Scaling displacement... does it cube?

[soundman]

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.

cheers

[Boatmik]

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

[soundman]

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.

cheers

[Boatmik]

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