Density Question?

Question

Ok here's the Problem....

I want to build a structure that will float on the water. Made of plywood, For me to stand on.

The material is going to be 1/8 inch plywood. I weigh 195 pounds. How much Plywood is needed, regardless of lengths, that being, that the plywood is one large piece. The water will be freshwater.

Answer 1

1/4 inch is too thin. Go with 1/2 or more. 3/4 would work great.

Unpainted plywood gets waterlogged in hours, will have little buoyancy. Build a boat instead, or use styrofoam under it. For styro, figure about 300 lb displacement minimum, or 4.7 ft^3.

A 4x8 sheet of 2" thick foam should do it. I would use marine grade plywood, but exterior grade would be OK for a while. Use waterproof construction glue to attach stryofoam to bottom. I would also glue 1' wide strips to the top side, for improved stability. Note that floatation along the edge of the boat (or even better, on an outrigger), and slightly above the water level (so it submerges as the boat tilts) is what adds stability. Could use 2x4's sticking out several feet from the sides with foam outriggers for much improved stability.

And, make sure you're wearing a swimsuit!

Answer 2

Are you going to build a "boat" out of the plywood?  This would be far more efficient than attempting to use the specific gravity of plywood as your floating medium.  If you are intent on building a "raft" with no displacement other than the volume of the raft itself, then consider using a plywood shell around a more buoyant core, such as styrofoam.

Assuming a specific gravity of 0.75 for your plywood, you'll need about 780 lbs of plywood to keep you completely dry.  At 30 lbs per sheet, this is 26 4x8 sheets of 1/4" plywood.

By comparison, if you build a boat you only need to displace 3.125 cu ft of water to equal 195 lbs, a 1 1/4"x4'x8' volume built up from a single plywood sheet (but don't splash or you'll capsize).

Answer 3

Too many factors are missing,it cannot be calculated. The best way is doing experiment. Get a piece of your 1/4 " plywood and put it into fresh water. Add weight on top of it until it starts sinking. Measure the weight....for example,it equals to one pound.  

The total pieces of plywood = 195 / 1 = 195 pieces

Add 20% risk factor, 195 * 20% = 39 pieces

Total minimun piece of plywood is 195 + 39 = 234

Answer 4

it wont work, you need to use displacement rather then straight up wood