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Post by the light works on Oct 20, 2013 13:54:16 GMT
Gun pointing backwards over the transom to fire to the rear of the boat. I know Punt guns face forwards, but it seems generally accepted on this thread that the larger gun or cannon should face backwards and make use of the transom as an anchor point, as the transom is generally used as the anchor point for may engines, and therefore accepted as the strongest part of the boat?....... So in mind the gun facing backwards will go forwards, if a bungee cord is attached to the gun and the transom, the recoil will "Pull" the boat forwards...?.. yes?... transom? the motor frame on an airboat is mounted to the structural frame of the boat. the transom is just there to stop water from running in when the boat is stopped. |
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Post by the light works on Oct 20, 2013 14:09:28 GMT
okay, yes, if you bury an anchor on each side of the test trench, and use heavy chains to chain the axle to those anchors, then the gun will react as you say. now put the toy car onto a cookie sheet. set the cookie sheet on a table. use tape or something to make sure the toy car does not move around on the cookie sheet. NOW push on the top front of the car. does it still rotate around the axle? Are you under the misapprehension that you cannot have static torque? Why would you say that? surely you don't want to bring that here. Or is your intent to sling shi........... stuff again normally yes although is it possible to float it on liquid nitrogen? Nope you clearly do not understand the concept of static torque I am trying to explain to you how your idea of a gun carriage including the force multiplying limber is going to effect the outcome. Also there is absolutely no reason you "cannot do the math or apply all the correct names to all the lines in the diagram" The WWW has it all. after reading some of the documentation on field guns, are you using "limber" to mean what the people of the time called a "trail"? you are trying to explain to me how it is that when you put a field gun with its standard wheels and trail carriage onto a sled, and anchor it in place on the sled; with the trail anchored to the frame of the sled in such a way as to transfer the force of the recoil to the sled, and the wheels generally secure so it doesn't bounce around; when fired, the force of firing will be applied to the wheels, which are beneath the balance point of the barrel; instead of the trail, which is normally designed so it slides along the ground and uses friction to slow the recoil of the gun enough that you don't have to go and fetch it after firing. if one is to compare a field gun carriage to a shoulder fired rifle, the wheels act much as your off hand on the foregrip, and the trail as the shoulder stock. please explain to me how, when firing the gun, the recoil acts as torque on the hand on the foregrip. |
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Post by the light works on Oct 20, 2013 14:15:12 GMT
why would I say that? I would say that because it looks an awful lot to me like you are more concerned that you get the results you want; than about understanding the mechanics of the scenario. the tone and manner of your argument leads me to believe that if I were to take my father-in-law's .50 caliber cannon, duct tape it to a cookie sheet, set it in my creek, and fire it off, without wrecking the cookie sheet; you would tell me I did it wrong because it should have wrecked the cookie sheet.
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Post by the light works on Oct 20, 2013 21:38:43 GMT
Airboats are shallow drafted boats that only plane when they get up to speed. It is unclear as to exactly how much draft an airboat has at rest, since that would depend on the overall weight of the boat. The figures I've seen appear to be around 2-4 inches. I'm guessing that you'd see around 4-6 inches with a six pounder cannon plus the additional reinforcement needed to protect the hull. (This shouldn't be a problem as it seems that the sides of an airboat are 10 inches high, and we're not talking about trying to take this thing to sea.) So you will always get inertia and resistance from the water, especially if the thrust is applied all at once as water is not compressible. Picture a cannon sitting on an airboat and tied in place with thick string. If you pushed on the cannon you would, eventually, get the boat to move backwards and once it was moving it would be fairly easy to keep it moving. The string would remain intact and the gun is going to stay where it is. Now imagine the same set up, but instead of pushing on the gun you decide to give it the same amount of energy as needed to push it a certain distance, but by hitting the gun once with a massive hammer. The force would snap the string, the cannon would go flying backwards and the boat is going nowhere, except possibly downwards as the cannon rips the stern of the boat apart. Because there is not enough room to allow it to do so - note both the distance the gun in the linked footage moved and that its carriage bleeds off the momentum by digging into the ground. On a smooth surface, such as the deck of a boat, such a gun would recoil even further. Far enough that its going to hit the stern with a lot of force. Besides, if the gun is allowed to move backwards then the thrust from the discharge is going into moving the gun over the deck with little or none of that energy going into moving the boat. Any movement would be due to the weight of the gun shifting not the recoil itself. basically, you are saying that they must make the boat stronger and heavier in order to sustain the force of the recoil against the resistance of the water. I am saying that I think the airboat hull is light enough, and of an adequate design, that a properly located gun will cause it to treat the water as a ramp, rather than be held in place by it. if you have a cannon tied to a sled with a piece of string, and you want to ensure the string does not break, you either make the string heavier, or you make the sled lighter. |
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Post by silverdragon on Oct 21, 2013 6:42:13 GMT
So no one ever puts an outboard on the transom of an air boat.... ever?...
Them big loud and can be heard many mile away, no one ever tried to move one on the quiet?...
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Post by the light works on Oct 21, 2013 14:41:02 GMT
So no one ever puts an outboard on the transom of an air boat.... ever?... Them big loud and can be heard many mile away, no one ever tried to move one on the quiet?... it has probably been done - but consider that the primary function of an airboat is to go where the water is too shallow for even a shallow water drive. |
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Post by Cybermortis on Oct 21, 2013 21:34:41 GMT
*Sigh*
The forces come from the movement - or attempted movement - of the gun against its mounting.
The gun gains energy practically instantly, and will start moving backwards almost instantly. The rest of the boat however does not start moving instantly. This is mainly due to the fact that while the gun is trying to move backwards in air, which has fairly low drag, the hull is trying to move backwards in water (which is not going to compress under this force) and will have a different amount of energy. The mounting, frame and hull will have to deal with this shearing force and the pressure of the water as the boat tries to move through the water.
With an engine the shearing force is marginal, as no engine is capable of creating that much force in an instant. So the frame, hull and connection points are not subjected to anything like the same amount of 'shock' force. With a gun however the energy being dumped into the boat may well exceed the structural limits of the design or materials - it is the same reason that you can't use high explosives as a replacement to gunpowder. The explosive may produce the same amount of energy as the powder, but it produces it all at the same instant which overloads the structural strength of the barrel.
It doesn't matter how you mount the gun, the boat is not going to treat water as a ramp because the boat is sitting IN the water. Airboats are SHALLOW drafted boats, not zero drafted boats - if they had zero draft they'd be hovering.
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Post by the light works on Oct 21, 2013 22:01:31 GMT
*Sigh* The forces come from the movement - or attempted movement - of the gun against its mounting. The gun gains energy practically instantly, and will start moving backwards almost instantly. The rest of the boat however does not start moving instantly. This is mainly due to the fact that while the gun is trying to move backwards in air, which has fairly low drag, the hull is trying to move backwards in water (which is not going to compress under this force) and will have a different amount of energy. The mounting, frame and hull will have to deal with this shearing force and the pressure of the water as the boat tries to move through the water. With an engine the shearing force is marginal, as no engine is capable of creating that much force in an instant. So the frame, hull and connection points are not subjected to anything like the same amount of 'shock' force. With a gun however the energy being dumped into the boat may well exceed the structural limits of the design or materials - it is the same reason that you can't use high explosives as a replacement to gunpowder. The explosive may produce the same amount of energy as the powder, but it produces it all at the same instant which overloads the structural strength of the barrel. It doesn't matter how you mount the gun, the boat is not going to treat water as a ramp because the boat is sitting IN the water. Airboats are SHALLOW drafted boats, not zero drafted boats - if they had zero draft they'd be hovering. we have now managed to reduce the argument to a question of degrees. you believe that the gun will be too powerful for whatever retention device they come up with to use to install it in a standard airboat hull. I believe it will not. keep in mind that a damascus steel gun barrel is not strong enough to withstand the barrel pressures of modern smokeless powders, but they were used for years with black powder with relatively little trouble. guns old enough to use black powder will have a slower recoil than guns using modern powders - but guns that use modern powders typically have a recoil compensator built into the carriage. similarly, when you place the gun into the airboat hull, I believe the airboat hull will be strong enough to survive the transfer of energy. now decide: is the water compressible, in which case the force has to displace the water enough to allow the boat to move, or is it incompressible, in which case the force has to bring the boat up out of the water in order to move it? taken to its component elements, we are striking a ball with a stick to move it. you want to use a cricket ball, I want to use a whiffleball. |
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Post by the light works on Oct 21, 2013 22:03:27 GMT
of course, if your intention is to keep ramping it up until the hull is destroyed, rather than just ramp it up until you get good movement, then yes, I have to agree that a borrowed boat is a bad idea.
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Post by Cybermortis on Oct 21, 2013 23:08:13 GMT
My concern lies with the structural frame of the boat, which is simply not designed to deal with some 600+ lbs of force moving horizontally in a microsecond - we are talking about 1/4tr inch steel tubing here. I'm concerned that the forces involved are simply going to prove too much, if not for the tubing them for the welds holding the frame together.
At best this could be inconvenient, as it would prevent the possibility of multiple tests thereby limiting the amount of footage they get - better to get too much footage than too little. At worst it would be very dangerous, as you could end up with a flying cannon or structural elements of the boat.
This is based on the weight and force produced by a six pound cannon of the type used on the show before - smaller guns may prove less problematic.
Sure, they can test to destruction if they own both the boat and the gun. But for practical reasons they would want to make sure that the repeated firings are not going to weaken the frame before then. They might also decide that testing to destruction is not practical, after all they would have to recover the boat and gun afterwards and they may feel that is more trouble than it is worth.
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Post by the light works on Oct 21, 2013 23:32:42 GMT
My concern lies with the structural frame of the boat, which is simply not designed to deal with some 600+ lbs of force moving horizontally in a microsecond - we are talking about 1/4tr inch steel tubing here. I'm concerned that the forces involved are simply going to prove too much, if not for the tubing them for the welds holding the frame together. At best this could be inconvenient, as it would prevent the possibility of multiple tests thereby limiting the amount of footage they get - better to get too much footage than too little. At worst it would be very dangerous, as you could end up with a flying cannon or structural elements of the boat. This is based on the weight and force produced by a six pound cannon of the type used on the show before - smaller guns may prove less problematic. Sure, they can test to destruction if they own both the boat and the gun. But for practical reasons they would want to make sure that the repeated firings are not going to weaken the frame before then. They might also decide that testing to destruction is not practical, after all they would have to recover the boat and gun afterwards and they may feel that is more trouble than it is worth. I am under the impression most airboat hulls are made from aluminum, and as I said during the squabble, they are built with the expectation that you will hit things with them, and might need to remain afloat. one of the clips I linked was a news item about two airboats that had a collision, with one towing the other back from it. in one of my posts, I did calculations based on a muzzle velocity between 1000 FPS and 2000 FPS as the range of velocities from 6 pounders (a Hotchkiss gun is midway between the two) and a gun and carriage weight in the 1600-1700# range, and the boat hull around 300-400#. while my calculations were fast and sloppy, it came to under 5MPH. they used to make cars to be able to run into things at 5MPH without damage; or damage only to the bumper. I haven't gotten up close and personal with an airboat, because we don't have that sort of terrain here. what we have are what we commonly call "Alumaweld Sled" which are used for river fishing and are also designed to be tolerant of being banged about. (Alumaweld being the manufacturer who pioneered the design) this is a later more refined design: they are not designed to plane as completely as an airboat, but they are still built with the expectation that you will bang them into the occasional rock. |
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Post by Cybermortis on Oct 22, 2013 12:12:07 GMT
Typically airboats have a 1/8th thick aluminium hull on a 1/4tr thick steel frame.
My figures were based on a six pounder smooth bore cannon of the sort used in the American Civil war. I picked this particular type of gun because this is what 'Old Moses' happens to be, and Mythbusters have used that gun several times. It is also a size of gun that would be able to physically fit on a small airboat hull without causing problems.
Muzzle velocity for such a gun is between 1300-1500 fps, assuming a full charge and a correct loading. Weight of such a gun with its carriage is 850 lbs.
Interestingly I also got a recoil speed of 5 mph when I did some calculations. Then I realised that I was forgetting that the distance I was allowing the gun to recoil and the time were based on ship mounted guns, which were brought up by the tackle. The force of the recoil was known to be quite capable of snapping the breaching tackle (which was at least one inch thick rope) even for the light and low powered carronade.
The frame of an airboat is designed to reduce flexing of the hull as it moves over and through the water. Impacts with rocks or other underwater objects would be applying force from the same direction. In the case of an impact with another boat the energy is fairly well spread out, and like the normal way force is dealt with it passes from the hull to the frame.
In the case of a recoiling gun on a boat, you can't apply the force directly to the hull so have to apply it to the frame. This makes the force more concentrated, and very likely means that the force is being applied at a slight upwards angle.
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Post by the light works on Oct 22, 2013 14:05:36 GMT
Typically airboats have a 1/8th thick aluminium hull on a 1/4tr thick steel frame. My figures were based on a six pounder smooth bore cannon of the sort used in the American Civil war. I picked this particular type of gun because this is what 'Old Moses' happens to be, and Mythbusters have used that gun several times. It is also a size of gun that would be able to physically fit on a small airboat hull without causing problems. Muzzle velocity for such a gun is between 1300-1500 fps, assuming a full charge and a correct loading. Weight of such a gun with its carriage is 850 lbs. Interestingly I also got a recoil speed of 5 mph when I did some calculations. Then I realised that I was forgetting that the distance I was allowing the gun to recoil and the time were based on ship mounted guns, which were brought up by the tackle. The force of the recoil was known to be quite capable of snapping the breaching tackle (which was at least one inch thick rope) even for the light and low powered carronade. The frame of an airboat is designed to reduce flexing of the hull as it moves over and through the water. Impacts with rocks or other underwater objects would be applying force from the same direction. In the case of an impact with another boat the energy is fairly well spread out, and like the normal way force is dealt with it passes from the hull to the frame. In the case of a recoiling gun on a boat, you can't apply the force directly to the hull so have to apply it to the frame. This makes the force more concentrated, and very likely means that the force is being applied at a slight upwards angle. with our aluminum boats, the frame is integral with the hull, and I am assuming airboats are of the same basic design. however, they have to have hardpoints for the installation of the motor mount; because there isn't really any good way to float something like that; and the hardpoints have to be secure enough to deal with the leverage of the tall riser. that provides you with a mounting point to secure your carriage adaptor. it looks to me like the standard field carriage focuses the force of the recoil on the trail; which is designed to not dig in; in order to prevent damage. I would build a sort of shoe that laid the length of the boat, and contoured to fit the hull; and set cross bracing to trap the end of the trail. this would transfer the push from the recoil to the shoe, while spreading any downward component along the length of the hull. I could then build out from that to secure it to as many hard points as were available. the wheels on the carriage, as I said earlier, serve much as your off hand on the foregrip of a rifle: the keep the muzzle up. they could have a simple slot to fit in, that would keep the gun in line, and be tall enough to keep it from jumping out. |
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Post by the light works on Oct 22, 2013 14:10:22 GMT
When you approach the puzzle with a "this might work, let's find out how" it becomes a much less impossible puzzle than if you start with a "This won't work, let's determine why" attitude.
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Post by Cybermortis on Oct 22, 2013 15:48:00 GMT
When you approach the puzzle with a "this might work, let's find out how" it becomes a much less impossible puzzle than if you start with a "This won't work, let's determine why" attitude. Unless you are planning for a TV show when you have to approach it from the 'What could go wrong and cost us money' angle, which is what I have been doing. The carriage used for field guns could be used on a boat, but does have both physical and practical (eg; TV related) problems. Physically the problem is that the gun, and its recoil, will be a fair distance above the boats centre of gravity which could affect stability. On (or in) a water filled trench this might not prove a problem, unless the gun shifts when it is fired. This leads us to the practical problem - you do not want to damage the boat or gun, or at least not unless or until that is what you intend. In part this is because ideally you want to be able to conduct multiple firings, cameras may not pick up footage or it may simply not look impressive enough. The other part is that unless they happen to own both gun and boat, the owners are not going to allow them to do anything that could damage or destroy their property. In the case of (say) Old Moses this would include damaging the carriage. MB could probably make them a brand new gun carriage if they had to, but that means taking resources and time away from other projects and episodes. |
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Post by the light works on Oct 22, 2013 16:02:55 GMT
When you approach the puzzle with a "this might work, let's find out how" it becomes a much less impossible puzzle than if you start with a "This won't work, let's determine why" attitude. Unless you are planning for a TV show when you have to approach it from the 'What could go wrong and cost us money' angle, which is what I have been doing. The carriage used for field guns could be used on a boat, but does have both physical and practical (eg; TV related) problems. Physically the problem is that the gun, and its recoil, will be a fair distance above the boats centre of gravity which could affect stability. On (or in) a water filled trench this might not prove a problem, unless the gun shifts when it is fired. This leads us to the practical problem - you do not want to damage the boat or gun, or at least not unless or until that is what you intend. In part this is because ideally you want to be able to conduct multiple firings, cameras may not pick up footage or it may simply not look impressive enough. The other part is that unless they happen to own both gun and boat, the owners are not going to allow them to do anything that could damage or destroy their property. In the case of (say) Old Moses this would include damaging the carriage. MB could probably make them a brand new gun carriage if they had to, but that means taking resources and time away from other projects and episodes. even "what could go wrong and how do we mitigate that" is not as pessimistic as "this won't work" certainly the concern over damaging a bona fide antique gun carriage is a valid. one thought is that History channel has access to a Hotchkiss mountain gun, or two, as well as a couple of bulldog gatling guns. if they would play nice together, they might be able to borrow one. |
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Post by Cybermortis on Oct 22, 2013 18:04:06 GMT
Reading back, and going away from the debate about design concerns, I wonder what else they could test that is related to this?
I'm thinking that it would be fun to see what happens if you fired a really big gun from a hovercraft. The recoil force should result in considerably more backwards movement as there is a lot less resistance to overcome. In this case the set up would probably be a little simpler than using a boat. All they need is some steel cables passing through loops at the side of the hovercraft to make sure it stays aimed in the right direction, and of course this would only require a flat area in which they can fire the gun.
The second is possibly a more interesting question - could you make a literal 'gunboat'? That is not a boat on which you have a gun, but a gun that could float? Thinking like a Mythbuster here, but if they do get backwards motion from the recoil of a gun. Then the next logical step would be to see if you could make a boat where the engine is replaced with a gun. I'd guess that for safety and time reasons such a boat would probably be fairly small, probably using a semi-automatic shotgun.
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Post by the light works on Oct 22, 2013 18:19:09 GMT
I think a gun hovercraft would be a much bigger challenge, because then you have a lot more potential for torque to have an effect. you would almost have to have the gun on the axis of balance, to keep it from tipping the hovercraft and spilling lift, grounding the bow, or inducing unwanted motion. or maybe you would want the motion - using the gun as a cyclic control as well as a thruster.
by gunboat, are you thinking gun propelled boat, rather than marine gun? I would think a gatling gun would be the best option for that, as it does not use any of the recoil for other functions. it will also give a steady rate of fire.
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Post by the light works on Oct 22, 2013 18:24:58 GMT
on reflection, I think induced motion from the hovercraft wobbling around might become an issue rather than a helper.
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Post by the light works on Oct 22, 2013 18:32:05 GMT
wonder if they could get access to a maglev track with a safe backstop behind it...
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