How many litres of air do I need for a raft in a Daft Raft Race?

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Discussion Overview

The discussion revolves around determining the volume of air needed in cylindrical metal barrels to support a raft weighing approximately 600 kg for a "Daft Raft Race." Participants explore various calculations and principles related to buoyancy and weight support.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that the weight of water the barrel can hold corresponds to the weight it can support, referencing Archimedes' principle.
  • Another participant questions the initial calculation, proposing that only three 50-gallon containers would be needed, which seems less than expected.
  • A different contribution states that one liter of water has a mass of one kilogram, thus requiring at least 601 liters of air to float 600 kilograms, noting that 600 liters of air has a mass of 0.7 kilograms.
  • Another participant calculates that to support 600 kg, approximately 158 gallons of container volume would be necessary, based on the weight of water per gallon.
  • Some participants express concern that the calculated volume may not be sufficient for proper flotation, suggesting a need for a larger volume to avoid being submerged.
  • There is a correction regarding the weight of water in gallons, with one participant clarifying that a UK gallon weighs approximately 4.54 kg, leading to a conclusion that three 50-gallon containers would provide adequate buoyancy.

Areas of Agreement / Disagreement

Participants express differing views on the calculations needed to determine the required volume of air for buoyancy. There is no consensus on the exact number of containers or the calculations involved, indicating ongoing debate and uncertainty.

Contextual Notes

Participants reference various assumptions about the weight of water and buoyancy calculations, but these assumptions may not be universally accepted or fully resolved within the discussion.

rob.oc95
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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.
 
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rob.oc95 said:
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!)
 


rob.oc95 said:
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.
 


berkeman said:
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...
 


Naty1 said:
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.
 
  • #10


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

It's 2.2lb per KG.

:smile: Good catch!
 

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