Concrete Can Float

How much concrete would you need to make a concrete cylinder float?

(image source is a wikipedia article of the worlds oldest concrete boat.)

If you wanted to make a concrete cylinder with 1 foot thick walls float, how big would it have to be?

Reinforced concrete weighs about 150 lbs per cubic foot. For each cubic foot of concrete used you would have to displace about 20 cubic feet of water.

One way to do this would be to make a wide shallow cylinder, like a short fat can. The opposite way (which I am personally more interested in) is to make it long and skinny, like a straw. I'd like to see how tall of a cylinder you would need if it were 5-10 feet in diameter with 1 foot thick walls.

What would the math be for that?
If it would need to be dozens of feet tall, how much pressure would be exerted on the cylinder walls at the bottom?