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Old 26 July 2012, 15:04   #1
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How much inherent buoyancy required to keep a boat afloat...

Evening all,

How much buoyancy would be need to keep 3 tonnes afloat?
I'm not sure if I'm looking too far into this but is it straight forward in that "x" amount of buoyancy will support "x" amount of weight or do you need to take into consideration that things are lighter in water so don't need as much buoyancy to support them?
Does the buoyancy need to support the weight of the boat or the displacement of the boat?
Any help would be great.

Ta
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Old 26 July 2012, 15:10   #2
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displacement of the boat - I believe

I remember taking two bits of clay - one rolled in ball an another made in boat shape....

The ball sunk ! the boat floated both weighed the same...

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Old 26 July 2012, 15:14   #3
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I understand why the boat shaped clay floats but wasn't sure on what weight I should be using to calculate the buoyancy required?
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Old 26 July 2012, 15:15   #4
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I'm not sure if I'm misunderstanding your question but if your boat weighs 3 tonnes then it needs to displace 3 tonnes of water to float (as near as damn it that = 3 m3).
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Old 26 July 2012, 15:31   #5
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If things weigh less in water though does that mean I don't need as much buoyancy or am I missing something.
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Old 26 July 2012, 15:34   #6
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not read this page but looks like the answers are there!

http://www.duckworksmagazine.com/04/...ning/Part1.cfm

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Old 26 July 2012, 15:46   #7
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not read this page but looks like the answers are there!

If you can use a calculator you can design a boat - Part 1

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Nope couldn't see it there, interesting read though.

To put the question simply how many m3 of air will be needed to keep a 3 tonne boat afloat?
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Old 26 July 2012, 15:48   #8
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If things weigh less in water though does that mean I don't need as much buoyancy or am I missing something.
Yes. They weight less in water as much as the volume of the material the hull (and rest of
the equipment)in the boat when fully under water. So the construction material of the boat is relevant when considering how much flotation you need. A 3 ton steel boat needs much more flotation than a sandwich GRP boat.

Can be a tricky thing to calculate and a thing to consider is also do You want the hull to be fully submerged but just floating or do you want it to stay slightly above the surface....
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Old 26 July 2012, 15:53   #9
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If things weigh less in water though does that mean I don't need as much buoyancy or am I missing something.
I think you are making it more complicated than it needs to be. Things weigh less because of their 'natural buoyancy' so if you fill a 1m3 IBC full of concrete and put it in the water it displaces 1 tonne of water so weighs 1 tonne less in water than it does out the water.

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Nope couldn't see it there, interesting read though.

To put the question simply how many m3 of air will be needed to keep a 3 tonne boat afloat?
Personally I'd ignore the volume the components on the boat occupy because you want to remain above, rather than just at the water line! However if you want to think like that imagine a scrappy took your boat and crushed all the air out of it so it was just a crumpled block. What volume would that occupy. You can deduct that from the 3m3 you need to keep the boat afloat.
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Old 26 July 2012, 15:59   #10
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So how much air in m3 do I need to to keep the boat floating?
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