basically you wont the largest surface area possible...
you could prob do this by rolling up the sides or scrunching them up..
when it comes to putting the coins in display scatter them evenly using as much of the base of the foil as possible. you need to spread out evenly.
due to the Archimedes theory, as long as the size of the water that your tin foil thing is in is large, the force of gravity on your tin foil thing will float because they should displace the same force.
(the boat's gravity against the water buoyancy = the same for it to work)
The boat has to displace less then it's weight in water otherwise it wont float.
As long as weight < displacement weight it will float.
Now with that said, I built one last night that based off of the weight of pennies (arround .1 ounces) my boat could have held about 500 pennies, with just one peice of tinfoil somewhere near 30cm x 30cm. Just build a Bowl with a flat bottom and make sure when loading pennies that the weight distribution is even!
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