Hello, this is my first post on these forums so I am not 100% sure if this is the right place to ask this question.

As a project for school, I need to design a cheap (under 80$) aquatic transport that can carry a 1-2lb object along the bottom of a shallow (2 ft) pool. It will be running for 20-30ish feet alongside a wall, and just needs to stay generally straight so it does not veer away from the wall into the middle of the pool. There will not be very much turbulence at all, and the faster it can travel the better.

My first idea was to just get a wide pvc pipe, slap a motor into the back, and cut out a hole to place the object in. But obviously it is more complicated than that, and I could use some help on the specifics (waterproofing a motor, how to keep it going straight, etc).

I have over two months to design it, but am trying to get a head start. Thanks in advance.

 

Have a look at AUV's. They are automated underwater vehicles, used alot by the oil and gas industry. They tend to be torpedo shaped, andare primarily designed for going forward.

I'd say that, for your project, a simplified version of an AUV would do just fine. Since you have a cheap budget I'd go for the pipe idea. Maybe put a nose on the front of it to streamline it. Put fins on the back and at the sides to keep the nose from deviating too much.

Regarding water proofing, does the cargo need to be kept dry? If not, you just have to have a waterproof container for the electronics that'll be onboard. You can easily waterproof wire holes with epoxy resin. For the motors, might be worth buying some bilge pumps and taking the motors out of those. They're water resistant, pretty much waterproofed, they've been used on hundreds and hundreds of underwater applications.

Have a look here :

http://www.homebuiltrovs.com/mayfair750test.html

There's also a tonne of good info on that website you'd find useful.

Good luck, let us know how you get on.

 

Thanks, that is a ton of good info. However I do have a stupid question, what exactly powers the motor? Is there a battery slot somewhere on it or do I need another part?

And would a 1000 GPH pump be enough to power more than one propeller? I am trying going to try and make the vehicle as fast as possible.

As for exactly what I am trying to do.....

http://www.youtube.com/watch?v=FKUu5uEa3yQ

Instead of having a floating vessel that goes all the way around, I wanted to try and make one that would slip underneath the barriers straight to the finish line.

The 'cargo' is an orange, which can get wet, but you need to be able to remove it quickly at the finish line.

 

A similar project done here: http://forum.allaboutcircuits.com/showthread.php?t=45623&highlight=submarine

John

 

Generally the bulge pumps are 12v DC and require a battery to run. The thing you have to test your motors for is current. Those 750 gph motors I showed you are rated at 3 amps, but when you put a propellor on them and put them underwater they draw more current, around 6 amps. In terms of a battery, if you choose a 12v dc 6 amp hour battery, the battery could continuously supply 6 amps for an hour. In practice, probably a bit less than that.

The motor could drive two props, but you have to think about HOW the props are going to connect to the motor. How are you going to transmit the power from 1 motor to 2 props? You'd need to make up a transmission setup with differential gearbox which is probably more complicated than it's worth. Be easier just to use 2 motors. With two props, you'd want to have one left hand prop and one right hand prop - to minimize the torque effect. Model boat shops tend to sell matching pairs.

If you want the craft to sink you want to do some buoyancy calculations. Basically calculate the volume of the craft and multiply by the density of water. That'll give you a mass. If the mass of your craft is less, it'll float and if more, it'll sink.

 

Does this thing have to dive from the surface, stay at a constant depth and hold its course, then surface again at the right time? You'd better have a plan.

 

There must also be some boundaries or rules? I mean, can you use non-electrical propulsion such as compressed gas? This would give you FAR more power:weight than a battery-driven system. Are you hoping to hug the outside perimeter wall? What will guide your craft safely to the finish. I think anything that big and that fast cannot rely on passive navigation.

Going underwater is a good strategy if the rules don't prohibit it. Your plan to "cheat" is also excellent, again, as long as you aren't DQ'd. An old business strategy professor of mine was fond of noting that "nothing beats an unfair advantage".

 

Does this thing have to dive from the surface, stay at a constant depth and hold its course, then surface again at the right time? You'd better have a plan.

If you watch the video I linked, you can see that you can get pretty close to the water. You are allowed to put your arms but not your body into the water. So it could just sink to the bottom, and glide along.

As for recovering it, I was thinking to put maybe a foot or so of buoyant string on the top. In theory the string would get pulled along parallel to the vessel, then when it comes to a stop(hits the wall), the string would float to the surface and could be pulled up.

There must also be some boundaries or rules? I mean, can you use non-electrical propulsion such as compressed gas? This would give you FAR more power:weight than a battery-driven system. Are you hoping to hug the outside perimeter wall? What will guide your craft safely to the finish. I think anything that big and that fast cannot rely on passive navigation.

Going underwater is a good strategy if the rules don't prohibit it. Your plan to "cheat" is also excellent, again, as long as you aren't DQ'd. An old business strategy professor of mine was fond of noting that "nothing beats an unfair advantage".

You are allowed to use any sort of propulsion bar liquid fuels.

As for navigation, heres a crude image of what I am trying to accomplish.

It basically just has to go in a straight line, as fast as possible, while staying close to the bottom of the water so it doesn't get snagged on the barrier.

There are steps leading down into the pool, and they go all the way across the entire thing (including under the overhanging walkway). So it would be possible to ride the side of a step the entire time if going in a straight line in open water isn't possible.

 

Are you sure of the water depth and clearance underneath the barrier? I love your strategy but the risk of a "cheating" strategy is that some unknown "extra" hurdle will be placed in your way, that you'll only discover to your horror on game day.

I think the traditional way of guiding a torpedo in a straight line is a gyroscope. Since you're going along a wall, you might be able to use another method, even something like a curb feeler. I think you'll need a servo on your fins, like R/C technology.

 

On a budget of $80 dollars, you may as well rule out any electronic control system (micro controller etc...) and start looking at the actual mechanics of what you want to do.

I don't think the task is impossible with some clever design work. If you balance the craft properly, ensure it can remain level, you could always put fins on that are 'banked' to give it a natural turning circle. This would require some tuning of course, and would work best if your craft floated.

If you're determined to go under instead of around, you want to make the craft neutrally buoyant - doesn't float or sink. I'd recommend you make it float then add weight to it.

I suggest you start with this:

Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

Or buoyancy = weight of displaced fluid.

Following you get the weight right, you have to think about where the various components are going to be in the craft, so it remains level (i.e. no moments).

This is the basics, after which you can start to consider the more finer details (fins, craft access etc).