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Mini-submarine buoyancy problem

  1. Jul 3, 2007 #1
    Hello!

    my group are designing ofwhat we called a mini-submarine..it is somewat like an underwater surveoyr..but in a submarine form of construction. our adviser told us, that we could use a buoyancy controlling device to address our problem of getting our mini-submarine to float and submerge..but a BCD would cost much..we would just like to know if there is any way we would improvise on buoyancy problem by not using a BCD? Is there a DIY buoyancy controlling device,and thus not buying the real BCD used by scuba divers? We're still deciding and designing what our mini submarine would look like..and it's size.

    Thank you..any inputs are very much appreciated.:smile:
     
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  3. Jul 3, 2007 #2

    Danger

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    I don't know how a SCUBA BCD works, so this might be repetitive.
    The simplest way that I can think of would be the same basic 'ballast tank' system that real subs use. I must assume that, for some reason (such as inadequate air supply), that isn't appropriate to your purpose.
    My next idea would be to fake it with a mechanical device. I'll try to just describe it, but might have to resort to a sketch.
    Start with a chamber that's open to the water. Line the chamber with a tough, flexible membrane such as inner-tube rubber which, when inflated, fills the entire volume. Inside that 'balloon', have an electrically driven expansion frame such as a screwjack. Connect a vacuum pump to the balloon and suck the air out so that it's all shrivelled up around the jack mechanism in the base of the chamber.
    Pre-weight the sub so that it's somewhat negatively buoyant when the chamber is full of water. To rise, activate the jack to expand the balloon, which in turn will displace the amount of water necessary to attain the neutral or positive buoyancy that you require.
    This is just an 'off-the-top-of-my-head' response which I haven't put any serious thought behind, so you'd best wait for responses from people who know more about it.
     
  4. Jul 4, 2007 #3

    russ_watters

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    A scuba BC is just like an inflatable lifejacket, but it is connected to your tank. Hit a button, and it inflates. Hit another button and it empties. You could make your own from scratch with very little effort or money.
     
  5. Jul 4, 2007 #4
    According to what Russ mentioned. I'd imagine the device consisting of a compressed gas tank. I'm sure, there are many places to find. Of course, look for the small ones like the small, lightweight disposable CO2 gas cans you use to clean electronic circuits.

    You wrap the whole can in an oversized sealed plastic bag so you can push the aerosol button without compromising the seal. Then install a valve in the bag, preferably in the topmost part. The valve can be a small gas pipe butterfly valve so it can throttled, I don't know what it's exactly called, it's frequently used on laboratory gas pipes and you turn it 90 degrees to close/open or throttle the valve.

    When installing the BCD in the sub, it's preferred to be installed right in the center of gravity(when submerged in water, negative bouyancy), and slightly above it, so during operation, it doesn't cause any dramatic attitude change. For practicality, install the BCD inside the sub body to not cause drag. Actually, alot a 'room' or section just for the BCD in the sub body. You should leave plenty of excess space for the tank, this excess space will be filled in by the expanding gas bag when the CO2 can button is pressed, so the sub will rise. In this manner, you should also put holes in the section where the BCD is, the expanding gas bag will push the water out or let water in if the bag valve is opened.

    Also, you shouldn't let the BCD solely be your source of bouyancy. There should be sealed sections inside the sub body filled with air. Just enough so there's a little hint of negative bouyancy but not positive. By keeping the bouyancy nearly neutral, but slightly negative so it sinks, you only need tiny adjustments to the BCD to make it rise or sink. Thus, you don't quickly exhaust your supply of compressed gas.

    Other than compressed gas BCD, there's no other simpler, more effective method.
     
    Last edited: Jul 4, 2007
  6. Jul 4, 2007 #5
    Thanks guys for you ideas..i'll be updating soon.We'll be starting experimenting this weekend..Thanks.. :)
     
    Last edited: Jul 4, 2007
  7. Jul 4, 2007 #6

    mgb_phys

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    If speed isn't a factor you can simplify the design by having an external boyancy device - just attach a bike innertube along the top outside the hull and add the inflations system described.

    Another tip - have the sub made negatively boyant by a weighted keel held on by an electromagnet. If the power fails or the battery runs out the keel is dropped and you get the project back - especially useful if you are using it in open water ( and remember that sea water is a different density to pool water )
     
  8. Jul 4, 2007 #7

    russ_watters

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    Yes, it is important to remember that there is no such thing as a stable buoyancy point with such devices. As the sub sinks, the air bladder will compress, and the buoyancy will drop. That makes buoyancy control fairly difficult.
     
  9. Jul 4, 2007 #8

    Danger

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    That's one thing that I rather hoped could be avoided by the design that I suggested. Since the displacement of water is forced by a solid mechanical device rather than a compressible medium, it shouldn't change with depth.
     
  10. Jul 4, 2007 #9
    You used a balloon, it's the same thing, at deeper water with higher pressure, it will collapse the balloon up to a point when the mechanical device will no longer be able to put enough pressure to expand the balloon.

    The problem is solved with a design that has only very little negative bouyancy. So a very little expansion of the bag is enough to make the sub rise, which is usually case at deeper depths where the water pressure is higher, even at full, let's say, screwjack extension, the bag will only inflate a little due to water pressure, but it should be enough to rise if the design has only very little negative bouyancy.

    I would suggest to keep the design as light as possible so proportionally, the slight negative bouyancy doesn't amount to significant amounts of weight in terms of water displaced. And the BCD doesn't need to work much to do it's job, thus you conserve energy or gas supply.
     
    Last edited: Jul 4, 2007
  11. Jul 4, 2007 #10

    russ_watters

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    I guess you could use an actual rigid ballast tank in almost the same way as the BCD, using compressed air to force water out and releasing the air to let water back in.
     
  12. Jul 5, 2007 #11

    Danger

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    We're having the same communication problem here that we are in the hovercraft thread. It is not a balloon; I merely used that term in reference to the rubber membrane. It can't collapse because its 'skeleton' is solid aluminum (or whatever). And I can't see that a 'mini-sub' is going to a depth where a 1/2 hp gearhead motor driving a lead-screw can't overcome the pressure.

    As noted in the second sentence of my post, that would be the appropriate method. I've been going on the assumption that he doesn't have an adequate air supply to do that (or he would have done it in the first place).

    Mgb, I do like that idea of a jettisonable ballast as a backup safety. It couldn't be used for control purposes, but it sure could save the equipment in an emergency. Good thinking.
     
    Last edited: Jul 5, 2007
  13. Jul 5, 2007 #12
    Yep I didn't understand you before sorry, so now, it all depends on motor power.

    Also, with Mgb's brilliant backup idea, it's not completely foolproof. I'd like to improve on that by adding a pressure switch. Simply, a device like piston with air inside, extended by a spring, at certain dept, the switch will interrupt the power so the keel is released. I could imagine the sub descending to depths that the BCD can no longer compensate or malfunctioned, it will keep sinking. Since you still have electric power left, it will hold on to the keel as it sink. The increasing pressure will crush the sub, and might cause total destruction even before the keel is released. The pressure switch avoids so if something went wrong and the sub happens to be sinking, the pressure switch will release the keel once the sub has reached critical depth.
     
  14. Jul 5, 2007 #13

    Danger

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    I agree that a pressure trigger would be an improvement, and very easy to make. It might be worth while to incorporate a leak detector as well, so the keel will drop before the thing fills up with so much water that it can't rise.
     
  15. Jul 5, 2007 #14

    mgb_phys

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    Could you make a mechanical BCD?
    Imagine a sealed hollow piston (like a soda can) which moved out of a sealed hole in the hull and so allowed the overall volume of the sub to be changed - since the mass is constant this gives a simple change in density.
    You can drive it back and forward wth a lead screw driven from a motor which might be easier to build and control than pumps and valves. You also don't have to worry about compression of the gas in a boyancy tank.

    Pressurise the sealed can with enough gas that it won't be crushed or fill it with foam (I assume you aren't going to crazy depths).
     
  16. Jul 6, 2007 #15
    Danger has suggested it already, read the replies he posted:smile:
     
  17. Jul 7, 2007 #16
    our depths limit is just around 15 to 20 ft. My group discussed the idea of having like a inflattable tube in between our inner and outer hull..and just have a pressure switch..in order for us to control the amount of pressure in order for the mini-sub to submerge and resurface.
    We also are confused on what material we should used for our mini sub. One of our classmates suggested that we used fiber glass. And also...do guys know any submersible dc motors? Our mini-sub would probably be 2 1/2 ft long.

    Thanks..
     
  18. Jul 7, 2007 #17

    Danger

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    I thought that you were talking about a real mini-sub, not a submarine sandwich. :tongue:
    For the depths that you're anticipating, fibreglas would be fine. It would be a lot easier, though, to form it out of thin sheet aluminum or even steel (think 'furnace pipe' thin). You don't need any moulds or curing time for that.
    Although I hate to discourage innovation, making the inter-hull space a ballast tank is not a good idea; it's way too complicated. Just dedicate a space for one.
    As for the submersible motors, there are lots of options. Someone in a previous thread pointed out that marine bilge pumps are waterproof and run on 12VDC. Someone else noted that flooding a regular motor with mineral oil will preclude the entry of water.

    edit: Incidentally, how deep is the water? I know that you mentioned the operational depth of the machine, but how hard would it be to retrieve? If the body of water itself is fairly shallow, you don't need recovery gear such as the jettisonable keel weight.
     
    Last edited: Jul 7, 2007
  19. Jul 7, 2007 #18
    actually when we will have the demo on a 3 to 5 ft aquiruim probably.but when we test our project may be we're going to a pool and have the mini-sub test if it would operate to the way we want it.
    We are also putting a pressure sensor on our mini submarine..do you guys know any pressure sensors that wud be applicable on our project? How do you actually make a pressure sensor?


    Thanks... :)
     
    Last edited: Jul 7, 2007
  20. Jul 8, 2007 #19

    Danger

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    The easiest way that I can think of to make a pressure sensor is to bond a waterproof rubber membrane (there's that inner-tube again) to the open end of a tin can. Have some sort of rod bonded to the centre of the membrane, which will change a variable resistor as the membrane flexes. A cheap ohmmeter can then serve as the depth read-out. It'll take a while to calibrate, but it shouldn't be too bad. A simple on/off pressure switch can be made by just sandwiching a rubber ring between two metal disks, such as the lids from soup cans, and soldering a wire to each side. When the pressure rises enough, the lids will touch and complete the circuit. The whole unit can be painted with liquid latex or silicone sealant to waterproof it.
     
    Last edited: Jul 8, 2007
  21. Jul 10, 2007 #20
    how much will it cost us if we purchase customize fiberglass?
     
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