A air Pressure question

[kermit911]

Hello all, it's been awhile :-) I am trying to make a little submersible remote controlled submarine. I am using two empty 50 gallon propane tanks welded together with a small frame between them. With the tanks empty, they will float. Now when I start adding weight to it, example "motors, cameras, motor control boards, all the other electronics. I am afraid it will not be able to hold it's own weight. If I pressurize the two tanks, will that give me more buoyancy?

Thanks
Kermit

[Rev. Green]

No, pressurizing the tanks adds air, which adds more weight. The only advantage of pressurizing them might be to prevent them from collapsing from the pressure of the water.

[jomifl]

No, less. Its a voume thing not the amount (weight).of air. Sucking the air out would increase the buoyancy but not so's you'd notice. 2x50 gallons at 10lb per gallon buoyancy would give you nearly half a ton of buoyancy. How are you going to make it submerge and how are you going to carry it? I know a little about these as I used to have one and at 50lbs it was a bu**er to launch and recover.

[kermit911]

I think the 50 gallons was a bad estimate, I am going to use two 100 pound propane tanks. Here is a link to what one looks like:

http://delaware.craig...g/for/1873037456.html

This is all still a work in progress, but what I think I am going to try is weld the two tanks together with a small platform between them. Drill a hole in the bottom of both tanks and connect a bilge pump "or a pump that is low power and can be submerged" to both holes, there will be a rubber membrane connected to each of the holes inside the tank. Drill another hole in the top of both tanks to release the air with a valve threaded to it. I will have two pressurized air tanks connected to the top holes also. I'm not sure how big to make the pressurized tanks yet. There is still a lot I have to figure out, and test, and research. Right now this is my information stage. It's going to take some work. :-)

[jake-the-peg]

The bouancy that you will get is equal to the weight of water displaced.

100 gallons (US) is 348 litres which is 348 Kg in fresh water about 354Kg in the sea (but it varies)

Now you have to subtract the weight these weigh about 70lbs each what's that? 63Kg in proper money giving you 285Kg of bouancy to play with.

Take away the weight of air you're putting in over and above atmospheric pressure, take away the weight of the kit, you get the picture.

Of course if you have too much bouancy you won't sink at all!

You've got to do the maths here

[beso]

The tanks need to be independent to control the pitch of the vessel. You need to be able to pump water into each tank separately to reduce boyancy of each end.

Doubling the pressure will halve the buoyancy of the tank. This is a massive change so really the pressure inside the tank doesn't have to change very much compared to the outside pressure. You don't need to be able to move water into the tank at much pressure.

Atmospheric pressure doubles at just thirty feet of depth.

You need to be able to pump the water out of the tanks into the outside pressure at the maximum design depth so don't underestimate the power required by the pump. If you get this wrong the submarine will be stuck at depth with negative buoyancy.

[jomifl]

fao Kermit..most if not all commercial remote submersible buiders try to keep everything as simple as possible so that there is less to go wrong. They do not control the buoyancy of the submersible at all, what they do is make the total device just buoyant enough to float, just breaking the surface of the water. To go down you have down thrusters and you hold it at a certain depth either manually or automatically using a depth sensor linked to the down thrusters. To go up you either reverse the thrusters or just let it float up. Using down thrusters means that the sediments on the bottom don't get disturbed. Stability is achieved by ensuring that the buoyancy part of the device is above the weighty part. The for and aft thrusters need to be aligned with the centre of drag otherwise it will have a tendency to go up or down when you try to go forward or back. I you want any more information give me an email address.

[jomifl]

200lbs of propane equates to about 160lbs of water or thereabouts, so your maximum buoyancy will be 160lb less the empty weight of the tanks.

[beso]

The thruster concept has one very important advantage. It will surface automatically when it runs out of power.

[kermit911]

I like that idea, now the question is how many propellers and where do I find them. :-) Another one of my problems is going to be how do I weld on a propane tank that has absorbed propane for the last 10 years without killing my self? I think I will hire someone to do that part for me. Unless maybe I could use 100 pound Co2 tanks or argon or something.

[beso]

Rather than welding you could use a clamping system. If you are using thrusters you won't need the pipe work for controlling the buoyancy. It also allows the tanks to be easily replaced.

Once you weld the tank they will have greater corrosion vulnerabliities. You can hot dip galvanize the framework or even use a stainless construction.

I would use four up thrusters located on the corners of the vessel. Using separate controls will easily allow you to adjust the pitch and roll to keep it stable.

[jomifl]

Re. the up/down thrusters. some commercial models have one central thruster positioned so that the line of thrust is through the vertical centre of drag. This way it doesn't cause any pitch or yaw when it operates either up or down. If you have vertical thrusters at each corner they have to be carefully matched in terms of thrust to avoid pitch and yaw. Re. buoyancy If you make the original body with a decent amount of reserve buoyancy to carry all the kit you are going to hang on it then can fit detatchable weights to the bottom of the chassis to adjust the buoyancy according to what bits you are hanging on it. I would suggest that before you go welding things together you get the camera system to work underwater. From my experience with these things you need to be able to rotate the camera up down and side to side because when the umbilical gets snagged on something you need to see how too unsnag it. You can buy propellors fairly easily try googling 'submersibles' and 'thrusters'

[jomifl]

Re. welding tanks. as they will not need to withstand internal pressure I suggest that you glue them together with either epoxy glue or silicone rubber. If you cover the glued joint with a collar it should have adequate mechanical strength

[RATTER15]

No problem welding them, just remove the valves and fill them with water to ensure there is no gas left in them, empty them and weld without the valves in place to ensure there is no expansion problems.

[jomifl]

Re. thrusters: As you will not require very powerful thrusters I suggest that you try using submersible pumps as your thrusters. They already have pressure seals on the shaft and cable entry and are in a water tight housing. All you have to do is bolt them on to the chassis.

[jomifl]

Try CPC online (if you are in the UK) for your electrical bits. they do a cable which combines a coaxial and several other cores which may make a good umbilical cable. This means you only need one cable entry.

[kermit911]

Awesome, thanks for all your help everyone. I'm sure I will have more questions, and I know the right place to come.

;-)

Thanks again