Reply With QuoteCould no one give me a yeah that's close or no your way off, in 3 years? Just saying.
I am thinking of building a propane tank submarine in a year or so and I'm trying to get some different perspectives on hull crush calculations. I've been researching the project for a few years and realise that hull crush depth can be difficult to calculate accurately. Basicly what I want to do is make sure my submarine hull does not crush in during unmanned testing which will probably be about twice the max diving depth. I also checked out one or two other related threads on this site and it seems like the topic isn't super common. That's to be expected, not everyone is building submarines.
Anyway, I have a friend that was an engineer and did work with ships, buildings, ect. He is better at figuring the math out than I am and also knows some of the terminology a little better, so I asked him to do a test calculation on a plain propane tank. No stiffeners, ribs, hull openings. Just a 500 gallon propane tank with semi rounded end caps. I guess they are called 2:1 heads. (Not perfect half spheres)
Specs he used:
Material - Mild steel
Thickness - 3/16" or about .190" (Heads and cylinder)
Diameter - 37"
Length of cylinder (to tangent of heads) - 119"
Using the formula for external pressure vessels on this site, he came up with a calculated crush depth for that tank of 177' fresh water or 80psi. I am just wondering if we are close to the mark or not. I know it takes some time to run through, but I'm just trying to make sure we're on the right track. Thanks.
Could no one give me a yeah that's close or no your way off, in 3 years? Just saying.
You have not built it yet? Assuming that your engineer gave you some numbers to think about, I would at a minimum do a remote test dive in ten ft increments to see how it performs. Also, be sure your life insurance is up to date.
Tell me and I forget. Teach me and I remember. Involve me and I learn.
Actually no, have not built it yet. Too many projects going on. Finally getting back to the project now. Not to sound like a jerk, because I'm not being cocky, but everyone always thinks small submarines are ridiculous. Actually, there are a ton of them out there. And there have been some made from propane tanks. It's not nearly as nuts as it may seem. There's a bunch Kittredge K-250 submarines out there and people still build them.
Anyway, maybe someone knows of a good book that discusses pressure vessels? Things like not putting a stiffening ring over a circumferential weld, that's the sort of things I need to know.
Working on a model of the sub to show what it will look like.
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Best book in the world is the ASME Pressure Boiler and Vessel Code. This engineering and design standard provides design details for all sorts of welded connections as well the calculations to determine strength characteristics.
There are a few common calculators on this website for pressure vessels.
https://www.engineersedge.com/pressu...els_menu.shtml
Not to sound like a jerk, because I'm not being cocky, but everyone always thinks small submarines are ridiculous.
Everyone?
Tell me and I forget. Teach me and I remember. Involve me and I learn.
Thanks, I'll see about tracking that down.
...And ok, not everyone but seems a good amount of people.
You should first figure out how deep you what to go then determine the water pressure applied to the hull that you need need to design for.
I used this chart for my go-pro mods
Water Pressure vs Depth Table
Last edited by Cragyon; 01-25-2018 at 09:49 AM.
I don't think I need to go much deeper than 100'. But I plan on building the hull as strong as possible for as large of a safety margin as I can get. If It turns out the calculated crush depth of my design is 500', then my max dive depth could be something like 150'-200'. I just think realistically, there's not much need or reason for me to go below 100'. Most other people who build small submarines want to look around and explore underwater. So they may have reason to go deeper. I am pretty much only concerned with the mechanics of it and how it works, not necessarily seeing outside. Thanks for the conversion table.
The factor of safety should be well thought out as you don't want to spend more on the design and manufacturing than is required.. Usually a proof load of about 1.3 to 1.5 x the maximum rated operating load is adequate.
For example, when I worked at Bell Helicopter Textron when a fixture or tool that was to used for a critical structural application if it was rated at a maximum load of 10,000 lbs we would proof load to 13,000 to 15,000 before actually using the tool. If the tool did not yield or display any signs of preliminary failure following the proof load it was a go.
Tell me and I forget. Teach me and I remember. Involve me and I learn.
At 100' your submarine would see 43.4 psi at a 2 F.O.S. you need to design for 87 psi with is equivalent to 200' and the proof load/depth should be about 130' or maybe 150" about 56 psi and 65 psi perspectively.
The issue I was thinking of is in the case of an accident or failure, should the submarine dive much deeper than expected, then the extra hull strength would be worth it. It might only be a difference of adding 3 stiffening rings along the length of the hull which wouldn't change the build cost that much.
For example, Seneca lake, the deepest of the finger lakes in upstate NY has a max depth of 618'. If you had some kind of sudden seal failure or something that caused the submarine to sink far below the normal depth before you could release the safety weight, you would want the hull to hold up. Adding thickness to the hull probably won't be an option as I want to stick with propane tanks if possible. But the thickness and number of stiffening rings could be changed fairly easily if needed. I don't need ridiculous overkill either, but there's no reason to skimp on materials at the cost of extra safety. At least that's what I'm thinking.
You are seriously planning on occupying this vessel during submersion?
I know that in the boiler industry the pressure is on the inside, and vacuum is not permissible, so boilers always are vented below a threshold lower pressure limit to atmosphere, to prevent steam collapsing to condensate during the cool down. Even a thick vessel such as a boiler drum can collapse with only about 12-14 psi directed inward.
Without stiffening rings, your vessel might collapse at a much lower depth than you think, especially if you bump into something. Are you able to place stiffening rings inside the vessel, or will they be outside?
Yes I'll be inside the vessel. And I plan on having as many stiffening rings as practical. These will all be internal. Although, I noticed that the stiffening rings on united states submarines from WW2 are external for much of the hull. So apparently, with full welds that is an acceptable method to use. And also I am aware that containers can withstand much more internal pressure than external pressure.
There are several examples of propane tank submarines that have been built by people online, some with questionable designs. The Kittredge K-250 and K-350 submarines are thoroughly tested design, but people that build them have the hull steel rolled for the application, they're not propane tanks. I think they even tested one in a U.S. navy pressure chamber. But anyway, point being, people do build small submarines although it's not the most common hobby out there.
I think that initial decent should be facilitated with remote control. I would configure a way to deep dive test with a wire line control from the surface. Additionally, if you could (afford) integrate strain gauges and read actual stress at test depth that would lend a degree of confidence in your design.
Originally Posted by mr.modified
Tell me and I forget. Teach me and I remember. Involve me and I learn.
Doing an unmanned test dive would be nice if I could find a good way to do that. The only issue with that is I am afraid of losing it if something goes wrong with the lines ect. I think any strain gauges are a bit out of the budget though. I think that if it's built correctly with known methods of construction, there shouldn't be any issues. I just need to get an idea of the calculations. Is there somewhere I could send my specs and have them calculate things for me?
I ordered a copy of the asme bpvc section VIII Division 2, but when I got it, there seems to be sections missing. Unless I'm just a total moron and missing something, but it seems like they didn't put everything in. It's a few years back not the current version, is there any reason pages or sections would be left out? It was wrapped in plastic too so I don't get it.
I strongly recommend against doing this.
However, understand that the reason the calculations are not out there is this is a highly nonlinear problem, as it's due to buckling failure that governs compressive stress of hulls and thin reinforced structures as you describe.
There are at least two nonlinearities:
- Nonlinear geometry
- Buckling problems are highly sensitive to the actual shape of the object
- Initial imperfections of the shell will determine the collapse load
- Investigate Roark's formulas on calculating collapse pressures for thin shells. They include a high percent knockdown due to differences between theory and test. Note that the knockdown does not consider the material nonlinear effects
- Rotation or deformation of the stiffeners, thermal distortions from welding and associated stress state, etc. can all affect the buckling load because it affects the geometry. As the loading increases, the rotation and distortion change the geometry, further affecting the collapse load.
- The effect is pronounced enough that small deviations from ideal, on the order of fractions of the shell thickness, can affect the collapse load
- Nonlinear material
- Depending on how highly loaded the structure is, the effective modulus will be closer to the tangent modulus for the loading
- This means that the closer you get to yield, the lower the modulus. This can absolutely destroy any prediction of buckling stress if it's unaccounted for
Again, I highly recommend against doing this without doing real testing and someone knowledgeable in the area oversee it. Calculations of collapse that look to show positive structure margins can be off by a factor of 4 or more due to the above nonlinear effects. Safety factors calculated with normal, linear methods or FEMs are completely unreliable and should not be used at all for something like this. Any solution to such a problem needs to be nonlinear, likely iterative, and address at least the two major forms of nonlinearity. It is highly likely that doing this wrong will result in a quick death with no chance of recovery. There's not much backup when the only thing keeping several atmospheres of water from invading the small bubble of air turns into a crushed soda can in a split second without warning.
Disclaimer: I have never worked on submarines but have seen very similar issues with aircraft and other thin structures analyzed under compression.
Give up before trying? I agree with much of your advice but seriously?I strongly recommend against doing this.
I say go for it, use caution and logic and be safe.
Build something, yes, it would actually be pretty cool. Mr.modified has been here for years seeking answers, and made a cool scale model.
But understand the need to test everything, without someone inside. Because these things are very hard to calculate properly. If the hull and fittings can survive depth with a proof test without collapsing, then the next thing are any and all secondary systems and everything maintaining buoyancy and depth. A failure of those could easily take the sub beyond it's design point.
Here is how quick things can go wrong. And this is less than 1 atmosphere of pressure on the outside. I believe it was a stuck valve allowing a partial vacuum inside:
A longer one with more stiffeners (linked as you can't do more than 1 video per post) that shows only a partial collapse:
https://youtu.be/AL4k9BGv_Gg?t=1m9s
Can it be done, sure!
But is it wise without proper guidance of someone who has actually don't nonlinear buckling analysis for thin shell structures under external pressure and the related problem? No. You're risking your life before you even know it. How close are you really pushing the limit of the hull? Only someone who has worked these problems before will be able to tell you with confidence.
I would like to find someone to calculate the crush depth of the finished hull if possible, I just have no idea how to find people that do that sort of thing. Obviously, I would be willing to pay for someone to do it.
But on the other hand, I have looked at specs of designs that have been calculated and tested so that's an indicator of what direction to go in. For example the kittredge K-250/K-350 submarines have all the information available such as hull diameter, thickness, number of stiffening rings and size of stiffening rings.
Unmanned testing would be a good way to ensure the design is safe before doing a manned dive. If I could find a safe way to test it without losing it in the bottom of a lake, then of course I would look into that.
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