Scaleing

Somewhere in the distant past, I read an article about scaleing of real boats. If my foggy memory serves me correctly it stated something to the along the lines of, "that if you doubled the length, you should increase the beam by 40% or so and increase the shear height around 15%".

If someone out there with a better memory than mine could point me to the article I would definitely be in your debt.

If not the article then correct me on the percentages.

Cheers
JJ - Tex Gulf Coast

 

JJ,

Reducing the scale of something means applying the same reduction to all dimensions equally. That is the only way I know of. Doing as you state will give you a smaller boat, but it will have a different shape than the original.

You may remember a formula for lengthening-enlarging a boat. I do not know if they consider that to be "scaling".

Take care.

TGoz

 

In Principles of Yacht Design, Larsson & Eliasson mention a work done by H.M. Barkla of the University of St. Andrews, Scotland.
There are a number of primary and secondary relations. If You scale the length by a factor L, then beam, depth and freeboard scales by L^0.7.

 

Several scaling formulas have been devised over the ages. Some work with small percentage changes, but none do (very well) if the original is much larger or smaller then the "scaled" new model.

This likely means you have a design you want, but it's not the correct size. Enlargements or reductions up to 15% (max) of overall length can be a simple matter of re-spacing the stations to get the length you desire. This is more common then most realize. Everything else remains the same, but for the station spacing.

Once the differences add up, to more then what station spacing can accommodate, you have to develop a new design. This new design can be heavily based on the original, but it will have new a new beam/length ratio plus all new dimensions for everything else.

Scaling the principle lines (sheer, chine, keel, etc.), proportionally or other wise is all well and good, but it doesn't address the scantlings, which will change also (a boat is little more then the sum of it's scantlings). In other words, building a 15' boat as a 20' using the scantlings of the 15'er will produce a fragile, possibly dangerous craft or building a 20'er as a 15'er with scantlings from the 20'er will generate a heavy vessel.

Is there a yacht you have in mind? What changes do you envision?

 

Thanks for the quick reply. Scaling can be up or down. For example "increase by 15%" and "115%" are the same thing, both give a greater dimension. While decrease by 15% will take an original dimension of 1 meter and return .85 meter. Sorry I was rather vague.
If I were to take the dimensions of a 10m X 3M X 1m boat and increase the length to 20m, I think that doubleing the beam to 6m would make the boat much to wide, then same can be said for the draft, hull depth, or shear height. That is why I was asking about the scaleing.

Actually scaling is just a quick way to change the designers scantlings with out getting to far outside of the design hydrostatics by an overly large amount.

The particular boat I have in mind is just a little to small for my use. But I like its design and have not been able to find a larger version by any designer. I have the contruction drawings from the original designer but until I have his approval I will not identfy him or his design.

 

JJ,

Thanks for the clarification. I was thinking in a purely mathimatical way.

If the members here cannot give you all the info you need they can certainly point you in the right direction.

Welcome to the forum.

Take care.

TGoz

 

I've got a question I've been wondering for a long while. How do you scale weight. Is there a formula? See I'm into modeling boat and weight doesnt scale.

For example I've got a jet boat that is a 1/12 scale Chaparral Villain IV The model is 30" long, weighs about 4lbs. That would mean the real boat would weigh 48lbs . Oh and it goes a mind blowing 300mph, but looks like it's going about 60-70mph scale, actually doing 25mph. Maybe I'm thinking too hard.

 

Weight=Volume=Displacement scales as the cube of your factor.
If you scale your boat with a factor 12, you scale in 3 directions,
12^3 = 12 x 12 x 12 = 1728.
4lbs x 12^3 = 6912lbs

 

Where is the water density?

The volume model x scale^3 x water density...= volume of boat in water

fresh water density = 1
salt water density = 1.026

 

Yes, but Quicksilver has a model boat scale 1:12 weighing 4 lbs, then a full scale boat should weigh 4 lbs x 12^3 = 6912 lbs to float in a simmilar way in the same water

You suppose the model boat is in a lake while the full scale boat is in the sea?
So the full scale boat should weigh 6912 lbs / 1.026 = 6739 lbs?
Maybe the model boat is in a salt lake in Utah?

 

very interesting, thank you, does that make sense for a boat that size. here's an earlier pic, pre paint with the original cover. 7000lbs seems plausible I guess

p.s. it is fresh water

 

It depends on the purpose of the scaling. If you are doing a model test, then all physical dimensions scale by the same constant. What is the purpose of the scale?

 

Raggi,

it is not correct to divide lbs by 1.026....1.026 is the specific weight of salt water but in metric units....

In fresh water no problem.. scale^3

 

Hi JJ
Here is the original thread you were looking for - http://www.boatdesign.net/forums/showthread.php?t=15226
I was the only one to reply, but I still stand by the advice in para 2 - "Try and get hold of the work of H M Barkla of the University of Saint Andrews, Scotland who has done calculations in this area. This info is also in chapter 2 of the book Principles of Yacht Design by Lars Larrson and Rolf R Eliasson – pretty much a standard among small boat designers."

 

Ranchi(?), This is maybe off topic but if the model is in water with density 1000kg/m3 and the full scale boat is in water with density 1026kg/m3, how would you scale? It doesnt matters what the weight of the model is, or if its in pounds or ounces. If the full scale boat is floating in heavier water it will float higher, now I see that... so we should take the weight of the model, multiply with the scale^3 and then multiply(!) with 1.026...
btw, I learned that fresh water is 997kg/m3