Scale down weight to test a model

Cubing LENGTH to scale WEIGHT.

Cubing Model Weight to get Full Size Weight is what you would have meant I am sure - as length and weight are not related.

 

I meant what I wrote. Length and weight are related for boats which are geometrically similar and float on the same relative waterline.

"Cubing Model Weight to get Full Size Weight" The numerical value of the model weight is not cubed to get the full size weight.

The weight is proportional to the length cubed. If the model length is 1/10 of the full size length, then the model weight is 1/1000 of the full size weight.
So a 1/10 scale model should weigh 1.2 lbs if the full size boat weighs 1200 lbs.

 

Sorry but they are.

This is the basics of geosim for a hull and data. Any naval architect can tell you that. Which thus begs the obvious question...

DC tells it correctly:

 

Perhaps you could provide a bit of a case study to illustrate the point for me.

Unless you are talking about displacement of water, where the volume of the scale model and the full size is uniform between the two, I am not understanding the concept.

 

What is the "obvious question" ?

 

Another try:

For geometrically similar shapes the volume of the shape scales as the length of the shape. If the length of shape A is 1/10 of the length of geometrically similar shape B then the volume of shape A will be 1/1000 the volume of shape B.

A 1/10 scale model floating on the same relative waterline as the full size vessel will displace a volume of water equal to 1/1000 of the volume of water displaced by the full size vessel.


The weight of a floating vessel at rest equals the weight of the displaced water. The weight of the displaced water equals the density of the water multiplied by the volume of the displaced water.


If the density of the water is constant then the weight of the displaced water of the model and thus the weight of the model will scale as the length of the model cubed. A model which is 1/10 the length of the full size vessel will have a weight of 1/1000 of the full size vessel.

 

Getting back to the boat of the OP, he'd be better to display a drawing of it to get some commentary on the likely utility of it, rather than make a scale model, which seems pointless in this case.

 

This is self evident. Yet you are answering a question you now admit you don't understand. It does not help the OP. one bit.

Ok...just like the Mr. Numbers of old (Rick W) who could not grasp simple concepts and left in a huff.

Here are some simple numbers for you.

Lets say you have a boat that is L = 10m (that is Lwl) and say has a displacement of 10 tonne.

Ok so far...??

Now, let's say we want to increase the size of the boat to an Lwl of say 20m. But, and here is the important point, the new boat at 20m length is a perfect scale of the original 10m boat. Thus all the dimensions scale perfectly in all 3 directions. So....what will the weight be?

The weight is related to its scale.

f = (new/original)^3

So the f = (20/10)^3 or simply 2^3 = 8.

The new weight is old weight x f = 10 tonne x 8 = 80 tonne.

If the hull is now scaled down from 10m to say 1m what will the weight be?

Again it is weight x f = weight x (1/10)^3 = 10 x 1/1000 = 0.010tonne or 10kg.

That is it, simple.

This says NOTHING..and i repeat NOTHING about anything else other than an exact scale of the original to another scale. If you wish to talk about different waterlines..or temperature of water etc...all red-herrings and nothing repeat NOTHING to do with the simple geosim, or exact scaling, of one hull to another.

 

Even I understood that, unlike Rick W, apparently. Keep up the good work !

 

Sigh - its amazing how you misread a persons answer and take great delight in berating the object of your disdain.

I imagine you have a group of a little heads on your Pc as you 'shoot down' person after person.

There is nothing wrong with any of the explanations or formulae I have given.

yes it is. I have no trouble following that logic, nor the reasoning behind it with that example. The only thing that confuses me is that it has nothing to do with the OP question.

He wanted to know how to calculate the weight in the model based on the planned weight of the full size boat.

That is simply what I said.

If you can tell me what the error is, I would be pleased to discuss it. And it DOES help the OP because it is the solution to the problem.

A totally unnecessary statement. I don't know why you fling in these silly non pertinent points. Where have I said anything about waterlines, temperature etc. ???????

You have obviously got serious attitude problems on occasions, which has been commented on before, and does nothing to help meaningful discussion.

 

Don't take it to heart, rwatson, I know I don't ! Couldn't find any sign of "Rick W" and his alleged flounce, using the search function !

 

It appears this thread has gone off track a bit and is becoming more adversarial rather than aiming to help the original poster at this point; therefore it's probably best to close this thread rather than have it turn personal. Hgraham -- if your wish for additional clarification on your question regarding scaling weight for a test model or to extend upon your question, please start a new thread. Thanks.