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20th February 2008, 10:43 PM #1
Scaling displacement... does it cube?
I currently building a scale model of a dodgy old pirogue I plan to build.
It occurs to me that the displacement therefor the load capacity will have a cube relationship with size.....it this a direct or an indirect relationship.
what I am getting at is can I model this sleek craft, test its displacement and directly calculate the displacement of the full scale craft?
Don't worry I wouldnt be trying this with any sort of serious design......this is a simple 3 slab pirogue.... it aint rocket science.
cheersAny thing with sharp teeth eats meat.
Most powertools have sharp teeth.
People are made of meat.
Abrasives can be just as dangerous as a blade.....and 10 times more painfull.
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21st February 2008, 12:38 AM #2
Surface areas vary with the square
Displacement and weights of items vary with the cube
Stability varies with the fourth power.
Double size
4 x surface area
8 x displacement
16 x stability
Half size
1/4 surface areas
1/8 x displacement
1/16 x stability
Building a model at 1/6 true size it starts to get pretty screwy
1/36 surface areas
1/216 x displacement
1/1296 x stability.
And don't start talking about resistance - you will start pulling your hair out!
MIK
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21st February 2008, 12:56 PM #3
I sat down and had a quiet contemplation late last night.
surely the whole thing would have to get very vaguely approximate....its not like a boat ( even one made of 3 slabs) is a regular shape.
just to make things difficult I'm working a 1/3 scale.
if it was a cube or even if it had straight sides it would be reasonably easy.
Oh well at least a model will give me some idea of poportion, angles and stuff.
for the fabric would I bebetter off with darnel or finishing cloth.....that needs to scale to about 70GSM.
cheersAny thing with sharp teeth eats meat.
Most powertools have sharp teeth.
People are made of meat.
Abrasives can be just as dangerous as a blade.....and 10 times more painfull.
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21st February 2008, 09:24 PM #4
If making a sailing model you need to consider the stability for the reasons I have run through above.
So I would be suggesting quite a deep keel with some lead on the bottom.
But only for a sailing model.
For example a racing yacht 10m long might have a keel 2 metres deep.
But a radio control model of the same approximate proportions 1m long might have a keel 300 or more deep.
If it is a coffee table model or a motor boat model it is not so critical - but for a sailing model you will need more weight and/or depth than expected.
Michael Storer
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