# Raft for a Daft Raft Race

• rob.oc95

#### rob.oc95

raft for a "Daft Raft Race"

Hi all,
Now I know this has probably been asked already, and it's probably a very simple question, but I'm building a raft for a "Daft Raft Race" and I was just wondering how many litres of air in cylindrical, metal barrels would i need to support around 600 kgs?
Thanks a mil.

Rob

P.S. Even just let me know how it's calculated/a formula and I would be able to work it out myself.

Last edited:

Hi all,
Now I know this has probably been asked already, and it's probably a very simple question, but I'm building a raft for a "Daft Raft Race" and I was just wondering how many litres of air in cylindrical, metal barrels would i need to support around 600 kgs?
Thanks a mil.

Rob

P.S. Even just let me know how it's calculated/a formula and I would be able to work it out myself.

Welcome to the PF.

In general, whatever weight of water the barrel would hold, is the weight that the empty barrel will support:

http://en.wikipedia.org/wiki/Archimedes'_principle#Archimedes.27_principle

.

Not to say you're wrong or anything but this doesn't seem correct.

To support 600 kgs or so I would only need 3, 50 gallon containers (plus a little extra) ?
This just seems a lot less than what i was expecting.

Thanks for the quick reply anyways...

One liter of water has a mass of one kilogram, so you need at least 601 liters of air to float your 600 kilograms. (600 liters of air has a mass of 0.7 kilogram!)

Not to say you're wrong or anything but this doesn't seem correct.

To support 600 kgs or so I would only need 3, 50 gallon containers (plus a little extra) ?
This just seems a lot less than what i was expecting.

Thanks for the quick reply anyways...

Check out the wikipedia article that I linked to. It should help out.

To support 600 kgs or so I would only need 3, 50 gallon containers (plus a little extra) ?
]

a gallon of water weighs about 8.3 lb, and there are 2.2KG per lb, so a gal is about 3.8kg
and 150 of them would weigh about 150 x 3.8 or 570 kg. so you really need closer to 600/3.8kg or 158 gallons of container to support 600 kg...

While what has been said in this thread is true, you are likely going to want a much larger volume than what you are calculating, otherwise your flotation device will be 99% under the surface.

Check out the wikipedia article that I linked to. It should help out.

Ok. I think I see it now. Very Sorry. Thanks everyone else too. Great help. Couldn't find out any other answers on the internet...

a gallon of water weighs about 8.3 lb, and there are 2.2KG per lb, so a gal is about 3.8kg

That's the exact thinking that caused the 767 to run out of fuel over Canada!

It's 2.2lb per KG.

A litre of water = 1kg.

There are approximately 4.54l in a UK Gallon.

That gives 4.54kg per gallon.

That means each 50 gallon container holds 227kg of water.

So for your boat you would need 3 of them to give you 681kg of buoyancy.

That's the exact thinking that caused the 767 to run out of fuel over Canada!

It's 2.2lb per KG.

:rofl: Good catch!