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Old 10-29-2014, 01:59 AM   #16
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Re: Hatch Custom Spearguns "Yacht Gun"

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Originally Posted by spearq8 View Post
What I can't wrap my head around is with a normal classic setup (even normal roller setup), the band contract and expand and thus there is no excess line, but in the case of an inverted roller or as is shown in the pix, the line being pulled does not expand and contract ... so where does it go once the bands stretch out ... I am guessing it just stays as extra stretched line hanging under the gun?. Also I think there must be a lot of losses with all the pulleys. Pulleys are great at reducing loading effort, but I am not sure they can efficiently transfer power at high speed and that loss in efficiency is certainly not zero.

I don't know, it looks interesting and as an engineer I am always a sucker for cool contraptions ... but I can't see this being better more robust, or easier or faster to load than a 2 or 3 banded classic gun. Maybe for the smaller guns where you absolutely need the power with a small size. But that just looks like too many things that can go wrong on a normal sized gun (say 120) to use as a go to gun on a spearfishing trip. I am willing to accept added loading time and a more complex and less robust system ... but there must be a substantial performance gain ... is there?
Watch the Roisub videos, spearq8, and you will understand. It is really very simple.

And, in this configuration, which is one of the most basic setups, the "line" is actually the wishbone, which is always in tension.

The long wishbone is cut to length by looping the wishbone around the rear sharkfin tab, pulling each leg through a muzzle pulley, then through the smaller, lower pulley, which is held in position near the muzzle, at the point of desired maximum band stretch, and fastened to the line keeper at the muzzle.

Each leg of a band consisting of two equal length latex tubing components and a wishbone is then attached to the pulleys.

The wishbone is then taken off the sharkfin tab and the band wishbone is pulled back towards the handle and pulled over the keeper, taking up all of the slack in the long wishbone. In order to give the band some pretension, the long wishbone can be adjusted at this time by shortening each leg at the line keeper at the muzzle by a few inches or so. See the bottom left picture in the first post.

Now, when the band wishbone is pulled onto the keeper near the handle, the band is pre-tensioned, keeping the long wishbone taught, yet the long wishbone, when pulled back onto the sharkfin tab, stretches the band to full band stretch. This way, there is no excess line. The long wishbone is just the right length when it is pulled onto the last sharkfin tab to stretch the band to maximum via the lower pulley. See the bottom right picture of the first post.

Once this long wishbone has been sorted out, additional bands, in this case, two, can be attached to the small, lower pulleys. When the speargun is fully loaded and ready to fire, all the bands will be stretched like the single band shown in the bottom right picture of the first post.

Yes, there are fictional losses of ~3%-5% attributed to the pulleys, but the gains in a lower cross-sectional profile, a clean sight picture, and near zero recoil easily offset those losses. Furthermore, the ability to pre-tension the bands means that when the powerstroke is completed, the bands are still applying force to the spear when they are arrested by the muzzle. In contrast, the bands on a conventional speargun quit applying the same amount of force when they contract to the same length as the pre-stretched bands. Also, the bands on a conventional speargun impart zero energy on the spear once they have fully contracted, which is ~1/3 the barrel length. When compared to a standard rollergun, the standard roller gun would need to have three sets of muzzle pulleys to equal the forces applied by this simple configuration.

Once you understand it, it really isn't complex at all. Wouldn't you rather swim with a 120cm speargun such as this one and hit your targets at 7m-8m with pinpoint precision and zero recoil than to lug around a 150cm-170cm bluewater cannon with 5 bands and some serious recoil?
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Old 10-29-2014, 04:37 AM   #17
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Re: Hatch Custom Spearguns "Yacht Gun"

Yes but does it give you 7m to 8m range with pinpoint precision? To be honest I have only tested a gun once at 7.5m and at that range the 7mm @ 160cm shaft barely had enough power to penetrate a 5cm foam even though it was loaded with 3 hot 16mm bands and had no flopper. I have a video of that if you look at very old videos I did. Accuracy was actually not bad at all ... but really no penetration power and lots of recoil. Personally I have no problem with heavy recoil if it doesn't affect accuracy. This was many years ago and today I feel the limitation was the mass of the shaft. Recently tested an Abellan Denton 130 with 3 x 14.5mm small ID bands and it has totally changed what I used to think was the maximum range of a 3 banded gun ... I used to think 6m was the limit for classical 3 banded guns ... now I feel it must be around 7m or 8m. No need for 5 bands or a huge oar to swim around with. Of course a well performing roller gun might be able to reduce size but when you reduce size, you are also reducing the length of the shaft ... which reduces the mass. To compensate for the lost mass you need to start really going into very high diameter shafts ... which increases drag (with flopper an 8.5mm shaft can end up with a 13mm cross section !!!) ... gain some lose some! I have seen the video from Seals test and I was quite impressed, and he seemed to have removed many of the problems inherent with rollers ... but I would want to have that gun and play around with it ... see how easy it loads and how quick ... how flat it shoots ... how tight does it group 5 continuous shots at 6 meters ... what the penetration is etc.

Most comparisons between roller guns and classic guns are using poorly setup classic guns. A classic gun with good band stretch 115cm+ is actually very capable if setup properly ... unfortunately the majority of classic guns are not setup correctly.
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Old 10-29-2014, 05:03 AM   #18
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Re: Hatch Custom Spearguns "Yacht Gun"

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some way to shoot something backwards at the same time with the same amount of momentum as the shaft, then you would have a recoilless gun...
This ^ right here.

Like I said, I kept it simple. Recoil is actually an impulse component vector, not a momentum component vector. The reality is that the impulse of the wishbone acting to accelerate the shaft mass is countered by the impulse of the band mass moving in the opposite direction, as the bands do in this configuration.

If the accelerating band mass were equal to the accelerated shaft mass, there would be zero net velocity for the speargun, as the two forces imparting onto the speargun in opposite directions would cancel out, right?

What is happening when the bands are moving in an opposite direction to the shaft isn't a whole lot different that what happens in an inline 6-cylinder engine, or when counterbalancers are used to get rid of vibrations, or when active mass dampeners are used to keep a tall building from swaying, or when thrusters are used to keep a ship from rocking. In an inline 6-cylinder engine, two pistons are always moving in opposing directions, cancelling each others impulse out, leaving no vibration or "recoil". Counterbalancers use an equal mass moving in an opposite direction to cancel out the impulse force of whatever it is they are counterbalancing. When wind forces act on a tall building, it wants to move in the direction of the wind, sensors then tell actuators to move a 30-ton granite slab on a near friction-less pressurized oil bearing in the opposite direction. End result. The building doesn't sway with the wind and make everyone sick. Which brings us to thrusters on a ship, which activate and counter any wave forces acting on the hull, keeping the ship from rocking. Yes, it is all about opposing forces, or rather opposing impulses.

In this application, there are no bands on the top of the speargun moving parallel with the shaft. The only thing moving in the shaft's direction is the long wishbone, which has relatively zero mass when compared to the shaft or the bands. The bands are contracting towards the handle, which is opposite the direction of the shaft. The line of interaction between the two opposing forces is at the muzzle pulley and the band keeper by the handle.

If you don't believe me, draw up a free body diagram accounting for the force vectors the instant the trigger is released, ignoring all vertical out of plane forces and solve for the impulse imparted on the speargun. To make it simple, ignore the division of forces conducted by the small pulley and assume the bands are connected directly to the long wishbone.

Now, a 7.5mm x 150cm shaft weighs in at ~500 grams. This speargun is ~ 120cm between the muzzle and the trigger mech, so use three 5/8" bands cut at 4:1 stretch ratio (300% elongation) which gives us 30cm per leg x 6 legs for a total length of 180cm of 5/8" latex tubing.

The cross sectional area of 1/8" I.D. x 1/4" Wall Thickness latex tubing is 0.29453 square inches or 1.9 square centimeters. Multiply that by the length of the tubing to find the volume, V = 1.9 sq. cm. x 180cm = 342 cubic centimeters.

The specific gravity of latex is 0.97, so the mass of the latex bands is 0.97 x 342cc = 332 grams. By my best estimates, this speargun weighs in at ~2.5 kg or 2500 grams. It may weigh more, it may weigh less, but we will use this number for this exercise.

According to the Primeline Force Calculator spreadsheet, the potential energy stored by a single leg of 1/8" I.D. x 1/4" Wall Thickness latex tubing at 300% elongation is 59.5 lbf (pounds-force) or 264.67 Newtons. For six band legs, that comes out to 1588 Newtons.

To find out what the force acting on the spearguns is, solve the following equation F = 1588 Newtons x (500 grams - 332 grams)/2500 grams. The force acting on the speargun is 106.7 Newtons.

To find out the acceleration of the speargun, divide 106.7 Newtons by the mass of the speargun, so 106.7 Newtons/2500 grams = 42.7 meters per second squared. This is the maximum instantaneous rate of acceleration applied to the speargun the instant the trigger is pulled.


Now, let us compare this to a conventional speargun with all else being equal.

F = 1588 Newtons x (500 grams + 332 grams)/2500 grams = 528 Newtons, or roughly FIVE times the force of this rollergun. The instantaneous acceleration acting on the conventional speargun is 211 meters per second squared.


To demonstrate the reduced recoil of the configuration shown, we will now factor in the effect of the gun tackle configuration of the small pulley, which basically adds another force couple, reducing the recoil further by a factor of 1.5. So, our 1/5th reduction in recoil using a single pulley now becomes a 1/7.5th reduction in recoil using two pulleys. Yes, Irma, that is 7.5 times less recoil.

With all things being equal, the band configuration used in this speargun has reduced recoil forces of a similarly configured conventional speargun by 86.7%.
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Old 10-29-2014, 05:42 AM   #19
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Re: Hatch Custom Spearguns "Yacht Gun"

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Originally Posted by spearq8 View Post
Yes but does it give you 7m to 8m range with pinpoint precision? ....To compensate for the lost mass you need to start really going into very high diameter shafts....
This is why I suggested skipping the 5/16" shaft and using the mass of an 11/32" (8.75mm) shaft. For the intended use of this speargun, the 5/16" (8mm) shaft is probably more than adequate.

When you look at the videos seal77 posted with his 130cm carbon fiber rollergun. http://spearboard.com/showthread.php?t=177601

He was accurately and consistently hitting his target at ranges out to 9 meters (29.5 feet) and he only used two 16mm (5/8") bands with an 8mm x 180cm (5/16" x 71") shaft with a slip tip to do it.

Once you have the configuration for accuracy dialed in, power can be added incrementally to provide penetration at distance.

As for accurately shooting and penetrating at distance, Ulusub's new slip tip would be my first choice. If only there was a way to get a Gamakatsu fish hook chemically sharpened point on a slip tip... I would save the floppers for the reef, and use a smaller roller speargun.

From my perspective, a four or five band 150cm bluewater gun can now be shrunk down to a 120cm - 130cm three-banded rollergun. The deep reef would no longer be handled by a 120cm two or three banded conventional speargun, but by a 90cm - 100cm mulit-band rollergun, and the typical reef gun will no longer be a 90cm - 100cm single or two banded conventional, but be serviced by 60cm - 75cm multiple band rollerguns.

Why? Because it is easier to dive with smaller, accurate, low-recoiling spearguns, making your bottom times last longer, which is one of the biggest factors to spearing quality fish.
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Last edited by SpearoTastic; 10-29-2014 at 05:56 AM.
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Old 10-29-2014, 10:42 AM   #20
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Re: Hatch Custom Spearguns "Yacht Gun"

Nice writeup. I'll have to check your figures when I have some spare time. I actually didn't pay enough attention to the pictures, and the first time I realized that the bands didn't loop over the top of the gun was when I was trying to figure out how you were saying there were no bands travelling in the same direction as the shaft.... hah.

And impulses impart momentum, blah blah blah, so we don't have to split hairs...

Quote:
Originally Posted by SpearoTastic View Post
....
If the accelerating band mass were equal to the accelerated shaft mass, there would be zero net velocity for the speargun, as the two forces imparting onto the speargun in opposite directions would cancel out, right?
Yes! As long as the *accelerations* of said masses were equal!!!!! But it's obvious that they aren't. As you demonstrated, the shaft's mass is almost double that of the bands, yet the speed of the shaft's center of mass is greater than that of the band's center of mass, so they can't cancel forces with this configuration, anyways.

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Originally Posted by SpearoTastic View Post
....
The line of interaction between the two opposing forces is at the muzzle pulley and the band keeper by the handle.
...
...draw up a free body diagram accounting for the force vectors the instant the trigger is released...
FBD! (well, what you make one from, anyways...)




For the rest of it, I need more time to ponder. But in the instant the shaft is released, before the shaft starts moving, I would say that the net force acting on the speargun is the same as the band tension: 1588 N, not 107 N per your later ratio calculation. The net force acting on the shaft is also the band tension. The net force acting on the bands I have to think about... The bands are creating the tensile force, and they will be contracting around their center of mass, so it feels like it should be zero at the moment of release.........

You may have to explain why you're using a ratio of masses to divide the forces up, because I don't recall that from Statics, and it's still not making sense to me.

So, my initial thoughts about your writeup is that the net force applied to the gun at trigger release should always be the band tension, no matter what the gun architecture. Therefore, the impulse would be [or, 'start out'] the same. [things might change as the rubber moves, seeing as how it's mass relative to the shaft is so much greater than I was thinking....... But I still think that the rubber moves so little, and the shaft moves so much, that most of the impulse must be borne by the gun stock, and very little momentum imparted into the rubber, because there is very little force acting on the rubber......]

I'm open to instruction, though! Thanks for the brain exercise.

Last edited by Tyson Brown; 10-29-2014 at 11:14 AM.
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Old 10-29-2014, 11:30 AM   #21
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Re: Hatch Custom Spearguns "Yacht Gun"

Some other random thoughts: the band's center of mass is going to move from 60cm to 15cm over the period during which the bands apply impulse to the shaft, a net distance of 45cm.

The shaft's center of mass moves the distance between the tabs when locked and the release point, which should be the same as the difference between stretched and relaxed bands - 90cm, by your example. Right?

So, the COM of the bands moves half the distance of the COM of the shaft over the fire-to-free period.

-----------------------------------------------

Also, I feel like the band's COM is going to accelerate at first, then peak, then decelerate as it comes closer to its relaxed state. Hooke's law - spring force is directly proportional to the stretched distance. As stretch goes away, the tension in the bands decreases, and so I think the acceleration of the band's mass is going to drop off.

-----------------------------------------------

And don't forget the whole "halving power" thing that you didn't address. Even if you get a perfectly "counterbalanced", recoil-free scenario where you have mass traveling backwards to counter the shaft's mass, you still have to use up half of your power to do it.

Last edited by Tyson Brown; 10-29-2014 at 11:50 AM.
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Old 10-29-2014, 12:48 PM   #22
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Re: Hatch Custom Spearguns "Yacht Gun"

Let's get one thing straight ... roller guns DO have recoil. The recoil is not as abrupt but is sort of a double recoil which almost feels like a misfire. Once you start using larger heavier shafts, you will of course get more recoil and although a roller setup will reduce the felt recoil, it will be substantially more than your average small single banded roller shooting a 7mm shaft. There is still one trick in a classical gun design that can decrease felt recoil without adding mass to the gun and improve shaft velocity with the same power load, but I need to test it out a little bit more as it is pretty radical design change. I really think that a classical banded gun still has some room where it can improve performance a little more.
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Old 10-29-2014, 02:32 PM   #23
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Re: Hatch Custom Spearguns "Yacht Gun"

So can we chill out with the nerd talk for a bit and just have a video of someone loading and shooting this thing lol
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Old 10-29-2014, 02:36 PM   #24
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Re: Hatch Custom Spearguns "Yacht Gun"

Thank you for the FBD, Tyson! And, yes, please ponder it some more as there is much to consider.

Upon firing, the bands will be contracting towards their fixed end, so there will be an impulse vector in that direction.

The ratio of masses comes from elastic collisions, which is a good approximation of what happens just after the trigger is pulled.

--------------
As for your random thoughts, I agree that the COM of the bands moves 45cm for a non-pretension band configuration. If the bands were pre-tensioned to the recommended 100% elongation, the COM of the bands would move 30cm. As for the shaft, assuming zero overhang, the COM should move 120cm at the completion of the powerstroke to match the displacement of the long wishbone, right? So, the COM of the bands would move 3/8 the distance of the shaft COM for a non-pretensioned band configuration, and move 1/4 the distance for a pretensioned band configuration. While I think this may be useful when calculating recoil energy, when it comes to recoil impulse, it is not so relevant.
--------------
Peak acceleration happens the instant the band acts on the shaft and diminishes from there. Peak shaft velocity, on the other hand happens when the bands are no longer imparting a force on the shaft, with pretensioned bands, this is when the bands are arrested by the muzzle. With conventional spearguns, this occurs before the bands return to their original shape at the moment when the hydrodynamic drag forces equal the band forces.
--------------
I am not sure what you mean by the "halving power", but the recoil, while not totally eliminated, is significantly reduced when the bands contract in the opposite direction of the shaft's motion.

But, keep at it, Tyson, and post up your thoughts. It may take delving deeply into ridid body dynamics to thoroughly explain what is occurring.

========

spearq8, explore the Roisub website. Check out the spearguns and watch the videos where they are used. After a while, I am sure you can easily convert one of your roller railguns or pipe guns to use one of the Roisub configurations and experiment for yourself. Winter is coming, after all.
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Old 10-29-2014, 02:45 PM   #25
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Re: Hatch Custom Spearguns "Yacht Gun"

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So can we chill out with the nerd talk for a bit and just have a video of someone loading and shooting this thing lol
Here you go. Now, this is not the Hatch gun, but the band configuration is the same. The gun in the video has hooks on the top of the barrel to retain the bands, instead of below, like the Hatch.

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Old 10-29-2014, 03:30 PM   #26
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Re: Hatch Custom Spearguns "Yacht Gun"

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So can we chill out with the nerd talk for a bit and just have a video of someone loading and shooting this thing lol
I must add: In real life to a live fish.
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Old 10-29-2014, 05:16 PM   #27
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Re: Hatch Custom Spearguns "Yacht Gun"

Cool proof of concept gun , or pool target gun ,but not a real world hunting gun . And very inefficient design the pully waste 1/2 of the band energy . A normal 2 stage single band roller will have more power and will be much faster to load .
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Old 10-29-2014, 08:00 PM   #28
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Re: Hatch Custom Spearguns "Yacht Gun"

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Cool proof of concept gun , or pool target gun ,but not a real world hunting gun . And very inefficient design the pully waste 1/2 of the band energy . A normal 2 stage single band roller will have more power and will be much faster to load .
I was kinda wondering the same thing. What is the advantage that this design could have over a simpler roller? I honestly think a conventional roller is complicated enough. I've jumped in the water more than once with an unloaded speargun and seen a wahoo sitting there.
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Old 10-29-2014, 09:38 PM   #29
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Re: Hatch Custom Spearguns "Yacht Gun"

At the risk of getting run off here....

Quote:
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...
The ratio of masses comes from elastic collisions, which is a good approximation of what happens just after the trigger is pulled.
...
I don't think that applies here. This is not a collision. It's a spring-mass system where the spring has mass and the rigid body to which the spring is attached is free to move. Anyways, in a collision, you can use mass ratios to describe what happens to relative velocities, but you can't use mass ratios to divvy up a force. Forces are always equal and opposite. the initial force in the setup above is always the band tension, no matter what the mass of the bands.

I know vaguely where to go from here, and it involves treating the mass of the bands as a point mass in the center of the bands, and treating the band sections on either side as separate massless springs. Then, conservation of energy and momentum apply. The initial force on the gun is still the band tension, so you can't go down the path you went down.

But, everyone knows that you always neglect the mass of the spring, so.....



Where's that video of mega fish death?
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Old 10-29-2014, 09:42 PM   #30
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Re: Hatch Custom Spearguns "Yacht Gun"

Just my opinion , but all this math is just math UNLESS someone actually CHRONOGRAPHS spear speed. And I'm not talking about some computer sound measurement. IMO, trying to evaluate spear penetration / energy transmitted to target is not possible without knowing TRUE chronograhed spear speed. I'm tired of seeing videos of pro ported roller gun speed (where the spear clearly hits the target LESS distance than a 2 wrap shooting line on a 90cm gun) and the shooter takes 20 seconds to load the roller gun. IMO, conventional vs roller gun comparison WITHOUT chronograhed spear speed is like arguing fin efficiency by personal opinion.
That said, SOME spearos will prefer roller guns and I am glad that the roller gun builders give these spearos beautifully designed guns to choose from.
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