"Buoy" science/math question

Yesterday, I was dropping some stuff off at the Salvation Army.  Since I was there, I decided to go in.  I found 6 retractable key chains (actually they look more like retractable security card holders) at $0.50ea.  I thought these would be great for little things like fishing pliers, etc.  I also ran across a small "Scanmarin" 6.5" net ball for $6.  A quick look on the internet came up with a buoyancy capacity of 8 pounds.  This is perfect for a used crab trap that I recently picked up.

After getting home, I placed this little buoy on the counter and then the kids placed a jug of milk on the counter beside it.  I started to wonder at how this little float is supposed to buoy 8 pounds???

1 Litre of water weighs 1 Kilogram.  So 4 Litres of water weighs 4 Kilograms (for you south of the border 4 Kilograms = 8.8 Pounds).  My understanding is that the jug would sink once 4 Kg (8.8 Lbs) was exceeded.  This little buoy is roughly half the size of a milk jug.  How can this have a buoyancy of 8 pounds?  What am I missing?

Okay, maybe saying half the size is a bit off.  It's certainly smaller than the milk jug though...

Google search:

A 6.5" sphere has a volume of 2356.35 cubic centimetres (2.35635 Litres). 2.35635 Litres = 2.35635 Kilograms which is equal to 5.194862515 pounds.

I am no math whiz, nor a marine float engineer by any sense of the word. 

I think what you are forgetting is that your ~8lbs of water in the milk jug won't weigh near that while submerged in water due to fluid displacement.

Check out the Archimedes Principle, it should get you on the right track to figuring it out with math.
http://docplayer.net/36702554-Lesson-applying-archimedes-principle.html

Or, you could weigh 8 pounds of rocks, put them in a bag with no air and see if the float holds it?  Wisconsin math graduate lol!

Maybe it's only rated for very cold water.

Just kidding. 

In non-technical terms, the buoyant force on any object in any fluid equals the weight of the fluid displaced by the object. Therefor an air-filled gallon jug in water can only produce a maximum buoyant force of ~8.34 pounds. Your ~1/2 gallon buoy can't support 8 lbs.  Sorry. :-(

Maybe that's the inflation rating.

Maybe that's the inflation rating.

+1

I believe everyone is correct when they say it wont hold 8 pounds. 

However, it should be  fine for your crab trap.   The weight of the crab trap should not be considered when calculating if the buoy is sufficient.  The buoy is only holding the rope itself vertical within the water column.  Should it be used for crab traps really depends where you are.  A smaller buoy could have advantages as it will not make a crab trap "walk away".

Could it be a "Buoy Light" designed to be allow greater holding capacity?  Dilly Dilly! 

Zach makes a good comment that a smaller/lighter buoy is unlikely to walk away with a trap and a round like that is no more likely to hook in kelp than a standard one.  It wouldn't be a legal crab pot buoy in WA.  We're special.

It could be a "Buoy Light"?  Dilly Dilly! 

Dilly Dilly!

I think it's the orange red color that makes it hold up so much weight.

This is where the metric system fails.

"A pint's a pound, the world around." Meaning a pint of water weighs a pound. So a gallon weighs 8 pounds. That's the displacement if it is the size of a milk jug.

This is where the metric system fails.

"A pint's a pound, the world around." Meaning a pint of water weighs a pound. So a gallon weighs 8 pounds. That's the displacement if it is the size of a milk jug.

1 Litre of water weighs 1 Kilogram.