a 75 gun, so I think it should be able to handle the forces.
Sadly, I eat through 1-2 spears a season, do to hunting in caves and on wrecks. So they will probably never get the chance to corrode.

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Originally Posted by kavachi View Post
Hey Anders!
pics below of Hunt 8.5mm shaft with tail chopped so rear tab sits right up snug with mech roof.

Easy job with a hacksaw and round file, and allows you to size the sear slot width precisely so there is zero slop when shaft engaged.

Couple of BIG qualifiers here tho, before chopping down that Hunt shaft!

1. you are messing with the geometry of the sear-shaft interface, and thus potentially the reliability and safety of the mech. I’m using Ermes DR mechs and the big roller sear makes this mod easy, less so with traditional sears that do not engage with the shaft slot so cleanly and thoroughly. Each time i mod a shaft tail i test it exhaustively in the pool before going anywhere near humans or fish
2. by taking the rear band ~30mm closer to the tail of the shaft, you increase the possibility of the shaft tail lifting as it exits the mech, and wrecking your accuracy, particularly as you increase the power with hot small id bands. big thanks to Majd for helping me understand this. 240fps video is really helping me to work out whats happening as that shaft comes out of the mech, and why with some rigs the shot wanders around the target. only one of many factors that need to be tuned in to achieve uber accuracy, but methinks this is a key one
3. bringing the rear tab 30mm closer to the butt pad significantly increases loading strain, and brings different muscles into play. No drama with lighter loads, but with 15mm small ID bands at 380% or more it does take some getting used to. Per your pic, the rear tab on the Bluetec is already quite close to the loading pad, could be hard work with hot 15mm bands, even with such a short gun
I guess I will just start with the fins in the position they are, and then see how it does with 2 x 14mm bands at 360%.
Its only a 75 gun, so it should be possible to load it, even if the fins are moved back (with 14mm). But wont 15mm bands be too much?

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Originally Posted by spearq8 View Post
That is just way too much shaft over hang and that will cause the shaft to shoot lower … especially if you move the shark fins even further back.
Its just a stockphoto from the interwebs. The overhang on my gun is about half of what is seen in the picture.

Quote:
The thicker diameter shaft you have the more you can get away with. Also the harder the metal your shaft is (stiffer) the more you can get away with.
Does the length of the shaft not play in to this at all? I was thinking that a 105cm 7mm shaft would be more resistant to whipping than a 150 cm 7mm spear. Is that not the case?

Quote:
and of course the higher the handle is and the closer the CG of the recoil is to the center of your palm the more you can get away with.
What is your experience with the bleutec handles in terms of handling recoil? Good, bad or average? Will I have to make a handle myself?

Quote:
To get a perfect setup where you have a gun shooting groups inside 1 inch at 4 meters or even 5 meters, with minimal shaft drop is easy if you know the basics.
Are those distances not a bit unrealistic with a 75 gun? or are you just speaking in general terms?

I will have a look at the video/s
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