This is my entry to the make it float contest. Before I started designing anything I conducted some research on buoyancy and displacement, with the essential question in mind: "How does buoyancy and displacement affect a boat's ability to carry weight?" The most significant piece of information I found was that “water will push upward against the object with a force equal to the weight of water that is displaced," (Archimedes). With this in mind I then proceeded to calculate the amount of quarters a sphere with a radius of 5 cm could hold. A sphere with a 5 cm radius has a volume of about 523.6 cm^3 and one cm^3 of water being one gram the sphere could hold 523.6 grams which is about 104 quarters, if they all fit and the sphere itself did not weigh much. With all this in mind I set out to design my first prototype.

Hydra 1: had a radius of 2.15 cm which should mean that it could carry a weight of 42 grams, but this design had a number of other problems, such as where the dome and the bottom connected there were gaps in which water could flow in through. This was because in the design file the two sections did not overlap but just shared one edge. In addition the coin hole was too small for any coins and for some reason the bottom half printed solid. The Hydra 1 was able to carry 21 grams in coins but altogether it weighed 42 grams, thus living up to Archimedes’ law.

Hydra 2: had various improvements over the Hydra 1 design, such as the dome and bottom were connected via an overlapping piece of plastic and the size was increased form 5 cm in diameter to 6 cm in diameter to allow more weight to be carried. The Hydra 2 could carry 9 quarters with the top coin slot plugged and 8 with it open, with quarters being about 5 grams each.

After testing the Hydra 2, I came up with a hypothesis: No matter how large the sphere is it is never going to be able to be completely filled with coins and still be able to float, because one cm^3 of a quarter weighs more than one cm^3 of water, the space the quarters would not fill up inside the sphere is negligible. With this in mind a sphere which would be filled up with quarters would never be able to disperse enough water to be able to float. That is why there needs to be an external flotation device.

Hydra 3: incorporated an external flotation device, the middle piece the boat was able to carry about 80 grams (14 quarters).

Hydra 4: Was only the bottom half of the sphere and the middle piece. When I was testing Hydra 3 I tested it without the top part of the sphere and I noticed that it was able to carry more weight about 10 more grams, 90 grams (16 quarters). I concluded that this was because for the most part the top was not displacing any water and instead was just dead weight. In addition when the Hydra 4 is full of quarters it can still float which makes it very efficient.

In the end I found that the only way to carry the most weight is to build the biggest boat or connect many smaller vessels together.