Mini-submarine buoyancy problem

[metz143]

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.

 

[Danger]

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.

 

[russ_watters]

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.

 

[gaming_addict]

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, a lot 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.

 

[metz143]

Thanks guys for you ideas..i'll be updating soon.We'll be starting experimenting this weekend..Thanks.. :)

 

[mgb_phys]

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 )

 

[russ_watters]

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.

 

[Danger]

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.

 

[gaming_addict]

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.

 

[russ_watters]

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.

 

[Danger]

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.

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.

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.

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.

 

[gaming_addict]

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.

 

[Danger]

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.

 

[mgb_phys]

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).

 

[gaming_addict]

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.

Danger has suggested it already, read the replies he posted

 

[metz143]

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..

 

[Danger]

Our mini-sub would probably be 2 1/2 ft long.

I thought that you were talking about a real mini-sub, not a submarine sandwich.
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.

 

[metz143]

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... :)

 

[Danger]

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.

 

[metz143]

how much will it cost us if we purchase customize fiberglass?

 

[Danger]

I couldn't begin to guess what it would cost to have the hull made out of fibreglas for you, but it would be a lot. My impression was that you would make it yourself. The way that you do that is to scavenge or make something in the shape that you want. In your case, you could maybe use a couple of beachballs for the ends, connected by a Bristol-board or similar tube taped to their equators. Support the assembly firmly on a table or a couple of saw-horses so it can't move.
Tape a 'T' shaped strip of cardboard or plastic around the midline of the assembly, protruding at least an inch from the surface, to act as a divider. Next, paint the top half (including the divider strip) with a coating of paste wax, followed by a good slathering of fibreglas resin. Lay your glass cloth over top, making sure to squeegie it smooth to get any air bubbles out. Keep layering resin and cloth in that manner until it reaches the thickness that you want (check with one of the structural guys to be sure, but I believe that 3/8" should be sufficient for your purposes). The set-up time shown in the resin instructions will determine how long you have to do this. More than one batch might be needed.
Once it has properly hardened (again, consult the instructions; full-cure hardening takes a lot longer than the 'working' set-up time), turn the thing over and repeat the process for the other side (again covering the divider tab).
When the second half is cured, saw or grind the bare minimum necessary from the outer edge of the divider strip to disconnect the two pieces. Drill appropriately spaced holes through the divider strip (about every 4" or so) for connecting bolts. Pry the halves apart gently, remove the form materials, and there you go.
You can build whatever you want inside, then bolt the two pieces together (using stress-relief washers) with a gasket in between. If you want to go to the extra trouble, you can sand it down to a smooth finish and paint it.
Again, though, I don't know what the materials would cost.

 

[gaming_addict]

And also...do guys know any submersible dc motors? Our mini-sub would probably be 2 1/2 ft long.

Thanks..

If your BCD is one that would have used a screw jack and water/airtight piston to mechanically increase or decrease the sub's density in terms of volume(like Danger suggested earlier). Then you only need to put the motor behind the piston, right inside the watertight body of the sub. You don't need special waterproof motors for that.

Also, are you familiar with the silicone sealant container? I think you could get an empty container, clean it off of any dried silicone, and use it as your BCD. You could also use a few or several large syringe for that matter. But you must lubricate them first with grease to not overwork the BCD motor

 

[metz143]

where can i buy fiber glass materials?

 

[Danger]

Try automotive specialty shops or marine suppliers. It's used both for body repair and boat building.
Incidentally, if you're tempted to use fibreglass mats rather than cloth to build up the thickness quicker... don't. The cloth is woven to make it extremely strong; the mats are just a bunch of spun fibres crammed together.

 

[gaming_addict]

Hmm, can't you just use PVC pipes? I meant, see that you don't have much time left.

I've worked with those pipes once. We used a transparent glue for working with PVC pipes-forgot what it's called, but it's waterproof and air-dried. But see that you can thread the ends and be sealed tight by a pipe caps, or join pipes by special 'joint' parts and using glue. This is the easiest and fastest way to get your project done. Not aesthetically pleasing sight though for a mini sub project(real life mini subs don't look great either)

I'd advice to ask an expert on PVC piping regarding this matter. Be practical. More people can give you advice on PVC piping than fiber, and it's been proven already that PVC could handle water pressures fairly.

 

[Danger]

Gaming Addict... we're not in a pissing contest here; we're trying to help Metz achieve his goals. Based upon the parameters that he/she has defined, PVC or ABS are not options. Likewise for the 'syringe' approach. While I might very well be wrong, the only approaches that I can see are my way or the compressed-air supply way. Given the price of solenoid valves, pressure regulators, and other sundry mechanical bits, I seriously think that my idea has merit. I have taken the liberty of draughting it up in a rudimentary sketch. I'm way too pissed at the moment to filter it through PhotoShop in order to post it. I shall do so tomorrow.

 

[gaming_addict]

Based upon the parameters that he/she has defined, PVC or ABS are not options

Fiberglass was only suggested. PVC is a good option because you could test it's pressure tolerance right away(simply get a pipe of similar diameter as would be used with the sub, cover the ends with endcaps and seal with glue, let it sink 20 ft in the water, let it stay for a while, recover and check for damage and leaks). And I don't think the tests will be done in boiling water that could compromise the PVC pipe.

To suggest that you PVC is not an option clearly suggests, you're not familiar working with these kind of material or their properties. No offense meant but I've seen so many projects on our school 'go down to the drain' with poor selection of materials and planning. I'm not saying that fiberglass is bad. But think that it's a much more complex material to work with. You had to carefully calculate the thickness as it would be too expensive to conduct destructive testing of the device. To use relatively 'exotic' materials isn't the right approach for beginners, unless, you've manufactured dozen subs already..

Also, PVC is safe to work with, not as toxic and doesn't produce a cloud of very fine toxic dust as fiber composites when you have to trim some portions or cut the material using electric saw.

. Likewise for the 'syringe' approach. While I might very well be wrong, the only approaches that I can see are my way or the compressed-air supply way.

It's still your idea Instead of just using one large mechanical BCD, multiple smaller tube would do the same. Of course if he can't find a suitably larger device that could be made into a mechanical option. I'm suggesting as an option only, a last resort.

 

[metz143]

my group are now choosing either to use fiberglass or just pvc..but were also looking a pvc that is also transparent. We made a sketch for our mini-sub..i'll try to post it this weekend. BTW, some submersible water pumps for aquariums have motors that are already enclosed and waterproof. We ask one of our teachers,if we could just dismantle those small water pumps and use the motors. We're actually having problems looking for the right motors and suppliers. Do you know any brands or models of motors that could be used in our project?

 

[Danger]

I agree that fibreglas is somewhat 'exotic'. The entire post about how to work with it was based upon Metz's question about it. I still think that using light sheet metal over a rigid skeleton is the easiest way to go.
In fact, I do have some familiarity with ABS and PVC, and agree that from a strictly structural standpoint they would be suitable. The problem that I foresee is in implementing the design. As you pointed out, the parts would have to be cemented together, unless attachment flanges are bonded to it. Then there's the problem of sawing the thing apart in the first place; it's not that easy to do without leaving ragged edges that would require a lot of smoothing in order to seal even with a gasket when it's reassembled. All of the foregoing is in regard to accessibility. If the dude was building a simple buoy or something, it would be fine... but he has to be able to open the thing up on a regular basis to install the internal bits and mess about with them for additions and/or tuning (even just replacing batteries). A metal shell would have screws or bolts for easy disassembly, and the fibreglas method that I mentioned incorporates connecting flanges.
The only objection that I have to the 'syringe' approach is that you would have to use a lot of them. The bodies themselves, along with the mechanical connections required, would take up a lot of space and add so much weight that the net buoyancy increase would be minor compared to a single large displacement volume.

I just remembered that Yanks use different terminology than we do. When I stated in the previous post that I was too pissed to continue, I meant 'drunk' rather than 'angry' (pissed off). I hope that you didn't take it the wrong way.

edit: Metz, you sneaked in that last post while I was composing this, so I just saw it. The problems that I see with using aquarium pumps are that they are very weak, and run off of mains electricity. The ones that I've used are also totally sealed and potted in epoxy. Taking them apart is not pleasant. I really think that your best bet would be to visit a marine supply shop and ask an expert what sort of waterproof DC motors are available for a reasonable cost. You might be able to get a used one in good condition.

 

[gaming_addict]

As you pointed out, the parts would have to be cemented together, unless attachment flanges are bonded to it. Then there's the problem of sawing the thing apart in the first place; it's not that easy to do without leaving ragged edges that would require a lot of smoothing in order to seal even with a gasket when it's reassembled. All of the foregoing is in regard to accessibility. If the dude was building a simple buoy or something, it would be fine... but he has to be able to open the thing up on a regular basis to install the internal bits and mess about with them for additions and/or tuning (even just replacing batteries)...

You could use lathe machine to either cut or smooth or both. It should give you a perfectly flat face for a number of benefits. It could be done in less than a minute. And for a proper design, there's really no need to join two or more pieces of pipe perfectly together, just one would suffice, since it's possible to just have one pipe section watertight that houses all your electronics and motors. The other pipes you can join which would allow intake of water for BCD don't need to be perfectly machined.

For accessibility, there's a threaded endcap available for PVC pipes, all you need is to thread the end of the main pipe that you must open and have a silicone gasket for the end cap. Or if you need to split the pipe in the middle, you could have a join with inside thread and use that to join two pipes together with a silicone gasket between the pipes.

I also agree using steel parts. They are easy enough to work with, you could even weld or use epoxy, only my concerns is it's weight. But I would also prefer it over fiberglass in the same manner as using PVC.

@metz
There's no transparent PVC tubes I think but there's transparent acrylic tubes.. And there's no need to use waterproof motors, you only need to get them inside the body of the pipe which I assume is watertight. It's simpler that way. My own semisubmersible project had a regular motor housed inside the watertight main body. It drove a propeller through a shaft that went into two layers of PP plastic fitting the shaft tightly with grease in between but I doubt you'd ever require building one given your requirements.

 

[Danger]

You have some good points there. Now that you mention a lathe, and internal threading, I see where our approach differs. I'm thinking that it should be split longitudinally, like a hot-dog bun; you're looking at an end-to-end joint. While I think that your way would still limit internal access, it would certainly be easier to implement.
The reason that I was thinking of a totally sealed hull rather than merely a water-tight equipment bay is that the whole machine then contributes to buoyancy. With a leaky hull, air bladders would have to be incorporated. That still might be a more practical way to do it, though, as long as there's room in there for them.
As you said, there's no particular need for the motor(s) to be waterproof; I just tend to be as redundant as is practical, in this case because I suspect that there will be accidental leakage no matter how careful they are. While the other electrical bits can be individually waterproofed with silicone, latex, epoxy, or whatever, it's tougher to do with a motor. Not only does the shaft-seal have to be proper, but there's then the possibility of it overheating if confined. That 'grease sandwich' shaft seal that you mentioned is quite a cool idea. I've never heard of that before. While it probably wouldn't take a lot of depth, it might work well for something like this. (How deep did your machine go, by the way?)

edit: I finally got the thing into ImageShack. This is just in case anyone didn't quite follow what I was talking about.
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