# Scaled model how to calculate weight

Discussion in 'Multihulls' started by Forecaddie, Oct 29, 2023.

1. Joined: Apr 2023
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Want to build scale model for multihull design. What is typical scale? 1:20? How do you scale weight? I have 37kg existing hull and cat hulls.
Scale- 1:20? 1:50? 1:100?

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### waterbearSenior Member

Weight goes up with the cube of the length. So if your model is 1:20 scale, just do ((1÷20)^3) x (weight of full size boat) to get the weight of the model. If the full size boat weighs 10000lbs, then the 1:20 scale model should weigh (0.05 ^3)* 10000 = 1.5 lbs.

I like to build in balsa, which means convenient sizes are 1-3ft. If it's too small its hard to get the details right. Don't worry about the scale except when calculating weight.

Last edited: Oct 30, 2023
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### gonzoSenior Member

You would have to scale the cores, laminates, cabinets, etc. It is not a good way to calculate weights. Simply calculate the surface area of each item and multiply it for the weight of the laminate, plywood, upholstery, etc.

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### rwatsonSenior Member

Question - Why do you mention other Hulls "you already have" ??

After extensive research for my own project, I recommend at least a ratio of 1:5, if you expect it to display any hydrodynamic potential.
Even at that scale, it is hard to make the model weight fit within the scale weight.
I found I had to use 3mm foam with 6oz fibreglass skins, which made the model weight just within scope.

I have attached a screen shot of the Weight scaling sheet I made. The Excel formulae is in Red

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### luckystrikePower Kraut

the question is ... for what do you want to use your scale model. Just for looking around the hull or do you want to let it swim and sail. Iam a amateur designer and I cannot build every crazy idea that finds it's way from my sick brain into my computer. So, sometimes I build a sailing scale model with RC. I normally choose a scale with results in a model around 1.00m to 1.30m long.
Sail Areas are a function from scaling the spars. To calculate, let us take a scale of 1:5, you calculate 5 x 5 = 25. If you have 17m² sailarea, your model will have 0,68m². 17m² / 25 = 0,68.
Wheight: you calculate 5 x 5 x5 = 125. If you displ. is 450kg your model, ready to sail (incl. payload), will weigh 3,6kg. 450kg / 125 = 3,6kg. Your hull wheight of 37kg will have only a minor effect of the model. What counts is the all up wheight of the boat with crew and payload.

In a monohull your model will have less righting moment than the original. To get comparable stability you have to add depth, a deeper keel. Water resistance is also greater in a model because the dense of water is always the same. imagine as if your model is sailing through oil. The waterstream around the boat, the wavemaking and the balance is as in the original.

In a multihull the resitence effect is the same as for a monohull, so pitchpoling will be more likely than original. Meaning ... if your model sails with the bow up the original will do it better!

Have Fun, Michel

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### rwatsonSenior Member

I was told the water on a model is like maple syrup. Oil is lighter than water, after all.

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### gonzoSenior Member

It is not the density but the viscosity increase that affects the behavior.

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### rwatsonSenior Member

But also, Density controls Viscosity.

"In liquids, viscosity increases with increasing density. The kinematic viscosity equation can explain the relation between viscosity and density. Kinematic viscosity is defined by the ratio of dynamic viscosity and fluid density."

Viscosity Tables

If everyone wore a breathing unit, you could run your tank models in Carbon Disulfide

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### gonzoSenior Member

That is not correct. Gear oil has greater viscosity than water, but lesser density. Kinematic viscosity is a ratio; totally different animal.

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### rwatsonSenior Member

It IS correct, and can be found in any engineering exercise.

Don't confuse "Greater than " comparisons to the effect that Density has on Viscosity In this diagram, it shows the relationship. Its not linear, but it is Relative
Like the last line says, if you heat a substance to reduce its Density, the Viscosity also reduces.

As you can also see, Kinematic Viscosity and Viscosity are not a "totally different animal". They are closely related, and BOTH dependant on Density.

And, as that graph shows, Kinematic and Dynamic viscosity are just two ways of measuring the same phenomena.

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### gonzoSenior Member

There is no significant temperature difference between the water a full size boat and a model operate in. I agree that ice is more viscous than water. However, it is irrelvant for this discussion. Kinematic viscosity is a normalized value.

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