ROV advice needed for BBC TV programme

I don't have ROV experience, but being able to use some sort of prebuilt pumps would definitely improve the chances of a successful contraption. Otherwise, they will need to have very good bushings around their prop shafts to prevent leakage and eventual failure under pressure. With a premade pump, they can use water-jet propulsion and rely on the seals inside the pump to prevent leakage. That's not to say that those bilge pump seals will hold up underwater...I wouldn't say the bilge pump solution is without potential problems. The teams choosing bilge pumps would have no control over the strength of the seals, or the pressure capacity of the casing, and might pay for it.

I have seen the use of trolling motor pods for shallow-water ROV projects...they're perfect, you just provide electricity and the pod is already sealed. However if you're concerned about making it too easy, throwing a few trolling motors in might be a problem. They also aren't meant to run deep underwater, so it might not be as easy of a solution either.

Though simply making it watertight, and sealing around cables and such, is definitely going to be a challenge in itself.

It might be neat if compressed air tanks and scuba equipment were available; one of the teams might figure out how to equalize internal pressure to keep the water out of the seals.

Scrapyard Challenge (Channel 4) did a similar thing, I think the two energy sources used on the one I saw (two teams, different approaches), were compressed air and electric motors. Provided the water is fresh water an electric motor will still work underwater (I think they used electric radiator motors plus the fan), the batteries of course will have to be sealed or kept out of the water and the power fed down on unmiblical cords. They have a web site and I think videos are for sale. I think your program is the one that did the steam rocket, good program but I don't watch much TV.

regards

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Well, if you want a wild idea - how about oars/paddles ! You put the drive mech in a box, with a leather/rubber gasket where the paddles stick out - no continuous rotation means much easier sealing.

Would paddles work - well, fish seem to manage ! Something like a swim flipper action might work quite well, needs only a simple crank on a motor - windscreen motors spring to mind.

Otherwise, a common method of running a motor under water is by placing the motor in a case filled with oil. It will leak a bit, but would probably hold up for a few minutes.

A real problem you will encounter is that anything built in a hurry will leak or compress a bit. As it does so, the effective bouyancy will change and handling will become difficult

Sounds fun !

regards, Dave

The best thing for the main structure of the ROV would be PVC plumbers pipes. My university does alot of projects using that kind of thing. Take a look at the site.

They are usually for AUVs without external control signals but you could certainly use a simlar method for actually making the ROV with a cord connected for control rather than being automated.

You could try getting in contact with the staff aswell, I wasn't on the AI and Robotics course so I didn't really do anything with them, but the members of the robotics group would know exactly what would be required.

Hope that helps.

You probably should limit the depth to something like olympic pool depth so external pressure issues don't require $$$ solutions, and so the TV cams can easily watch the action. Control it via one or two pieces of cat3 4 conductor phone wire from an old computer with a web server so I can control it over the web from the house (put a webcam on it so I can see where I'm going too). Can some parts be purchased with X $$$, or must every thing be found as free junk? Have you established the "rules" and what the ROV must accomplish?

Thanks very much everyone, I'll pass all these ideas onto the team - they're great. I think ideally we want our scientists to use propellors as we like the idea of making them build the whole propulsion system using a motor etc but as you guys mentioned it's going to be the seal between motor and prop that'll cause us problems. Maybe we could try a magnetic coupling, anybody heard of doing this? Thanks lots for your help. Jon Andy: am in touch with your Uni' - thanks for sites

For a decent magnetic coupling, you will want strong magnets. A couple of old hard drives should do the trick.

Cheers!

Chip Shults

Easier to keep the internal parts pressurized to the outside water pressure. Aquarium air line tubing and a chunk of dry ice would be a cadillac solution, but baking soda and vinegar might be easier to get. ;-)

Heh. I like the way you think. Though it would certainly be funny if the ROV overpressurized and exploded underwater.

It shouldn't over pressurize. The chunk of dry ice sits in a sump like bowl in the bottom of the ROV. The bottom of the sump bowl has a small open tube in the bottom sticking down three or four inches. As the ROV goes down, the internal gas starts to compress and water comes up the small tube and comes in contact with the dry ice. The co2 released from the dry ice increases the pressure in the ROV, forcing the water back down the tube and off of the dry ice. Internal pressure is equal to outside pressure. As the ROV surfaces, the excess gas just bubbles out the tube. Baking soda and vinegar would be a little more of a challange, but these are scientist. ;-)

I personally would just make a compressor on the surface and switch the compressed air into differnt tubes which would provide thrust at differnt points on the ROV. You could either just have an open tube venting the compressed air or a prop driven via the compressed air to provide thrust.

That sort of system has the advantage that there is no electricity involved but also it's very simple. The compressor and so on would need to be made and the main ROV body, perhaps some form of sealant could made by the chemist and biogist.

in addition to the other ideas ...

off the shelf trolling motors will work, at depth the pressure will defeat the sealsdue to presure differential between the air and the surrounding water ... if the seals are in good condition, dielectric oil inside the motor housing can change the issue from water intrusion into oil retention (replacing the air and making pressure differential a non-issue) .

gotcha of course is to make sure the seal will retain the oil ... if the oil bypasses the seal you have possible pollution issues (so finding an environmentally safe dielectric liquid might be important)

I've seen small VLCROV make it to 100ft with dry seals around motor shafts ... not sure how they accomplished the seal ... the motors spun pretty quick (the seal was tight enough that you could feel the drag if you spun the prop by hand)

magnetically coupled props will work for small rov's , thought he low end designs tend to slip . Probably not a big issue for your program (?)

- Ed

-- refillable drysuit talc bag $9.95 ppd

Why is not using electricity an advantage?

Mitch Berkson

--There was another TV show a while back in which a team recreated what was thought to be the first submarine; it was driven by oars and supposedly went something like 2 miles, just below the surface of the Thames. The hull was massively thick and the sub leaked where the oars entered the hull, but it worked, sort of...

Firstly becuase they will make them generate the electricty first. Compressed air is easyier to generate.

Secondly a leaky air line or air powered motor is one thing, one which is easily spoted and fixed also, a leak into an electrical component is another.

--A bit of an aside: this is one kewl program; glad to hear it's still going! One of the rare "learning experiences" I've had lately on TV... ;-)