I'm planning a boat made from plastic bottles

How does the water don't go through the holes between the bottles ?

Either that's fake ... or isn't only bottles.

And i guess you could put helium in the bottles, so you would need less bottles XD

 

I want to see it when it's finished

 

It's because the bottles are empty and thus have a lower density than the water, enabling them to carry a certain amount of weight, just like any other boat. You could also think of a normal boat as a single bottle -- if you tie a lot of normal boats together they will still swim and not sink, right?

@Berat Sword, you could either just try around or make calculations based on Archimedes' principle using the mass and the volume of the bottles. (This idea is coming from a physicist. I have no idea of boat construction whatsoever.)

 

I understand how they float, what i don't understand is how the water doesn't get in.

A bottle is not a square, is like ... bottled shape So there will be little gaps in which the water will enter the boat

 

Alright, there might be some water getting into the boat so you get wet feet once you step in, but the boat will float nevertheless.

Let's build it in our mind:
- You take the first bottle. It floats.
- You connect the second bottle to the first bottle. Regardless of how they are connected, the boat still floats.
- You connect the third bottle. It doesn't change anything about the fact that the bottles are floating.
...
- After n bottles you are done. Congrats! When you step in, each bottle will carry a tiny bit of your weight. When you have enough bottles they will carry you just like you will start levitating when you hold a sufficiently large amount of helium-filled balloons

Assuming that the bottles have zero weight, the total volume of the bottles that are in direct contact with the water has to be equal to the volume of your body for the boat to carry you. As the bottles have some mass themselves, there need to be some more.

EDIT: @Berat Sword You could mentally approximate your body by a cylinder, calculate its volume (which is pi*r²*h with r being the radius and h the height of the cylinder) and divide this volume by the volume of a single bottle. That's the number of bottles (at the floor of the boat) which you need to carry your weight, assuming that the density of your body is equal to the density of water (which is a fair approximation). For additional people you will need additional bottles, so you need to multiply the number by the number of people. I think the weight of the bottles can be omitted. Just make the cylinder a little larger so you can comfortably fit in.

 

I was assuming that