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Name: Sammy
Status: Student
Age: 8
Location: N/A
Country: N/A
Date: N/A

My Dad and I are going to build a sail boat using PVC pipes for flotation. Is there an easy way to caculate how many 8 foot long, eight inch diameter pipes we will need to support our weight and keep the boat floating?

The weight that the pipes will support is the weight of the water they can displace minus the weight of the pipes themselves. To calculate this, you need to know the volume of the pipes, the weight of the pipes, and the density (weight/volume) of water.

Fresh water has a density of 1 kg/liter. Salt water has a higher density. The volume of the pipes can be calculated from their diameter and length by the formula

      V = L pi r^2, where
      L is the length of the pipe, 8 ft = 96 in.,
      pi = 3.14159,
      r is the radius of the pipe, 4 in.

So the volume of a pipe is

     V = (96 in)(3.14)(4 in)^2
        = (96 in)(3.14)(16 in^2)
        = 4823 in^3

Now you need to convert cubic inches to liters. 1 cubic inch = 0.01639 liters, so

     ;V = (4823 in^3)(0.01639 L/in^3)
        = 79 L.

So, in fresh water, an 8-ft length of 8-in diameter PVC pipe can float 79 kg minus its own weight.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois

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