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Post by the light works on Oct 16, 2013 15:15:29 GMT
True, but in that case there was no risk of the boat being damaged. I think even with a modest sized cannon, the odds are still pretty slim. |
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Post by Cybermortis on Oct 16, 2013 15:40:03 GMT
As I noted, a six pound cannon (a small cannon as things go) masses around 650-700lbs and would recoil some six or seven feet backwards in less than a second on a ship. Imagine what that amount of force would do to the hull of an airboat.
Even the big boats used by the Royal Navy circa 1805 could not handle the recoil of guns that size - and these were vessels that were capable of sailing halfway across the Indian Ocean. These boats could handle a six pound carronade, which was not only a lot lighter than a cannon but notable in that the gun carriage was designed to absorb much of the guns recoil, rather than replying on the frame of a ship to do that.
That is the reason that they would want to buy or build their own large boat if they want/can test something a lot bigger than a punt gun. It eliminates any concerns about what happens to the boat, so as Dragon noted would give them the option of testing to the point that the ship ends up at the bottom of the trench. (Assuming they can find or build a gun to do this with...somehow I doubt that the owner of Old Moses would let them do such a thing with his gun.)
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Post by the light works on Oct 17, 2013 2:34:25 GMT
As I noted, a six pound cannon (a small cannon as things go) masses around 650-700lbs and would recoil some six or seven feet backwards in less than a second on a ship. Imagine what that amount of force would do to the hull of an airboat. Even the big boats used by the Royal Navy circa 1805 could not handle the recoil of guns that size - and these were vessels that were capable of sailing halfway across the Indian Ocean. These boats could handle a six pound carronade, which was not only a lot lighter than a cannon but notable in that the gun carriage was designed to absorb much of the guns recoil, rather than replying on the frame of a ship to do that. That is the reason that they would want to buy or build their own large boat if they want/can test something a lot bigger than a punt gun. It eliminates any concerns about what happens to the boat, so as Dragon noted would give them the option of testing to the point that the ship ends up at the bottom of the trench. (Assuming they can find or build a gun to do this with...somehow I doubt that the owner of Old Moses would let them do such a thing with his gun.) probably something along the lines of this: keep in mind a stock Chevy 350 V-8 weighs around 575 pounds. also keep in mind airboats are made to slide on top of the water instead of made to provide a stable gun platform. |
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Post by silverdragon on Oct 17, 2013 8:25:38 GMT
The top of a Van can be used.
If you take the fibreglass top of a small "One-Ton" (Up to 3or5 ton max) van, with a little bracing and fixing, it floats, I know a few people have done that, its quite effective.. you do have to glass-fibre in supports, but that isnt exactly rocket science... even I could do that?....
Going one further, say a week, I could mock up a mould big enough to make a glass fibre hull.
Three days to build the hull, you have to let it cool between layers or it overheats, glass in some supports at that stage, and make sure you have some serious air pockets built in for un-sinkability.
One of my first jobs was building RIB Rigid inflatable, you could make a full one in a week.
Just.
--The worst part is trimming the fibreglass with a chainsaw... the dust gets EVERYWHERE... and I do mean everywhere... and it itches... that job is done in a sealed room with NBC suit and breathing apparatus.
So saying that, if you take the top off a scrap van, its a good place to start making an air-boat type hull thats lightweight, add wooden bracing, and Bob is a well known relative.
Yes its a "Big build", but whoever said it wouldnt be?...
Edit..Additional, on something that big, see previous question on keeping grant bust about adding a pod at each corner for geo-stability.
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Post by the light works on Oct 17, 2013 14:03:29 GMT
The top of a Van can be used. If you take the fibreglass top of a small "One-Ton" (Up to 3or5 ton max) van, with a little bracing and fixing, it floats, I know a few people have done that, its quite effective.. you do have to glass-fibre in supports, but that isnt exactly rocket science... even I could do that?.... Going one further, say a week, I could mock up a mould big enough to make a glass fibre hull. Three days to build the hull, you have to let it cool between layers or it overheats, glass in some supports at that stage, and make sure you have some serious air pockets built in for un-sinkability. One of my first jobs was building RIB Rigid inflatable, you could make a full one in a week. Just. --The worst part is trimming the fibreglass with a chainsaw... the dust gets EVERYWHERE... and I do mean everywhere... and it itches... that job is done in a sealed room with NBC suit and breathing apparatus. So saying that, if you take the top off a scrap van, its a good place to start making an air-boat type hull thats lightweight, add wooden bracing, and Bob is a well known relative. Yes its a "Big build", but whoever said it wouldnt be?... Edit..Additional, on something that big, see previous question on keeping grant bust about adding a pod at each corner for geo-stability. airboat hulls are welded aluminum. much more resistant to punishment than fiberglass. keep in mind the traditional method of launching an airboat is to shove it off the trailer on to the ground. excuse the moron cameraman: |
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Post by Cybermortis on Oct 17, 2013 14:50:59 GMT
They have a week to film an entire segment, including any builds required. The time constraints are why they usually work with wood and metal, they are faster and more forgiving of mistakes than fibreglass as well as being materials/rigs that can be recycled even if damaged.
I have seen fibreglass airboats for sale, but they are considerably more expensive than their metal counterparts. I did some checking and found a 13 foot aluminium airboat hull for sale at $750. The cheapest fibreglass hull I could find was $2000.
A metal hull would be preferable, as they can build a support structure and weld it onto the existing hull so the recoil from a large gun would be spread evenly throughout the entire frame. Rather than having a separate frame that could put excessive force on a fairly small part of the hull and end up punching a hole in it. This, btw, is another argument for either buying or making their own in the shop rather than trying to hire one out for the day.
I'm leaning towards the idea of building their own boat, as this would allow them to design the frame so that it has a mount for a heavy gun (rather than putting the weight directly on the keel) and so that it can handle the recoil forces far better than a normal ships frame could. They would have to modify any boat to do this if they intended to supersize things anyway, and I'm guessing that it would be faster to design and build a new frame than to try and modify an existing frame.
Besides which footage of them building the boat can be included in the episode to pad things out and to hark back to comments about how they never show the builds any more.
A home made boat could also be built to the most convenient size, both in terms of what they can transport easily and in terms of what is needed for the guns they are able to use for testing.
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Post by the light works on Oct 17, 2013 15:46:26 GMT
They have a week to film an entire segment, including any builds required. The time constraints are why they usually work with wood and metal, they are faster and more forgiving of mistakes than fibreglass as well as being materials/rigs that can be recycled even if damaged. I have seen fibreglass airboats for sale, but they are considerably more expensive than their metal counterparts. I did some checking and found a 13 foot aluminium airboat hull for sale at $750. The cheapest fibreglass hull I could find was $2000. A metal hull would be preferable, as they can build a support structure and weld it onto the existing hull so the recoil from a large gun would be spread evenly throughout the entire frame. Rather than having a separate frame that could put excessive force on a fairly small part of the hull and end up punching a hole in it. This, btw, is another argument for either buying or making their own in the shop rather than trying to hire one out for the day. I'm leaning towards the idea of building their own boat, as this would allow them to design the frame so that it has a mount for a heavy gun (rather than putting the weight directly on the keel) and so that it can handle the recoil forces far better than a normal ships frame could. They would have to modify any boat to do this if they intended to supersize things anyway, and I'm guessing that it would be faster to design and build a new frame than to try and modify an existing frame. Besides which footage of them building the boat can be included in the episode to pad things out and to hark back to comments about how they never show the builds any more. A home made boat could also be built to the most convenient size, both in terms of what they can transport easily and in terms of what is needed for the guns they are able to use for testing. I still think you're trying to reinvent the wheel. the motor mounts on a large airboat are made to hold a 600# engine, transfer the thrust to the hull, and survive the vibration involved. certainly you couldn't put a cannon at the elevation the airboat engine rides at, but they could certainly make a gunmount that would connect to the mounting points in the hull - and still show that process as a build montage. |
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Post by privatepaddy on Oct 18, 2013 13:48:56 GMT
So has any one sat down, drawn a picture. A diagram of what your trying to achieve? You appear to want to put a six pounder on a small boat. Forget momentum of gasses your talking forces and not the forces of a V8 driving a propeller, Shear..... sorry forgot my place as an observer. Carry on  |
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Post by Cybermortis on Oct 18, 2013 14:21:17 GMT
So has any one sat down, drawn a picture. A diagram of what your trying to achieve? You appear to want to put a six pounder on a small boat. Forget momentum of gasses your talking forces and not the forces of a V8 driving a propeller, Shear..... sorry forgot my place as an observer. Carry on I used a six pounder cannon as a basis for the heaviest weight the boat would have to take simply because this is the largest size of cannon they have used on the show. I doubt, however, that they would get to use this gun since the carriage it rests on is totally unsuitable for use on a boat and making a new carriage is probably far more work than they have time for. Paddy has a good point about the forces the boat has to withstand. The weight of the gun is only part of the problem, the other is that the 600+ lbs of weight is going to want to move six feet or more backwards in less than a second. This force would cause considerable damage to the frame and hull of a boat unless it was heavily reinforced. A (historical) example comes from HMS Speedy in 1801. In an attempt to increase her firepower (Then) Commander Cochrane had a pair of twelve pound cannons mounted as chase guns (her broadside guns were four pounders). Test firing these guns, even with a reduced charge, came close to ripping the frame and decking apart even though the ship had no problems carrying the weight of the guns alone. What you'd see if you tried a six pound cannon in a small boat (or even a fairly large one) would be just as bad, if not worse if you used anything close to a full charge for the gun. As I said, the nature of the boat they use and its size will probably rest on what guns they can use for testing. But it is easier to consider the best/worst case scenario in terms of hull strength than to ignore it and end up watching the boat sink as the gun fires. Re; Stability - The stability factor is about where the weight of the gun is located in relation to the centre of gravity. If the weight is too high up the vessel is liable to roll far more than is safe. Although this might not be considered a problem for a boat that is resting in a water filled trench, if the gun was recoiling off the centreline of the ship there is a chance it would roll. The best case is simply that the boat floods and sinks, but it is also possible that the gun will be dismounted as the boat rolls and either smack into the side of the hull or worse the side of the trench. Both could damage the gun, which is something that should be avoided until/unless MB make the decision to do something that could damage it - such as overloading it. The safest way to counter this problem is to place the weight as low down as possible on the centreline - which also means that almost all of the recoil force created by the gun will go into moving the boat backwards rather than in other directions. Note that, as I mentioned, a canoe has something that can partly offset instability caused by the weight it is carrying - the rower. However MB are unlikely to consider having someone sitting in the boat when the gun is being fired as safe - at least not for a larger gun. So they will need to try and keep the weight as low down and centred as possible. |
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Post by the light works on Oct 18, 2013 14:30:28 GMT
So has any one sat down, drawn a picture. A diagram of what your trying to achieve? You appear to want to put a six pounder on a small boat. Forget momentum of gasses your talking forces and not the forces of a V8 driving a propeller, Shear..... sorry forgot my place as an observer. Carry on something like this: only in one of these: |
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Post by the light works on Oct 18, 2013 14:37:41 GMT
so here is an airboat hull with, if I am doing my math correctly, about 2000# of payload on it. note that it is designed to slide easily rather than resist sideways movement. the dynamics of recoil are, in my own uneducated perspective, slightly different between this and a sailing vessel firing broadsides.
(note that I am counting everything that would not be there if they had a gun carriage secured to the hull)
addendum: It looks like the average empty airboat hull weighs between 200 and 300 pounds. - so that is the mass you are accelerating with the recoil. consider that the question appears to be whether the recoil will carry the airboat with the cannon, or tear it apart.
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Post by privatepaddy on Oct 18, 2013 14:49:42 GMT
So has any one sat down, drawn a picture. A diagram of what your trying to achieve? You appear to want to put a six pounder on a small boat. Forget momentum of gasses your talking forces and not the forces of a V8 driving a propeller, Shear..... sorry forgot my place as an observer. Carry on I used a six pounder cannon as a basis for the heaviest weight the boat would have to take simply because this is the largest size of cannon they have used on the show. I doubt, however, that they would get to use this gun since the carriage it rests on is totally unsuitable for use on a boat and making a new carriage is probably far more work than they have time for. Paddy has a good point about the forces the boat has to withstand. The weight of the gun is only part of the problem, the other is that the 600+ lbs of weight is going to want to move six feet or more backwards in less than a second. This force would cause considerable damage to the frame and hull of a boat unless it was heavily reinforced. A (historical) example comes from HMS Speedy in 1801. In an attempt to increase her firepower (Then) Commander Cochrane had a pair of twelve pound cannons mounted as chase guns (her broadside guns were four pounders). Test firing these guns, even with a reduced charge, came close to ripping the frame and decking apart even though the ship had no problems carrying the weight of the guns alone. What you'd see if you tried a six pound cannon in a small boat (or even a fairly large one) would be just as bad, if not worse if you used anything close to a full charge for the gun. As I said, the nature of the boat they use and its size will probably rest on what guns they can use for testing. But it is easier to consider the best/worst case scenario in terms of hull strength than to ignore it and end up watching the boat sink as the gun fires. Re; Stability - The stability factor is about where the weight of the gun is located in relation to the centre of gravity. If the weight is too high up the vessel is liable to roll far more than is safe. Although this might not be considered a problem for a boat that is resting in a water filled trench, if the gun was recoiling off the centreline of the ship there is a chance it would roll. The best case is simply that the boat floods and sinks, but it is also possible that the gun will be dismounted as the boat rolls and either smack into the side of the hull or worse the side of the trench. Both could damage the gun, which is something that should be avoided until/unless MB make the decision to do something that could damage it - such as overloading it. The safest way to counter this problem is to place the weight as low down as possible on the centreline - which also means that almost all of the recoil force created by the gun will go into moving the boat backwards rather than in other directions. Note that, as I mentioned, a canoe has something that can partly offset instability caused by the weight it is carrying - the rower. However MB are unlikely to consider having someone sitting in the boat when the gun is being fired as safe - at least not for a larger gun. So they will need to try and keep the weight as low down and centred as possible. I get sick and tired of being the person that rains on the parade all the time, its a good idea which if the MB's take it up they will figure it out. A canon imparts its force in fractions of seconds, Newtons third law & f = p x A as the black powder burns. Any frame , mounting and vessel has to withstand the shear forces that will happen when the canon is fired. Ok an example pick a weight up with a thread of cotton gently no problem. Then jerk it as you pick it up, thread breaks. |
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Post by the light works on Oct 18, 2013 14:58:53 GMT
Is it completely lost on you that airboat hulls are specifically designed to have MINIMUM resistance to movement and MAXIMUM resistance to damage?
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Post by privatepaddy on Oct 18, 2013 15:00:49 GMT
so here is an airboat hull with, if I am doing my math correctly, about 2000# of payload on it. note that it is designed to slide easily rather than resist sideways movement. the dynamics of recoil are, in my own uneducated perspective, slightly different between this and a sailing vessel firing broadsides. (note that I am counting everything that would not be there if they had a gun carriage secured to the hull) addendum: It looks like the average empty airboat hull weighs between 200 and 300 pounds. - so that is the mass you are accelerating with the recoil. consider that the question appears to be whether the recoil will carry the airboat with the cannon, or tear it apart. so what is your best calculated guess, what is the instantaneous force rating of every rivet, channel and bracing? the boat has, I would really like you for a change to back your statements with hard engineering facts instead of wild guesses. What does happen to the hull of your preferred test boat if the firing angle is off by a few degrees(not exactly on the centreline)? and as for the shear forces what happens per distance that the cannon is above the centre of gravity/waterline? |
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Post by Cybermortis on Oct 18, 2013 15:09:15 GMT
Chase guns are mounted in the bow and stern of a ship, therefore fired along the forward axis of the ship - they were not placed directly on the centre line as the bowsprit was in the way but their weight was offset by having another gun of the same size and weight next to them. (There is a good reason why sailing ships had even numbers of guns.)
Regardless of where they were mounted, the limits on what size and weight of gun that could be carried and used on a ship or boat rested on the width of the deck and the strength of the frame and hull. In regards the chasers, which technically had more space to recoil in, the limit was a mixture of the strength of the frame, hull and decking and how unstable the weight made the ship.
As noted, HMS Speedy (a Sloop massing around 150 tons, armed with 14 four pound cannon) was simply not strong enough to handle the recoil of a 12 pound cannon even when the guns were not fired with a full charge.
It was (and is) possible to mount heavier guns on ships of this size if the hull and decking is reinforced to take the weight and recoil. HMS Wolverine was a good example - she was a sloop aimed with a couple of 18 pound cannon. However putting such weight high up made her very unstable, in even fairly moderate seas it was not unusual for the crew to have to run the guns on the windward side out and those on the leeward side inwards to prevent her from rolling so much she lost her upper masts or worse the guns broke free. (Nothing says 'This is not a good day' than three tons of cast iron sliding over the deck towards your legs.)
MB would not have quite the same problems, after all they are not sailing the boat. However the stability problem is not something that could be ignored since if the gun is even slightly off the centreline it could force one side of the boat down enough to flood it or end up dismounting the gun.
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Post by privatepaddy on Oct 18, 2013 15:10:29 GMT
Is it completely lost on you that airboat hulls are specifically designed to have MINIMUM resistance to movement and MAXIMUM resistance to damage? No but your taking the comparatively gentle force of a V8 off and putting a bloody canon in its place..............  |
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Post by privatepaddy on Oct 18, 2013 15:15:23 GMT
Chase guns are mounted in the bow and stern of a ship, therefore fired along the forward axis of the ship - they were not placed directly on the centre line as the bowsprit was in the way but their weight was offset by having another gun of the same size and weight next to them. (There is a good reason why sailing ships had even numbers of guns.) Regardless of where they were mounted, the limits on what size and weight of gun that could be carried and used on a ship or boat rested on the width of the deck and the strength of the frame and hull. In regards the chasers, which technically had more space to recoil in, the limit was a mixture of the strength of the frame, hull and decking and how unstable the weight made the ship. As noted, HMS Speedy (a Sloop massing around 150 tons, armed with 14 four pound cannon) was simply not strong enough to handle the recoil of a 12 pound cannon even when the guns were not fired with a full charge. It was (and is) possible to mount heavier guns on ships of this size if the hull and decking is reinforced to take the weight and recoil. HMS Wolverine was a good example - she was a sloop aimed with a couple of 18 pound cannon. However putting such weight high up made her very unstable, in even fairly moderate seas it was not unusual for the crew to have to run the guns on the windward side out and those on the leeward side inwards to prevent her from rolling so much she lost her upper masts or worse the guns broke free. (Nothing says 'This is not a good day' than three tons of cast iron sliding over the deck towards your legs.) MB would not have quite the same problems, after all they are not sailing the boat. However the stability problem is not something that could be ignored since if the gun is even slightly off the centreline it could force one side of the boat down enough to flood it or end up dismounting the gun. You want to put a sloop  in as a test vessel? a whaler maybe, but this thread is talking a captains dingy. Since I have rowed both, never mind  . {I was using a sloop as an example of what can happen even with something a lot larger and stronger than a boat if you use guns that are simply too large for it. I used HMS Speedy and HMS Wolverine simply because I know of these ships and the effect oversized guns had on their hulls. - CM}And I forgot the smiley's was late my time |
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Post by Cybermortis on Oct 18, 2013 15:19:42 GMT
Is it completely lost on you that airboat hulls are specifically designed to have MINIMUM resistance to movement and MAXIMUM resistance to damage? No but your taking the comparatively gentle force of a V8 off and putting a bloody canon in its place.............. Have to agree with Paddy on this one*. The force an airboat hull has to withstand from the weight of the engine and the thrust are fairly minor, and spread over a long period of time. The force it would have to withstand from a cannon is considerably larger and almost instantaneous. An analogy would be a car roof. I could stand on a car roof without causing any real damage to it, I can even walk on it or push down in it without doing anything notable. If however I fall on the car from two stories up there is going to be a mess. (*Try not to look surprised) |
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Post by privatepaddy on Oct 18, 2013 15:21:56 GMT
I'm backing out of this discussion just about as fast as I can, I have finals soon and lots of other things going on. I am sure you will sort it.
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Post by the light works on Oct 19, 2013 0:44:36 GMT
so here is an airboat hull with, if I am doing my math correctly, about 2000# of payload on it. note that it is designed to slide easily rather than resist sideways movement. the dynamics of recoil are, in my own uneducated perspective, slightly different between this and a sailing vessel firing broadsides. (note that I am counting everything that would not be there if they had a gun carriage secured to the hull) addendum: It looks like the average empty airboat hull weighs between 200 and 300 pounds. - so that is the mass you are accelerating with the recoil. consider that the question appears to be whether the recoil will carry the airboat with the cannon, or tear it apart. so what is your best calculated guess, what is the instantaneous force rating of every rivet, channel and bracing? the boat has, I would really like you for a change to back your statements with hard engineering facts instead of wild guesses. What does happen to the hull of your preferred test boat if the firing angle is off by a few degrees(not exactly on the centreline)? and as for the shear forces what happens per distance that the cannon is above the centre of gravity/waterline? based on the fact that the punt turned slightly as it drifted backwards in the video I posted, and airboats are not very resistant to lateral drift, I would guess that it will essentially be driven straight away from the axis of the gun barrel. as for the welds, since by definition, a proper weld is stronger than the base metal, then assuming the recoil is not distributed evenly throughout the boat, it will not be substantially effected. here is a film clip showing the sort of damage that occurs when two airboats collide. I know a head on collision between two boats is not quite the same as firing a cannon on a boat, but it should give you some idea of the durability of the material. I guess if you wanted to so preliminary small scale testing, you could test with a "toy" cannon on a baking pan. it would be about the same scale, I think |
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in as a test vessel? a whaler maybe, but this thread is talking a captains dingy. Since I have rowed both, never mind 
.