# Scale Model vs. Full-size

Discussion in 'Boatbuilding' started by AstroTux, Jul 20, 2006.

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### AstroTuxJunior Member

Hi,

If I were to build a scale model of a not-yet existing full-size design that I wanted to build, would the model be sufficiently representitive of the full-size in such respects as waterline etc..? I can see such things as density of material used in the model, if different to that used on the full-size, would in itself produce different results.

I was also considering scaling such things as weight to match that of the model (e.g. if I'm using a 1:10th scale model, and a full-size engine weighed 1 ton, then a weight of 100g would be required in the model). Would this be representitive enough to test a design??

Any thoughts on this?

Best regards,
Robin.

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### marshmatSenior Member

The process of scaling is governed by what is known as "dimensional analysis". Basically, the idea is that things like length, area, speed, etc. do not matter (as long as the shape is the same)- what matters is that key combinations of these things match up. For instance, the Reynolds number (density*speed/viscosity)*(some characteristic length) relates inertia and viscosity, thus governing how turbulent the flow is. The Froude number (speed^2 / (grav.acceleration * length) ), very important for boats, relates inertia and gravity, and governs wave-making. If you're intending to get valid data on how a boat moves, using a model, you need to match the appropriate dimensionless groups.

If you're just looking for static how-it-floats data, the best way is to use the built-in hydrostatic capabilities of something like FreeShip, to find the key points of the hull shape (centres of buoyancy, etc). Then you can make a spreadsheet or database listing all the equipment, its weight and location, and calculate the centre of gravity from that (the method's not very complicated). Building a model looks cool but the computer method is a lot more realistic.
If you do build a model: Areas scale with the square of length, volumes and weights scale with the cube. Ie. a 1000 kg prototype at 1:10 scale would yield a 1 kg model. Usually models are built light, and then ballasted so the centre of gravity and the inertia tensor scale correctly to the prototype.

Model testing is a very well developed science. Almost any general fluids textbook will cover the basics, there are many more detailed papers available if you're still interested.

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### AstroTuxJunior Member

Hi,

Looks like I have some studying to do for sure. Sounds like the spreadsheet method is more what I'm after. As I'm not going to be designing/building ships for a living, realistically, do I need to know every last detail of the boat?? I know that sounds like I'm avoiding study, but I'm not; I know from personal experience that I can get stuck in the technical detail and not get on with what I'm trying to do!

I found n interesting problem with FreeShip. I'll see if I can find the link to my post on it. FreeShip thinks my hull isn't submerged.

I think I have a hull design I'm happy with so far, but as I mentioned in a previous thread, I need to know more about some of the technical details of the hull so I can refine it. From what I've been reading, a semi-displacement hull is most stable, and can attain speeds in the region of 25kts.

If you would be happy in taking a look over the design, I'll post the file for you to look at.

Best regards,
Robin.

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