# Weight capacity of new design

Discussion in 'Boat Design' started by Ron Skelly, Feb 25, 2014.

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### Ron SkellyRonS

I am building a 1/10th size model of the small boat I have designed. I am thinking there must be a principal or formulae involved here that I am not aware of. I have calculated, using the water displacement method, the size of the pontoons required and calculated the full scale finished boat will easily carry a load capacity of over 1000 pounds. However the 1/10th size model will not handle 1/10th if this weight - no where near it.
Question - is there a phenomena that applies that as the boat gets bigger it can handle more weight? ... Can I test the weight capacity using the model?
Ron S

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### gonzoSenior Member

The load carrying capacity will increase to the cube. That is a 10 times larger model will carry 1000 times more because it is 10 times longer, 10 times wider and 10 times taller.

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### Ron SkellyRonS

Thank you very much. Is there a principal involved here that I can read about?

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### lewisboatsObsessed Member

Just mathematics. Volume is length x width x height. Your model pontoons only displace 1/1000th the amount of the full sized design because you reduced all dimensions to 1/10th the original size or 1/10th the length x 1/10th the width x 1/10th the draft or height. 1x1x1/10x10x10 = 1/1000. So your full sized boat will displace 1000 times more than the model. Structural considerations between the two are another story. You will have more than 10 times the weight in structure to support the large one as opposed to the small one.

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### Ron SkellyRonS

Thank you Steve for this explanation.

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Ron you can build to any scale you like and calculate the scale displacement quite easily wuth your handheld calculator.

Take the scale factor which in your OP is one tenth (1/10) Now raise the number ten to the third power. That is ten times ten times ten. You will get one thousand. Use that number to determine the weight that your model must be to simulate the weight of the full sized boat. Let's say that your full sized boat is to weigh 1500 pounds including passengers and everything else. Divide that number by the cube of the scale factor ....1500/1000 = 1.5 pounds for the model.

You can see right away that a small scale model is not the best way to go. The larger the model the more likely that you will be able to hit the target weight. Not only that but you will be able to build the larger model with a higher degree of accuracy. For example if you used a scale of two inches to the foot that would be one sixth scale. Six cubed is 216. DIvide the weight of the full sized boat (1500) by 216 to get 6.9 pounds.

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### Ron SkellyRonS

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### PARYacht Designer/Builder

Is this a model of your trimaran?

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### Ron SkellyRonS

Yes it is. Small craft for slow cruising.

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### Ron SkellyRonS

1/10 th scale so that I can try and forsee some problems ahead of time.

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### rwatsonSenior Member

I built a 1:5 scale model because I read that this is the smallest you can go if you expect any serious data about performance.

To be honest, I should have gone even bigger, not just because it was more realistic, but because fiddling with stuff at such small sizes is awkward.

With a trimaran , having separate hulls to make transport easier - you could almost benefit from a 1:3 scale, so you can get your kids to sit inside and sail it, and save the cost of expensive radio controls, batteries etc.

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### PARYacht Designer/Builder

A 15" long model of your little tri-toon will not provide much useful information. You'll want to work to a considerably larger scale (as previously mentioned) to get anything meaningful. First I don't think a tri-toon is particularly efficient in the size you want, just extra wave making and drag, where a cat will do just as well and there's one less hull to build. In this vain, a monohull, given your speed desires, would simplify things further. Building a 'glass sheathed foam toon will be more costly and require more effort than a plywood toon. Lastly a round toon is about the least efficient shape to employ.

Maybe a better understanding of your SOR will help drive the focus a little better.

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### rwatsonSenior Member

Oh yes - and a big PS.

Getting a proper scale weight for small models is really hard.

Even very thin plywood for 1:10 scale will make the hull triple the desired scale weight.

I had to go for 4 mm foam, with very light f'glass on both side. Even then it was a battle at 1:5 to come close to scale weight.

For example, fire up Excel and type 800 ( kilos ) in cell A1

For 1:10 scale, in cell B1 type =A1/(10*10*10). It should equal .08 ( Kilos) . So you model has to come in at 80 grams to correspond to the 800 Kilo full size weight.

For 1:5 scale, at cell C1 type =A1/(5*5*5). It will equal 2 ( Kilos ). Thats a whole lot easier weight target to meet. You can build a lot stronger hull that way, so it wont be fragile for carting around.

The problem gets even worse for scale mast weights and rigging etc. Dont forget, you may need to have batteries and RC gear in the hull for control, so suddenly you lose 600 grams 'off the top'

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### Ron SkellyRonS

Thank you for this. I am going to do some serious calculations.

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### Ron SkellyRonS

Mr. Watson,

Are you saying that in a 1:10 scale model, the finished weight of the full size boat will be 100 times the weight of the model assuming everything is accounted for and to scale?

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