SHOCK CORD FOR SPEAR LINE

(Click on any picture for larger size)

Going to fashion a shock cord for shooting line where it attaches to speargun, and want a stretchable link between line attached to spear and the speargun, securing the line with moderate tension before shooting and providing some degree of stretch after, to minimize the spear tearing from the fish.

Used a punctured bicycle inner tube I had, 1⅛-1¼ seems to be a good size; used before with success. You may want larger or smaller. Your local bike shop will probably grab you one out of their trash for free if you ask nicely. The inner tube rubber is compounded to be under pressure so it works well as a shock cord and does not stretch out of shape and the stretch is gradual, ideal for tensioning your line and not getting floppy.
One speargun maker sells a 5” shock cord, so thought go for ~6” final effective length.
Cut out three 12” sections of tube.

Cut 3 of these 12” sections.

Then insert each into the other so that you have 3 ply rubber for the shock cord. I have use 2 ply and recommend it too, but wanted to make this more difficult for illustration.

One of my great tools is bicycle spoke I found on the road, great hook to pull things through.

So pull one 12” section of tube into another with the hook. Repeat for the third 12” section, pulling it into the other two make a three ply section of inner tube.

You now have a nested 12 inch piece of inner tube, 3 sections, one within the other.

Next we need a cord, you can use Dyneema, heavy mono-filament, or parachute cord or just regular cord as I am using here. If you use mono, you will need to use double barrel crimps covered in shrink wrap so it doesn't cut the rubber. Mono works great, have used it successfully, little harder to work it into the rubber.

So cut a 48" piece of cord, you will need the length for the doubling inside the tube and for the knots and loops at the ends.

Form a double strand knot with a ~4” loop, pull very tight, trim the little bit of excess if needed

Now, double the cord so you have ~5” doubled line to insert inside the tube. This allows the rubber to stretch and if really strong fish, keeps it from breaking and if does break, you don't lose fish and spear. It is a stretch limit and will let the shock cord stretch to more than double original length.

Next use a little Scotch Tape to hold things together, not too much and not too little. It needs to hold enough to facilitate inserting into tube and be loose enough to break free when done.

Use a spear shaft to make sure the center opening in the tube is clear and open as much as possible

A plastic straw makes a nice fid, also, tape the end of the cord, like a shoe lace.

Insert the loose end with straw (the other end has the loop and knot) into the 3 ply inner tube section.

Use the shaft to keep the tube open as you push the straw and line through

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Now, very gently pull on the straw and line and work the doubled line into the tube so that the knot is at the entrance to the tube. The shaft may help to guide. Be careful that you don't pull the doubled line loose.

So now you have the line inserted in the tube and the knot ready to be worked into the tube.

A squirt of silicone type lubricant helps and wouldn't attempt this without it.
Use the shaft to work the knot down about 3” from the end o f the tube.
You may need to squirt a little more 303 from time to time
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Fold the layers of inner tube back to help getting the knot into the center of the tube and keep them stabilized so they don't slip

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So now we have the knot at ~ 3 inches from the end of the tube.

Some waxed twine and our old friend the constriction knot comes in. Tie the constriction knot like a band, just at the knot. Tie another one just to be sure.

Then tie another waxed twine with a constriction knot at the other side of the knot in the cord. So this is what you have.

Now work the different ends of the inner tubes back down over the constriction knots and cord knot
Now you have a very finished looking end of your shock cord.

NOW FOR THE OTHER END

If you used a 48” piece of cord, you should be able to form a ~4” loop with a good tight knot. I inserted a swivel in the loop, It's optional and maybe a heavier swivel too depending on what fish you are going for. You can customize your tackle to your spear gun

So we just repeat what we did on the other end.
Lubricate with 303 and use shaft to push the knot down ~3”

Tie a constriction knot at the loop end of the cord knot

I use chop sticks for leverage on the waxed line

So now we have a constriction knot above and below the cord knot

Work the three ends of the inner tubes back over the constriction and cord knot and now you have a finished looking shock cord.
Pull and stretch the finished cord to free the doubled line from the Scotch tape

It will stretch to ~ double length and keep the line tight on the gun without getting floppy.
I've had one on my speargun for more than 5 years without the rubber deteriorating.

Your fingers might a little sore after you make this, but it is without par or peer.

You can be proud of this on your speargun.