buoyancy of marine plywood box

[Shraddha]

Will a 4 ft x 4ft x 1ft marine plywood (thickness - 12mm) box float on water? The box is hollow. Also, will it sink if multiple boxes are joint and someone has to walk on it?

 

[BlueBell]

Shraddha,

Welcome to the forum.

Yes, it will float on water, and, it depends how big the person is...

4 x 4 x 1 = 16 cubic feet x 62 pounds per ft^3 = about 1000 pounds buoyancy minus the weight of the box, is how big a person it will hold.

Why do you ask? School project?

 

[Ike]

that's 16 cubic feet. its 48 square feet of surface 12mm thick (0.472 inches) which is approx 1.9 cubic feet of plywood. Fir plywood weighs about 55 lbs per cubic foot which means your box weighs about 104.5 lbs. However wood floats so the wood alone would support 85 lbs. 16 cubic feet of air will support 16 times 62.4 lbs per cubic foot in fresh water. If the box is watertight it will float approximately 978 lbs . 16 x 62.4 = 998.4 - 104.5 + 85 = 978.9 . Walk on a single box ? Yes, but it would probably roll over because your center of gravity is about 3 feet above the surface you are standing on and that would raise the CG of the box about 2.5 feet. If you sat down it would probably be ok. I may have made a math error somewhere but someone will come along and point it out .
If you have multiple boxes that are joined together it could be walked on. That is the basic principle for floating docks and floating pontoon bridges.

 

[rwatson]

If you are from India, the metric system is your friend.

4ft (1.2 metres) * 4 ft (1.2) metres x 1 ft (.3 metres)

= 1.2m x 1.2m x .3m
= .432 cubic metres

x 1000 (litres)
= 432 litres

1 litre = 1 kilogram

So, that box will float up 432 kilos (952 pounds), less the weight of the box.

 

[Shraddha]

Why do you ask? School project?

yes university project.

 

[BlueBell]

yes university project.

So, did I pass?

 

[jehardiman]

Walk on a single box ? Yes, but it would probably roll over because your center of gravity is about 3 feet above the surface you are standing on and that would raise the CG of the box about 2.5 feet.

As Ike said, stability is going to be your real issue. Just how the floats are constrained or joined can be problematic.