# Making a rubber duck swim in a tabletop pond

I plan to build a small round tabletop pond (radius 15 cm).

I want to put a small rubber duck inside, and I want it to "swim" constantly, preferably moving randomly in the pond (as opposed to a fixed swimming path, like in circle).

What would be a good way to do it?

One idea I had is having 2 small pumps in strategic points on the circle (in the bottom of the pond), so that they create water circulation. But I don't really know how to place them. I'm also concerned that the water surface would be too bubbly because of that, so not sure.

I'd appreciate any direction. It doesn't have to include pumps or any electric device, so please suggest anything you find suitable.

• With a radius of only 15 cm any amount of movement is very limited, especially "random" one. The surface tension of water will always drag the duck right to the edge once it gets close enough. Or do you have a tiny rubber duck in mind (like 1 - 2 cm)?
– Elmy
Mar 27, 2023 at 5:11
• @Elmy, not that tiny: it's 5cm length, 5cm width, 4cm height. What would be an ideal ratio of pond vs. duck? I can make the pond larger. Mar 27, 2023 at 7:32
• Well that makes the pond 6 times as wide as the duck. I assume it will end up floating along the edge in circles, unless you somehow tether it to a string that is anchored to the center of the pond. Might actually work out well like that. Since the surface tension draws the duck to the edge, it makes it harder to move along with a water stream. If the duck cannot touch the edge, it should move more easily and with less force.
– Elmy
Mar 27, 2023 at 11:01
• If mechanical devices are going to be involved it seems simpler to have one big thing at the bottom that spins around slowly, than multiple pumps. Quieter, too Mar 27, 2023 at 16:28
• FYI: All answers are great, and I debated which one to mark as accepted. I eventually went with the marked one, because of the additional creative ideas to achieve a random swim. Mar 29, 2023 at 16:36

I've run a lab experiment (article in MagPi, the Rapsberry Pi magazine) in the past that had some rather neat rotation in the reaction vessel. We did this with angled hose fittings, one inlet and one outlet, and a small pump. The angle is important to get good rotary flow, i.e. the hose fittings should be as close as possible to tangential to the round tank. We used a miniature water pump run at the lowest voltage that it worked at reliably (5V on a nominally 12V pump), and the pump introduced no bubbles. I tried some cheap pumps from eBay. They were fine to start with but not robust enough.

This shows what happened when we added dye (methylene blue) to the system. The water is flowing in at the top of the image, out at the bottom. The flow isn't much, but it gets a good rotary motion going.

The beaker is about 7cm/3" in diameter here. Note the bubbles come from squirting in the dye with a syringe from out of shot.

To upgrade this for your use I have a few ideas, which may be comined, and coudl easily be controlled with some simple electronics of a Raspberry Pi or Arduino:

• Run the pump intermittently at random
• Run it off different voltages to get fast/slow movement. In our case we had to run really hard to get bubbles, unless there was a slight leak admitting air.
• Use a reversible pump, and reverse it (ours wasn't reversible, some of the cheap ones are - I've just looked on AliExpress). Reversing the flow wont give a mirror image of the movement on the surface.
• Have one outlet, multiple inlets, multiple pumps (or valves), with T pieces as required for the hose. In this case you'd probably want one tangential inlet, and one radial, so you could, for example, start it swimming round in circles, switch inlets, then it would dart across to the other side. A tangential inlet in the opposite direction could also be good.
• An inlet in the base might give some interesting effects if run briefly (even better if you can detect when the duck is over the inlet and turn it on, which could be done with a camera and OpenCV)

If you can't fabricate the connections like this, you can try sealing the hose straight through the wall and finishing with a 90° elbow to get the tangential motion. You can even run the hose in from above with an elbow. Depending on any decoration you have around the pond, it might be possible to conceal this.

You'll want to experiment with the water depth, so put the inlets high enough that the duck will float if they're at the surface, but low enough that you can raise the water above them by a few cm.

If your pond is square, you'll get a less circular flow, and there's a risk it will get stuck in the corners. But you can get a tangential flow more easily as that's square to the walls. Using 3 T pieces to inject flow at all 4 corners should ensure it keeps going in the same direction, probably sucking the water from the middle of the base if you can.

I think your 6:1 pond:duck ratio is the minimum worth going for; 10:1 would be better I think. You might also get good effects with multiple ducks.

Here are a couple of plumbing ideas, to combine, modify etc..
Pumps marked * would be good to have reversible, with appropriate drive electronics. Arrows indicate the direction of flow at in-/outlets that I think should be better if you can't reverse the pumps

• I should be able to sketch the plumbing later if the description isn't clear, but I'll need a reminder Mar 27, 2023 at 10:30
• I guess your lab is missing magnetic stirrers. Did someone steal them all for a tabletop duck pond? Mar 27, 2023 at 16:26
• @user253751 we're a physics lab playing with a bit of chemistry and our only magnetic stirrer is built in to the hotplate. This reaction beaker was where the phoyocatalysis happened, and the plumbing went off to the optical measurements. The mixing was designed to ensure the reaction happened evenly Mar 27, 2023 at 16:39
• ... Sadly I dont work in that lab any more, because I might be able to find a rubber duck of the right scale to try it. Mar 27, 2023 at 16:40
• @HeyJude top-right out and bottom left in are meant to be as close to tangential as practical (and to fit the flow arrows in line with the hose). That's enough to get a flow going round the perimeter, as in the gif - after all it's not to scale. Or to think of it slightly differently, tangential to a circular stream. Middle left is the radial one I mentioned for variety Mar 28, 2023 at 6:04

This honestly started as a fun idea, but it might actually work to integrate a magnetic stirrer into the pond.

Magnetic stirrers are usually used in a lab to (you guessed it) stir liquids. What's special about them is that the mechanism that does the stirring isn't in contact with the liquid that's being stirred. So even if your pond runs completely dry, the stirring mechanism won't take any damage. The disadvantages are that the stirrer itself adds height to the pond and you need to put a magnetic stir stick in your pond, which might not be visually appealing.

In a lab, you usually want to stir things quickly and thoroughly, so even the lowest setting of most stirrers is quite quick. If you don't want a vortex in your duck pond, you'll probably need to look for a low speed stirrer (I saw one with 5 rpm online, which is 1 revolution every 12 seconds) or an analog one. If a lab stirrer doesn't work for you, any tool that can rotate a magnet beneath the pond will do.

I would tether the rubber duck to the stir stick in the pond to keep it a constant distance from the very edge. Then it should start floating in circles when you activate the stirrer.

• I wondered about a magnetic approach. It strikes me that you could put a magnet under the duck, and use a bigger, more complicated mechanism underneath rather than a stirrer that rotates in place. Something made with Lego technic for example, which could have a rotating gear at the end of a rotating arm Mar 27, 2023 at 11:57
• I think with the right depth gluing magnet to the duck itself will additionally benefit to stability of the duck by making it bottom heavy. Mar 27, 2023 at 20:27
• Magnets are a great way to induce seemingly random movement: youtu.be/J9DlFONwMpI?t=25
– vsz
Mar 28, 2023 at 7:50
• Put the magnet in the duck and slow down the spin speed by several orders ? Mar 28, 2023 at 23:31

The question describes wanting random motion rather than a fixed path. Here's an idea to do that.

Instead of circulating the water around the pond, use a small pump to pump water straight up from the bottom on opposite sides of the circumference of the pond, with very little force (call that 12:00 and 6:00). From each pump outlet, the water will reach the surface and spread out in all directions. The "fan" shaped flows will meet in the middle, and the water will flow toward the sides, perpendicular to the line that the water outlets are on (call that 3:00 and 9:00).

Anything floating in the water will move in some random direction away from the 12:00 or 6:00 water discharge. Momentum will carry it a little into the flow from the other side, and it will be pushed in another direction. Eventually, it will be herded by the water flows toward the circumference on one side or the other (3:00 or 9:00).

At 3:00 and 9:00, hide a magnet below the surface (possibly in some scenery). Put a magnet on the bottom of the duck, with the same pole interacting on all three magnets, so the duck is repelled on both sides.

When the duck is herded toward one of the magnets, it will be deflected in the general direction of one of the pump outlets, and will have enough momentum to be carried a little bit "upstream", where the cycle will be repeated.

This kind of action might stabilize into a simple loop, where the duck gets deflected by the magnet, and the flows return it to the same point, so the motion repeats. That can be avoided by combining this with the magnetic stirrer idea in Elmy's answer, with the duck's magnet being the only free magnet inside the pond. The stirrer will give the duck a periodic kick that will alter its path. Adjust the water level so that the stirrer attraction on the duck's magnet is weak (so it influences the duck, but the duck doesn't lock onto that magnet and become the stirrer; the water currents should be the main drivers).

• So two pumps (at 12 and 6), or one pump with two outlets (I've actually never encountered a small pond pump with two outlets, hence asking)? Mar 27, 2023 at 23:44
• @HeyJude, you could get two outlets by using two pumps, but you wouldn't want a huge amount of flow. I was envisioning one pump with the outlet split (pump feeds the leg of a tee, and the two arms of the tee feed water to 12 and 6). Mar 27, 2023 at 23:50

Use only one pump, split the exit into two. Somehow, the two resulting exists will be able to move inside the pond, at least left / right.

Also, you might want to try to enable the duck to be able to change the position of the water exits (i.e., the exists are long enough for the duck to hit them occasionally).

Note: changing the relative position of the water exists from the pump will change the movement pattern.

You might want to install some elements on the two water exists, to adjust the pressure - different pressures, different reactions of the duck when in the area.

Making the pressure adjusting elements to change automatically while in operation for extra randomization will not be possible to be done easily, as far as I can tell.

Bonus info: water pumps for aquariums tend to be more silent than other pumps. And usually you can find tubes and splitters also, where you find the pumps.

Additional gear: you might want to install a tiny motor inside the duck, connected to a propeller installed outside the duck. If you are technically inclined, you install the propeller in a tube inside the duck, so the propeller would not be visible outside. This propeller will add extra randomness combined with the pump.

For supplying electricity to the motor inside the duck, you can use some photo-element, and a small lamp above the setup. If the lamp changes colors / intensity (in a pattern, or randomly), the duck's movement will become even more random.

I have in mind the kind of motors that provide vibration to mobile phones, or similar.

Another bonus: if you do not mind the extra sound and splashes, use air pumps (aquarium pumps should be fine) to create movement in the water.

If you have a sufficiently long and flexible hose, attaching a pump to it and putting it into the pool will result in the open end snaking around in the water in a seemingly random motion. The resulting changes in the inlet water vectors will then propel the duck in similarly random ways.

• Did you read how the question is about a very small "pond" with a 15 centimetre radius? I'm not sure this method is of any use for a tabletop decoration.
– Joachim
Mar 28, 2023 at 11:54
• @Joachim The video depicts a hose on this scale, so I don't see why wouldn't it be usable. Mar 28, 2023 at 12:41
• Immersed in water the motion will be damped quite a bit, and small hoses are effectively stiffer as their walls are a greater proportion of the thickness. I think you'd need a really floppy (silicone?) hose pointing upwards from the bottom of a deeper tank than I was thinking of, with a bit of weight on the end, and more flow than might be ideal - and if it went vertical you'd have a fountain. It might be doable on this scale but would take a fair bit of experimentation - and random flow wouldn't necessarily mean very good random duck movement Mar 28, 2023 at 14:04