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Old 05-06-2012, 07:21 PM   #1
Ric.Fallu
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Light tuna gun

In an earlier thread, Iamasalmon posed some questions about a tuna gun. As I am currently making a light tuna gun, I thought I might post info on the build.

I wanted a gun to take southern blue fin tuna off the western coast of Victoria (on the southern side of the Australian continent).

A gun should be tuned to match the fish and their environment. Where I hope to go, the tuna can be large, but it will be unlikely to encounter a real monster, so I built the gun with a 20 to 30 kg tuna in mind. The tuna are found near the edge of the continental shelf (but substantially higher in the water column) so it’s a true blue water environment. I am unsure whether they will be encountered head on/from the top, or from the side, so I don’t know if the stalk will be “extend and shoot” or “swing and track”, or both.

I wanted a powerful gun, but nothing too heavy. My thinking was a four rubber gun, pushing an 8mm (5/16 inch) diameter spear, fronted by a slip tip. The stock length (from front of mech to tip of muzzle) should be about 1.35 metre.

Before starting the build, I named this gun “Death Descending”.

Since it was an experiment/prototype, I didn’t want to use fancy timber or joinery. I made it from DAR, KD, radiata pine, purchased from the local hardware. I tapered the central strip, and laminated it between two others. I used fibreglass plasterer’s tape as stringers. West epoxy holds it together. The pine strips came in a width of 40 mm, but I wanted a bulkier gun, so I laminated-on some side strips. Cheap timber or no, I still think the pine looks OK.

I don’t care what other people say in other threads, but real men make their own handles. It is mortised into the under plate, and secured by a dowel. It fits my hand (and fits it exactly). I angled it back a little more than usual to assist in dissipating the recoil.

Seems to me there is a 1950s “spacey”, “ray gun” aesthetic to this gun’s appearance, and I like that. Not that that will impact on how well it shoots. But it did influence the name I gave it.

I used a Riffe mech, with a 5mm stainless steel push rod.

There are four 16 mm (5/8 inch) rubbers at 3.5 to 1 stretch. The bridles are “starter motor” braid.

The spear runs in a channel, about 8 mm deep. This was cut with a hand held router. In three places along the channel are “spear whip inhibitors” mini enclosed tracks, about 20 millimetres long. The spear hole through these is 9 mm. I fabricated them from acetal (delrin) plastic, fitting each individually to align the spear in the channel. This is an unconventional system, but it has worked for me in the past.

I plan two wraps of shooting line.

There is a hole above the handle to take a rubber break away, but I think that for my tuna fishing expedition, I will use a tethering rig.

I have not yet balanced and ballasted the gun. It has yet to get wet. I don’t know how much it will finally mass, and how it will shoot. Sticking my neck out and making predictions is a sure way to look silly, but it’s also the fastest way to learn how to build a better gun. My guess is that:
Death Descending will shoot a little low, but once that is compensated for, it will be consistently accurate. I make the prediction based on the distance of the handle below the axis of the spear, and the length of the spear overhang at the front. I also suspect that if the spear overhang was a little shorter it should shoot flatter.
• there will be noticeable recoil, but nothing too obnoxious. If I had have had the benefits of Phil’s comments on inertia from another thread, I might have opted for a little more mass in the gun, but I had laid-up the stock by the time I read them.

But, time will tell. I need to finalise the gun, and target shoot.


Ric
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Old 05-06-2012, 07:23 PM   #2
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Re: Light tuna gun

I took the gun to the Southern Freedivers club meeting. (Its not always wise to show off an unfinished gun, but what the heck!) Anyway, there were several comments:
• The spear, trigger and line hook were stiff – this was due to swelling caused by epoxy and varnishing, and should be rectified by judicious sanding and filing later
• It was in !#@!*! pine. If this gun shoots well, I will prolly make another with flasher joinery and timbers
• The spear whip inhibitors may not withstand the rigours of firing the gun. Time will tell on this one, but similar arrangements have lasted OK in other guns.

Am always interested in other's comments - especially if they challenge what I think is correct.

There has been a big dump of rain over the last few days, no doubt dropping the salinity in nearby seawater, so I will have to wait a few days before I try to ballast and balance the gun.

In the meantime, I might start another, built to the same specifications, but without the spear whip limiters – that way, I can shoot targets and compare and see what effect, if any, the limiters have. If a gun with just a spear channel shoots as well as one with the limiters, then there is no need to install them. If it doesn’t shoot as well, its not too much trouble to install some. I am toying with names – perhaps “Forbidden planet” or maybe something else.
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Old 05-06-2012, 11:39 PM   #3
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Re: Light tuna gun

please send more pics of details
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Old 05-07-2012, 12:26 AM   #4
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Re: Light tuna gun

Watch that radiata pine- we call it Norfolk or cook island pine here, and it is just about the weakest timber I have worked with. I understand that you will be making another gun, this being a test, but I worry about the integrity of the stock under 4 bands. Also, I would think the flex in the weaker pine might cause the gun to shoot quite different than a gun of a solid hardwood.
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Old 05-07-2012, 01:42 AM   #5
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Re: Light tuna gun

Shaft whip will not be a problem with 8mm rod and 4 x 5/8" bands unless the rod is really soft metal. If it is spring stainless steel or the new super stiff carbon coated shafts. I would have preferred a 7.5mm shaft with 3 stiff bands for 20kg to 30kg fish. You might even be able to get away with a 7.1mm shaft if you enclose part of the track. I like you idea of "whip inhibitor" but really you only need to do it 20cm or 30cm near the trigger part to theoretically shorten the shaft whip profile. If you do it in sections, chances are that the fins are going to hit them. For a light gun a 7.1mm will have much less recoil, and 3 bands will really give that shaft a lot of velocity that will keep the shaft shooting flat. In open water it is hard to judge distance, and if you are taking long shots, a fast velocity shaft with little shaft drop is really nice.

By the way the gun looks really nice and would be great to get some more detailed close up photos.
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Old 05-08-2012, 12:37 AM   #6
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Re: Light tuna gun

Cool looking gun. Thanks for posting!
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Old 05-08-2012, 01:47 AM   #7
Ric.Fallu
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Re: Light tuna gun

I went down to the water today to ballast and balance the gun. Fifteen degree air and fifteen degree water. There was a stiff northerly chopping the surface too. It was bloody chilly but I got the job done. At least there wasn’t a horde of families looking at the weapon with disapproval.

I needed to add 1.280 kilogram of ballast. Seems a lot, but pine is light wood (SG around 0.3 or 0.4). The in-water balance point for the ballast was 9200 millimetres (about 3 foot) from the tip of the muzzle.

The gun, including spear and rubber, massed at 4 kilograms so, once ballast is added, it will be about 5.28 kg. Converting this to imperial measure, that’s about 12 ½ pounds.

In the previous thread on inertia needed for powerful guns, Phil suggested that the appropriate mass was likely to be between 3 and 4 pounds per rubber. Death Descending is a four rubber gun (so that means it has just over 3 lb per rubber) and just gets into Phil’s category, but might be a tad light-on.

Of course, this pre-supposes that the rule of thumb given by Phil includes the mass of the spear. If not, the spear and slip tip are about 700 grams and this would make the gun just a little too light. (If Phil reads this, I would be interested in his comment, should he care to give it.)

I came back home, chopped mortices in the stock, inserted the ballast, and epoxied it in. There wasn’t a lot of room for all that lead, but it did fit in the underside of the stock, and will be covered by the handle under-plate.

Once the epoxy has cured, I will reassemble the gun and slap on (whoops, I meant “carefully apply”) a finishing coat of varnish. Then its back to the water to:
• shoot targets,
• see how well the inhibitors stand up
• see how well the spear flies,
• feel how much thump there is on the wrist and,
• gather other general impressions.

Pictures soon.

Thanks for comments. Radiata pine (Pinus radiata) is not, as I understand it, the same as Norfolk Island pine. Its plantation grown around here and used mainly for house frames. Its not the strongest stuff in the world, but adequate to the task if the cross section is 40 millimetres and the grain is clear of checks and knots. You have to sort through a big stack to get the clear mouldings. I have used it in the past and it has stood up well.

The comments about spear whip and the use of one inhibitor only are food for thought. Especially since the Riffe spears I use have rest tabs. I plan changes/experiments with the number two incarnation, and this might be one. I can’t change too much though or else I can’t compare.


Ric
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Ric Fallu started spearfishing on the southern coast of Australia in the 1960s, and never really stopped. His other passion is building wooden spearguns.

Last edited by Ric.Fallu; 05-08-2012 at 01:52 AM. Reason: duplicate text
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Old 05-13-2012, 07:15 PM   #8
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Re: Light tuna gun

I attach pic of whip inhibitor - seeking comment.

It has been removed from the stock of the gun prior to varnishing, and sits just behind its slot. I fabricated it from acetal (delrin) plastic.

I have put the final coat of polyurethane onto the gun, as well as some antislip on the butt and handle (sawdust, stuck onto wet polyurethane, then covered with two more coats of polyurethane). I allowed it a weekend to cure, and will re-assemble the gun today.

Then, as soon as the water looks calm enough, and clear enough, I plan to try some target shooting.

Ric
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Old 05-13-2012, 11:30 PM   #9
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Re: Light tuna gun

Quote:
Originally Posted by Ric.Fallu View Post
I attach pic of whip inhibitor - seeking comment.

Ric
Terry Maas called then "shaft capture guides" on his Tuna gun, I guess the spacing needs to be right for them to "iron out" the shaft as it zooms out of the gun. Whether they pick up the wishbones is something to look out for, metal wishbones will leave marks on the guides if they get hit, otherwise you will have to check the wishbone cords for signs of hits on the guides.
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Old 05-14-2012, 06:05 AM   #10
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Re: Light tuna gun

I once tried an enclosed muzzle, with a shape very similar to your capture guides, on the end of an open track hybrid tube. It was pretty much destroyed on the first shot. Examining it at the time, I thought that one of the metal crimps on the monofilament shooting line caught the top of the enclosed muzzle as the shaft exited.

I know it's not quite the same as your setup, but it seems like the same thing could possibly happen. It's probably a case of the divil being in the details, where similar looking constructions might work in one instance and not in another with only minor differences.
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Old 05-14-2012, 04:05 PM   #11
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Re: Light tuna gun

Spear whip limiters
I got the idea for the “spear whip inhibitors” reading Terry Maas’ book. The guns he had pictured had lumps along the stock, and I wondered what they were for. I drank beer, pondered, and then experienced an epiphany.

I have tried them on other guns (which consistently shot straight) but made them from laminated wood (being an “If I can do it in wood, I will” kind of person). This is the first time I have tried plastic screwed in place. On the other guns, so far, they seem to work OK, and don’t seem to wear. I gently taper the entries to guide the shark fin. If, on Death Descending, they wear, it should be easy to replace them. Perhaps the rubbers hold the spear straight, reducing collisions between the limiters and the fins. Terry Maas wrote (if I remember correctly) that once the spear has left the rubbers, it starts to rotate on its axis. That is why I had the foremost limiter about 30 cm from the muzzle. The rest are equidistant between it and the mechanism. The other benefit of having the foremost limiter set that far from the tip is that it should remove any stress that might occur from muzzle flip.

Earlier in this thread it was suggested that only one limiter might be needed to collapse the spear whip wave. The idea seems really attractive. I can place a single limiter by the rest tab 30 cm in front of the mech. But this will have to be subject to empirical testing (target shooting).

One of the virtues of having individual limiters (as opposed to an enclosed track) is that the stock can be formed with a hand held router with a cheap round nose bit (I take two passes with a 6 mm bit, then clean up the channel with sandpaper wrapped around a drill bit). It takes a little bit of fiddling to position the limiters properly, but you check it with the spear in the channel, and its not rocket science/engineering.

I will report back on the guides when I test shoot. It would be interesting if anybody else has experience with them, or wants to try out the idea and report back. Thanks Tin Man.

Progress
After varnishing and putting antislip on the handle and butt, I reassembled Death Descending. The spear and line release are still a tiny bit stiff. I can easily move the spear with my little finger. But better a little tight than too loose: they will most likely wear a little. As well, when its in water it will have lubricant (the water) and move a little better.
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Old 05-14-2012, 10:30 PM   #12
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Re: Light tuna gun

While my tuna gun is very heavy, the 22 pounds (weightless in the water) is used to drive the shaft forward, not the rear into my face. It makes a very stable platform..

Regarding the capture guides. The art is in the taper of the guide as the tabs approach before the nylon (or Spectra) wish bone line hits the guide. Each guide has a 3-inch ramp to prevent it from being destroyed.

I've never had a rubber, or a shaft hang and with only 3, 3-inch points of contact, the shaft is very fast and accurate.
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Old 05-15-2012, 02:42 AM   #13
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Re: Light tuna gun

I did a lot of pool testing last year to test how shaft whip affects accuracy. What was interesting was that it happened very predictably and could be induced by changing a few parameters. In some instances I was able to cause shaft whip by simply doubling up the bands on the back notch on a shaft that was right on the edge. So if you are on the edge of a shaft whip profile, simply reducing the effective length of the shaft can prevent shaft whip. Also there was one instance where an identical length shaft and identical thickness (both 7.1mm) where I had one whip and the other was stable. It turned out that the whippy shaft was a batch that was not heat treated properly (according to the supplier). So even a change in the metallurgic composition of the shaft can change the shaft whip profile. One thing is absolutely certain is that you do not want a shaft that whips as it dramatically hurts accuracy and the shaft impacts the target at strange angles.

My advice would be to avoid any shaft whip inhibitor if possible but if you really need it have it only at the first 20 or 30cm. I found absolutely no difference in accuracy in an enclosed track and non enclosed track ... so long that there was no shaft whip on the open track gun. Up to 160cm length @ 7.1mm using a Riffe shaft you will not get shaft whip with 3 x 15mm stiff bands. If I was going to do a new gun today, I would put the inhibitor all the way to the back of the trigger and have it extend to the furthest fin with a "V" shape to allow easier shaft insertion. This will avoid any chance of having the fins smack into the inhibitor and most likely your effective shaft length would be 20cm or 30cm less ... which would change your shaft to being effectively around 130cm. I tried very hard to induce shaft whip on my Beuchat Carbon 95 using a 135cm shaft with a 6.75mm shaft ... but even with 2 x 20mm stiff bands I couldn't do it. The recoil was hellish but the shaft impact on target was flat. So a small change in shaft length goes a long way.
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Old 05-15-2012, 05:00 PM   #14
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Re: Light tuna gun

Information right from the source itself! Thanks Terry. (This is not to belittle the experience and input of other forum members)

As Terry has written, and Spearq8 implies, nothing substitutes for target shooting when evaluating the accuracy of spear flight. Death Descending is ready to shoot, but poor weather means I can’t get it into the water for a little while yet, so I won’t find out.

The next incarnation/progression of this design (I have named the gun Ming the Merciless) is already under fabrication. I want to try two experiments: assessing the impact of inertia on muzzle flip, and the effect of the limiters on spear whip. The gun will be heavier. I will build it with spear channel only, target shoot, then add one limiter/inhibitor and see how it shoots then. If I still don’t get repeatable results, I will go to more inhibitors.

Personally I am not a fan of enclosed tracks or deep spear channels (as they reduce functionality if you bend a spear whilst out on the water) but it seems to me that the way to go is to put the limiters in a channel that is as deep as the spear, that way there is no bump for the bridles to hit. (But also, there may be more friction from the bridles rubbing on the top of the stock).

In previous guns, I have built shorter inhibitors than Terry has described, with more abrupt tapers, and they seem to work. The ones in Death Descending are shorter still (19 mm) and time (or rather, repeated shots) will tell how well they stand up.

But I have to build and test before I can evaluate.


Ric
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Old 05-18-2012, 07:50 PM   #15
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Re: Light tuna gun

During the final re-assembly of Death Descending, a teething problem occured. The shooting line release hook is too short for the two wraps of thicker shooting line. One wrap slips off.

When I previously thought of shakedown issues, this is one thing I wouldn’t have envisaged.

I decided to fabricate a longer hook. I started with the idea of a hook shaped more or less like the ones Riffe use, then I cut and fitted, cut and fitted. I cut one from 3 mm stainless, then from 2.5 mm. It took me a while, and several iterations, but what I finally ended up with was something more like the shape of the line release hook from an Undersee mechanism, only a bit longer.

Problems were compounded by the mortise (which holds the hook) not being neat. I chopped it in from the side with a narrow chisel, and the surface was a bit rough. Judicious use of a warding file helped.

Seems to work OK now (on the garage bench at least).

Ric
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