Water Displacement calculation

Posted on by

Pontoons are the heart of the vessel since they are the ones that keep the vessel afloat.

The first step is to calculate how many PVC pipes I will need for my pontoons in order to properly float 2 adult passengers + weight of cat + additional 20lbs of weight (lunch box, life floation device, etc). I am using standard 10′ x 4″ PVC pipes found at local Home Depot, as seen here,

Charlotte Pipe 4 in. x 10 ft. PVC Sch. 40 DWV Pipe

Water Displacement

Water displacement is the formula used to calculate how much water the PVC pipe will displace and how much weight in pounds that we can store on top of the cat.

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, we need to know the volume of the pipes (V), the weight of the pipes (W), and the density (weight/volume) of water (D).

Fresh water (at 70 degrees F) has an approximate density of 1 g/mL. (~ 8.239 lbs/gallon)
Salt water has a density of 1.025 g/mL (~ 8.554 lbs/gallon)
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, so in our case its 10 ft or 120 in.,
pi = 3.14159,
r is the radius of the pipe, ~2 in. (4″ total diameter)
V = 120 * 3.14159 * 2^2
V = 1507.96 cubic inches

convert cubic inches to liters (and pounds, where 1 L = 2.2 lbs)

freshwater V = (1507.96 in^3)(0.01639 L/in^3) = 24.72 L (~54.38 lbs),

saltwater will be approximately 55.73 lbs

In order to carry my projected goal of 2 adults (~300 lbs) + weight of vessel (unknown) + 20 lbs of additional weight = ~350 lbs, so I will need about 8 PVC pipes in order to have enough buoyancy.

If I can find PVC pipes that have a larger diameter than 4″, I will use these instead, a larger diameter is more preferable since it has a larger Volume and less surface area (less friction). If I can find PVC pipes like these I will adjust this post.

If anyone finds error in these calculations (my math skills are pretty awful), please post corrections. Thanks!

1 thought on “Water Displacement calculation”

  1. thanks for this post, i’m also considering using charlotte pipe for pontoons, but recently I’ve been considering using rubbermade roughneck 52 gallon totes, placing three in a row, filling them with foam and attaching them together to form solid pontoons, they should be way more buoyant, and considerably lighter,

    Reply ↓

Leave a comment