How much inherent buoyancy required to keep a boat afloat...

[chewy]

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

[SPR]

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...

S.

[chewy]

I understand why the boat shaped clay floats but wasn't sure on what weight I should be using to calculate the buoyancy required?

[Poly]

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).

[chewy]

If things weigh less in water though does that mean I don't need as much buoyancy or am I missing something.

[SPR]

not read this page but looks like the answers are there!

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

s.

[chewy]

Originally Posted by SPR

not read this page but looks like the answers are there!

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

s.

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?

[C-NUMB]

Originally Posted by chewy

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....

[Poly]

Originally Posted by chewy

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.

Originally Posted by chewy

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.

[chewy]

So how much air in m3 do I need to to keep the boat floating?

[chewy]

Found this on another forum:

A Guide to Buoyancy in Trailer Boats

[EnglishLes]

According to the famous Greek - Archimedes - with his Eureka moment, things float when the weight of water they displace is greater than the weight of the object. This is called positive bouyancy. When the weight of the water displaced equals the weight of the object the object will stay at the same position in the water, this is neutral bouyancy Divers use a BCD to create neutral bouyancy so that they stay at the same level in the water. Negative bouyancy is when the weight of the object exceeds the weight of water displaced and means the object will sink.

If the 3 ton boat is on the bottom then 3 tons of lift (3 cubic meters of air) will be required to counteract the weight of the boat but it will still sit on the bottom. So 3+ tons will be required to lift it. If it is on the top then 3 tons will stop it sinking but only just.

[Boatnomad]

Originally Posted by C-NUMB

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.

Big difference of actual-weight down there!

[m chappelow]

Why what are you trying to lift Phil lol
Rough idea when we removed the clubs old concrete slipway using tidal lifts down at the Gare in 1 ton slabs it took 8 empty plastic 55 gallon drums to float them about or to get them in neutral bouyancy
would have took less but we contained/ captivated the drums together in a steel framework and the lifting chains and shackles weighed a fair amount.
Think on Average in seawater a 55 gallon plastic oil drum will support around 250 kgs.
You have to remember that the drums will be just under the water to get full neutral bouyancy .
A cubic metre of set concrete weighs 2483 kgs,
A cubic metre of lead weighs 11342 kgs
A cubic metre of seawater. weighs 1024 kgs
A cubic metre of fresh water Weighs. 999.9 kgs
Mother inlaw A lot .

[geir]

Assuming the density of the hull ++ is approx 1750 kg/m3 and the engine is 500 kg, the volume of the hull++ would be 2500/1750 ~ 1,4 m3. This gives it a negative bouyancy of 1,6T when flooded, so 1,6 m3 of flotation should keep it (barely) afloat. Due to lack of information I have disregarded the influence of engine and fueltanks and the fact that seawater is denser than freshwater etc.

[C-NUMB]

Originally Posted by Boatnomad

Big difference of actual-weight down there!

No, the weight of a 3 ton steel boat weights in air exactly as much as a 3 ton sandwich GRP boat. But for flotation You need much less volume for the sandwich GRP one....if any.

So for an accurate reply, one needs more specific details regarding the construction/ equipment of the boat

geir's method of calculation is looking good.

[chewy]

Morning Mart,

Not trying to lift anything, I have a Tremlett 23 which is been "restored" and want to fit some buoyancy so worse case scenario it still floats.
From what I can gather we need 3m3 of buoyancy to keep it afloat and a bit more to keep well afloat.
Looks like we're going down the closed cell foam route which can be poured in....

[tonto]

Originally Posted by C-NUMB

No, the weight of a 3 ton steel boat weights in air exactly as much as a 3 ton sandwich GRP boat. But for flotation You need much less volume for the sandwich GRP one....if any.

So for an accurate reply, one needs more specific details regarding the construction/ equipment of the boat

geir's method of calculation is looking good.

Nope!

Mass does not vary, so the mass of a 3 tonne boat is the same wherever it is, but the weight is different. Weight is affected by the environment it is in, Mass is not.

For example somethings, such as gasses are measured as weght in Air, or weight in vacuum, the former taking into account that the gas displaces a certain volume of air, and therefore has less weight.

Technically a boat that floats has no weight in water.

to find your true amount of bouyancy to float an object you would need to sink your boat in a test tank, to determine it's volume, i.e. all the construction material, and fittings, paint, anchors etc etc. all the tanks/void spaces etc would need to be flooded as well.

that volume would be it's basic displacement, and for arguments sake could be 1.0 cubic meter.

If the boats Mass is 3.0Mt, then you would need 3.0 /1.025 of bouyancy to make it float, or 2.92m/3 of bouyancy.

If the basic displacement was 1.0 M/3 you would then need to increase the basic displacement by 1.92 M/3 by adding closed cell foam.

However the closed cell foam also has Mass, so the mass of that has to be added to the boat.

The bottom line is that 3.0 of reserve bouyancy will keep your boat afloat, with some to spare, but depending where you put it may not stop it capsizing, or floating with the bow pointing vertically.

That is a whole different ball game!

[wavelength]

Without mixing imperial/metric units( or involving US gallons )
a pint of water (20 fl oz) weighs a pound and a quarter
therefore a gallon (8 pints) weighs ten pounds
Displace 1 gallon of water by sinking a gallon container and the upthrust is the weight of water displaced (ten pounds)
Sink a 50 gallon drum ( or indeed fill it with air whilst diving as a lifting bag) and the upthrust is 500 pounds.

[SPR]

be care full with foam - I known people to do this in sailing dinghy's - never ends up well - ends up with heavy boat and handles crap.

1 litre of water = 1kg


S.