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Dear Brummipapiteddy,

my kid enthusiastically talks about "our" upcoming paddle boat trip. He is super excited about it and insists that I should come aboard. But what if we capsize and we all go overboard? Or if I fall off the boat? While I know theoretically how to swim we all know that teddies get soaked full of water and will soon sink. What if I sink to the ground and will never be found and seen again?
- teddy with a sinking feeling

Dear teddy,

first of all, don't panic. You have written to me and we will come up with a solution so that you can enjoy the boat trip together with your kid.

🌊 Will you float or sink?

So first the positive news: If you fall into water, you will actually float — at least for a while!

To understand what happens when you fall in water, we need to talk about something called buoyancy.

Buoyancy is the upward push that water gives to anything placed in it.

Whether something floats or sinks depends on one simple question: Is the object lighter or heavier than the same volume of water? Most teddy bears are made of synthetic fibers (polyester) and hollow stuffing, which trap a lot of air. Since air is much lighter than water, your overall density is less than that of water when you are dry. So if you fall in, you are not sinking immediately!

But here's the catch: You are a teddy, not a real bear. A real bear has lungs full of air that keep it buoyant from the inside. You don't have lungs! The only air you have is trapped between the fibers of your stuffing. And over time, water soaks into your fabric, fills those air pockets, and the air escapes. Once that happens, you become waterlogged and — yes — you start to sink.

Even though the surrounding water helps support some weight, if you are waterlogged you sink because soaked stuffin is heavier than water.

If you are lucky, you'll be rescued during the time that you float. But well, we can make you float much longer!

🚀 Idea 1: A propeller backpack (looks cool, but tricky!)

A fun idea would be to build a waterproof propeller that you can put on your back. The propeller spins and pushes water downward, and — thanks to Newton's Third Law of Motion (every push has an equal push back the other way) — the water pushes the propeller (and you!) upward. It's the same idea as how a rocket works, except you're pushing water instead of fire!

How well this works however depends on how big and heavy you are. As the propeller needs a battery, it lasts less long the bigger you are.

⚖️ How much force do we need?

Gravity pulls everything down. The strength of that pull depends on your weight: But here's the tricky part: a wet teddy doesn't just sit still in the water. Unlike a real bear with air-filled lungs that stays partly buoyant on its own, a waterlogged teddy has almost no natural buoyancy left. The propeller has to do nearly all the work of fighting gravity — not just helping out a little.

Now, a toy propeller is not a magic wand. When electricity spins the motor, a lot of that energy gets lost as heat in the motor, and a lot more gets wasted swirling water around in chaotic bubbles instead of pushing it neatly downward. Because of these losses, a small home-made propeller needs roughly 1 Watt of electricity for every Newton of upward thrust it creates. Sometimes a bit more, sometimes a bit less, depending on how clever the design is.

So to fight those 9.8 Newtons of gravity, your motor needs to draw roughly:

~10 Watts of electrical power


(If you want to know how to get to 10 Watts: Electricity going into a motor does not turn into pure upward lift. It gets lost in three places: Because of these losses, a toy-grade motor+propeller needs roughly 1 to 1.5 Watts of electrical power for every Newton of thrust it produces. (A large, professional underwater thruster is more efficient; a tiny toy propeller is less.)

So the estimate is:

Power~9.8N*1W/N~10W

Think of it like a garden hose trying to keep a beach ball in the air. A tiny perfume spray bottle doesn't have enough "oomph," no matter how hard you squeeze it. You need a proper hose! A small, weak toy motor is the spray bottle — it simply cannot push enough water with enough force. You need a stronger motor.

🧸 Where can a teddy find a strong motor in the house?

Don't worry, you don't need to order anything special from a laboratory! If you want to use a 10,000 mAh power bank (which outputs 5V), then you should look for 5V motors: 🔌 How much power does a real motor give us?

A typical 5V USB gadget you might find around the house — like a small desk fan, a computer cooling fan, or a little aquarium pump — usually draws about 0.5 to 1 Amp (500 to 1000 mA) when running. Let's say you find a decent one that pulls 1 Amp.

At 5 Volts and 1 Amp, the motor uses:

5 V × 1 A = 5 Watts


Remember our rule of thumb: roughly 1 Watt of electricity gives about 1 Newton of thrust. So 5 Watts gives about 5 Newtons of upward push. Since 1 kg needs 9.8 Newtons to stay up, 5 Newtons is only enough to support about 0.5 kg — a small or medium-sized teddy, or a bigger teddy who still has a little air left inside and isn't completely waterlogged yet.

Since this 5-Watt motor only provides 5 Newtons of lift, it is only sufficient for a teddy weighing about 0.5 kg (or a larger teddy that still has some air left inside). If you are a bigger 1 kg teddy and fully soaked, you would need a much stronger motor — closer to 2 Amps (10 Watts) — to get those 9.8 Newtons. Those exist (some stronger USB water pumps draw 2A), but they are less common in the average house.

So for most teddies, the propeller idea works best if you are on the smaller side, or if you can find one of those stronger 1–2A pumps.

🔋 Battery life: How long will the fun last?

Think of a battery like a bucket of electricity, and the motor like a hole in the bottom that lets it drain out. The bigger the hole (more Amps), the faster the bucket empties!

The formula is simple:
battery life (in hours) = battery capacity (in mAh) / current draw (in mA)


Let's say you found a decent pump that draws 1 Amp (1000 mA) and you power it with a 10,000 mAh phone power bank — the kind your kid's parents probably have in a drawer.

10,000 mAh / 1,000 mA = 10 hours in theory


In real life, motors waste energy as heat, weeds can tangle the propeller, and power banks don't give their full label amount when working hard. Realistically, you'd get about 6 to 7 hours of gentle zooming. That sounds wonderful!

But wait — the vicious cycle!

That 10,000 mAh power bank weighs about 200 grams. If you were 0.5 kg before, you are now 0.7 kg — 40% heavier! Gravity pulls 40% harder, so the motor needs 40% more power, which drains the battery 40% faster. Your 6–7 hours suddenly drops to about 4 hours. It is like packing a huge lunch to run a marathon — at some point, the lunch weighs more than the energy it gives you!

And if you are a bigger teddy using a strong 2 Amp motor? Then:

10,000 mAh / 2,000 mA = 5 hours in theory, or about 3 hours realistically


Add the 200 gram power bank to a 1 kg teddy, and that 3 hours drops to about 2 hours because of the extra weight. Still fun, but not forever!

So, is it worth it?

A propeller backpack won't keep you floating forever, and finding a motor strong enough for a big teddy is tricky. But for a smaller teddy with a modest USB pump, it would look absolutely awesome, and I'm sure your kid would love watching you zoom around with a little motor on your back! Just don't forget to come back to shore before the battery runs out.

🛟 Idea 2: Foam floaties (simple and reliable!)

My next idea is to strap a small foam pool noodle or cork blocks around your tummy. Unlike the propeller, foam doesn't need batteries and never runs out! The air trapped inside closed-cell foam bubbles can never escape, so it floats forever.

To know how much foam you need, we can use Archimedes' Principle — named after an ancient Greek scientist who supposedly jumped out of his bathtub shouting "Eureka!" (which means "I found it!") when he figured this out.

His discovery was simple: the buoyancy force pushing up on something in water equals the weight of the water that object pushes aside. In other words, if you push 1 liter of water out of the way, water pushes back up with a force equal to the weight of 1 kg of water — about 10 Newtons.

Here's the rule of thumb:
1 liter of closed-cell foam provides about 1 kilogram (1000 grams) of buoyancy in freshwater (slightly more in saltwater — about 1025 grams — because saltwater is a bit denser).

But that's the total buoyancy. To keep floating, the foam must support your weight plus the weight of the water that soaks into your fabric.

So the practical formula is:

Foam volume needed (in liters) = (Weight of the wet bear in kg) / 0.9
or
Foam volume needed (in liters) =(Weight of the wet bear in kg)* 1.1

Why this formula? Because we want a safety margin: the foam should work at only 90% of its maximum capacity, leaving 10% extra lift to keep your head nicely above the water. It's like packing a bag that holds 10 books but only putting 9 in — just in case!

Example:
You weigh 500 grams (0.5 kg) dry
When soaked, you might hold another 300 g of water in your fabric = 0.8 kg total
Foam needed = 0.8 / 0.9 = ~0.9 liters of foam

That's roughly a block of foam the size of a 1-quart milk carton (about 10 cm * 10 cm * 9 cm). Strap that to your tummy, and you'll bob like a cork forever!

If you don't want to calculate: just use twice the volume of your head in foam. That's a safe overestimate.

🦺 Idea 3: A life vest (the easiest solution!)

But honestly, a much simpler solution would be to simply ask for a life vest! Your kid will probably be wearing one, so maybe they can find a little one for you too? Life vests are designed to do exactly what we need: keep someone floating even when fully soaked!

Assuming that it fits properly and you don't fall out it also has the advantage that it will keep your head upright since swim vests are designed with more foam on the chest and less on the lower back, so they naturally turn you face-up in the water.

I hope you see now that there are many ways to stay afloat and actually enjoy the trip! And now you understand exactly what options you have. Everything tested — no magic, just science and math!

Rest easily and have fun! Many teddies will envy you for such a great excursion!

Warm hugs,
Brummipapiteddy


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