scaling down a submarine?

Discussion in 'Boat Design' started by tugboat, May 27, 2010.

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  1. tugboat

    tugboat Previous Member

    I have thought a lot in the last couple months when i was investigating design characteristics of submarines on the surface-was the need to have more boat than sub.
    Yes i am an advocate of concrete for a sub hull..but a few things came to my attention after doing models of the sub i had designed-

    and...(laugh)-no im not yet admitting defeat with regards to fc or concrete...

    here is the first part of my thread- about scaling.

    the UC3 nautilus uses 1/2 inch plate. rolled into a tube and using circular transverse frames spaced about 3-4 ft apart. the crush depth on her is 300 meters. they only go 100 meters for safety.
    so using the UC3 as a guide- if i know that 1/2 inch-(assuming) mild steel was used can go to X depth--then would 1/4 inch of the same steel built the same way go to X times 0.5 ??? or half the crush depth and S.O.D.?

    hence- can it be scaled that way?

    i also think the Kittredge 250 and 350 designs may be using 1/4 inch plate for those pressure hulls?

    part two-

    sadly i need to redesign the hull to run more on the surface than below since i only plan to dive occasionally - maybe 40% or less of the time if that.

    what would be the simplest way to do this?...i was thinking of going with a ww2 type VII b or C class sub hull. but this is much more complicated than a skipjack hull, which is designed for underwater running. could i even take the skipjack since it has a round bilge already- just form the bow section into a rake?
  2. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    NO! Pressure hulls do not work that way. In pressure hulls it is not only about strength but also about stiffness, design tolarance, and load transfer. If you really want to do this, study the math of stiffened thin shell cylinders under external pressure. Chapters 4 and 6 in Submersible Vehicle Systems Design from SNAME has more than enough needed start you off.
  3. kerosene
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    kerosene Senior Member

    and read it quick if you are going to lay the keel on your own design sub in few months... just saying

  4. tugboat

    tugboat Previous Member

    Kerosene--ill take that under advisement--Its still a 50 50 im not too still investigating it. but i do have the systems worked out.
    I am also not an "authority stamped kind of guy" low tech for me all the way.
  5. samcool111
    Joined: Jul 2009
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    samcool111 Dr.

    A U series sub used in WW-II may be a good example.
  6. tugboat

    tugboat Previous Member

    Hi there samcool111- my brother in law is second mate on the Princendam!
    (I think)...

    he lives in Ottawa with my Sis.

    he is from Holland-- i will check out the a u sub ww2..thanks for the info!
  7. apex1

    apex1 Guest

    NO, there is a big fault in that "calculation".

    A hull plate does not double its strength when it doubles its thickness. (to go the other way round)

    A 1/2 inch, or stay with SI units, 12mm plate has a strength of 12x12x12 = 1728 times the given materials properties.

    A 6mm plate has only 6x6x6 = 216

    This calc. would have been part of a design!

    Not to bother you, but stop please telling us you "designed" any boat!
    You may have produced a drawing, but what you are doing is not designing, when even the most basic calculation is´nt part of your experience.

  8. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Concrete is even more difficult to calculate and control production since it is not a homogenous material. I suggested you go to Oconomowoc, which is close to you , and check that WWII sub. They will give you a tour with explanations on all the systems. The oldtimers that serve on them will talk your ear off. It is not just a pipe that goes down. They can capsize under water and pitchpole and corkscrew. You need to study basic submersible design and operation to be able to at least ask relevant questions.
  9. tugboat

    tugboat Previous Member

    Apex1--i have DESIGNED 5 boats and built 6 all of which floated on thier lines...
    no problem. no they werent perfect or hard designs but |I can do what mnay boat builders do and have been successful at - and that is usingh eperience.
    I understand about what your saying as to the difference in plate thickness-
    thanks for helping out...
    All that needs to be done is to reinforce the steel by increased longitudinals and frames. that will give a stronger pressure hull.
    Not everything has to be NASA engineered. as some people here are proposing.(others not)
    sometimes intuitive engineering works as good or
    better than DRAWINGS- as apex1 calls them.

    just see how the polynesians intuitively built(not designed) the most efficient crafts of all time??.my guess is they didnt use any math or had any inspectors or the chief medicine man/ mechanical engineer stamp their approval on the proa's. they didnt have any "drawings" as you call them. maybe they etched the designs out on the sand and calculated the simpsons mulitpliers and perhaps youngs modulus or calculated the LCB using an abacus and papyrus roots?
    yet somehow (wierd huh) they managed to DESIGN boats that travelled around continents in the some of the worlds worst seas! anyone can design a boat with a little knowledge and studying plans and common sense. you dont need to be a NA- in fact APEX1--you know a lot --yet you arent an NA? so how is it you can know so much??.from experience right?

    Gonzo--ill look for that sub...thanks--

    I disagree with the statement the question wasnt relevant- its very relevant imo.
    as long as the sub is ballasted properly it should be fine and my model experiments proved that. no corkscrews or turtles.or pitchpoles...
    ill repeat ad infintum- the only way to know is to build it I think.
    but i agree there are factors to consider certainly...

  10. TeddyDiver
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    TeddyDiver Gollywobbler

    Not comparable... You don't believe it yourself either that the polynesians draw their crafts out of a hat ;) instead they had maybe thousand years of tradition in boat building.
  11. apex1

    apex1 Guest

    Right Tug,
    from experience......

    ...which you do not have. After nealy 35 years of building, operating, restoring and maintaining boats, I will tell you that I would not "design" my own yacht.
    And sure I have forgotten much more about boatdesign and boatbuilding than you probably might ever know.

    It IS NOT as easy as you phantasize.

    But good luck anyway, and when you need another hint on scantling rules, do´nt hesitate to ask.

    It is not my intention to get you down, but you have to realize that you are dreaming far above your abilities!

    Sorry, so easy is colour TV.

  12. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Submarines operate on something like a mix of a boat and an airplane. They use dynamic lift(positive and negative) and ballast. A positive floatation sub goes down only when moving forward. They will surface as soon as you stop. They are somewhat safer, but you may come up against something you don't want to. Also, they can't hover over a wreck or other interesting sites.
  13. tugboat

    tugboat Previous Member

    apex1- i understand your point--may i ask what does it matter to you whether i fail or not? what happens if i succeed?...i dont think im being unrealistic..but the way i see it if i fail i still have learned...more experience. i do know a little more than you give me credit for. ok what would you suggest for plate thickness if i want to go to 120 ft? safely. and what about ferro? the hulls are tough on fc hulls. easy to build up to alot of compressive strength.

    Gonzo--thank you- I thought using a dynamic system would be much better. since I wouldn't need to worry about problems with negative buoyancy changes.i.e. trim and the pump system etc. what do you think about using a net cutter for safety??..
    i did reason that the problem with that type of design would be - if you ever took on any water while submerged its big trouble since your sub is not designed for release of ballast submerged-so you would need and emergency ballast system.
    Getting precise buoyancy changes seems like a lot of headaches.
  14. MatthewDS
    Joined: Mar 2010
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    MatthewDS Senior Member

    About design. Anybody can build a boat. It's trivially simple, all you have to do is keep the water out of a box. Viola! Instant boat.

    However, the importance of DESIGN is establishing the limits of your vessel, so that you can stay a reasonable distance away from them. This is called a factor of safety.

    Back to your submarine. Of course you could build a submarine, but without calculations, how would you know what the safe operating parameters were?

    You can't exactly "test it" and learn from your experience, because only a failure will show you where the limits are, and frankly, a failure probably kill you.

    I'm not saying that you have to be certified, although that certainly helps. What I am saying is that this sort of folksy, elbow-grease and enthusiasm approach to building a boat (or submarine) is no substitute for actually opening a book and learning what the hell you are actually doing.

    I can't even begin to express my disagreement with your statement that "the only way to know is to build it".
    You can't possibly believe that this is the case. Do you really think that as a building is being erected, the engineers are all standing around with their thumbs in their pockets, saying "Well by golly, I hope this holds!" Of course not.

    Obviously with boats there is scale model testing, but this is largely to examine complex hydrodynamic interactions with the hull, not to test the structural elements, which were surely designed prior to the model hitting the water.

  15. apex1

    apex1 Guest

    Welcome here,

    rarely I agree with any new attempt to contribute without having a proof of experience at least.

    In this case I do.


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