# Scale

Discussion in 'Boat Design' started by chandler, Dec 9, 2006.

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### chandlerSenior Member

I have a question for the pencil and paper designers.
When creating offsets how do you come up with fractions of an inch when using say 1/2 inch scale?

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### Wynand NRetired Steelboatbuilder

why do you think so many countries uses the metric system:?: Simplicity at it's best and no fractions, only decimals

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### chandlerSenior Member

Ok Wynand
so how do read millimeters from a 1/2 centimeter scale??

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### Eric SponbergSenior Member

Chandler,

You have to buy an architect's scale (a triangular section one is best) which usually has divisions from 3" = 1' down to 3/32" = 1'.

An engineer's scale divides feet into 1/10th divisions, or multiples/fractions thereof, and generally goes 1/10 to 1/60 or 1/80.

A metric scale also divides meters in similar divisions to an engineer's scale from 1/10 down to 1/100.

Look in any artist/architect supply store, or you can look in the Charette catalog (online???) to see what would suit you.

Eric

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### PoidaSenior Member

G'day Wynand, it beats me why anybody would want to use inches, we had them at school bloody headache.

Anyway I reckon metric boats float better than the imperial ones.

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### boltonprofilesSenior Member

By just a fraction....................

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### marshmatSenior Member

I see I'm not the only one with an SI preference.... now and then you come across an old (or in the case of Americans, sometimes new) steam plant or something, and it's all in imperial units... so many correction factors everywhere to fix the unit inconsistencies, etc... the same thing in metric often takes less than half the time to solve....
Chandler, it's well worth it to invest in the right type of scale rule for the plans you're working from..... the scale is about \$9, and you'll likely make at least \$500 of errors if you use the wrong one

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### chandlerSenior Member

Thanks guys
You're missing the point.
I have scales up the yin yang.
Someone please tell me how with a 1/2" scale you can determine 1/8ths of an inch in a table of offsets?
A 1/2' scale is divided into 24 parts in it's smallest portion. That gives at best a mark for each 1/2", I can see half way between 2 marks, that gives accuracy to 1/4", where do you come up with 1/8ths?
From the lofting? Sure, but how many boats are designed and never built, therefore never lofted? Most of them I'd bet.
Eric, is there a trick I'm missing?
Also Eric what scale would you draw a 34' sailboat on 24x36 paper?

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### marshmatSenior Member

My 1/2" to 1' scale has markings for 1/2", but not 1/4" or 1/8". You can interpolate to the 1/4", but it doesn't seem feasible to go to 1/8" precision on a drawing done at 1/2" to 1' scale, that would be thinner than the thinnest lines on the drawing. If offsets are done to eighths, they might have been done from a bigger drawing or with mathematical interpolation methods. Not until I go to 1":1' do I see markings on the scale rule for 1/4", which would make interpolating to 1/8" reliable.

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### longliner45Senior Member

ok im out of my comfort zone on this one but I think this might help,,,1 inch contains 1000,,,,,,,, 1/2 inch = .500,,,,,thousands,,,,,,1/4 inch = .250,,,thousands1/8 inch = .125 thousands this is machine shop lingo ,,,hope it does some good,,,,longliner

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### jehardimanSenior Member

Yes, you are missing a trick. First, use a very sharp pencil. Second, when laying out the prime dimensions, measure from the edge of the CL and BL and use the divider points to prick them in the paper. Now draw your lines, ensuring that the lines just catch the prick to the inside. Now when you measure, measure from inside to inside you should be able to "read" to 1/8" +/- 1/16". But remember that at this scale, the drafting spline and/or lofting batten is more likely to be off than your measurement. I've done 60' shapes at 1/2"=1' with offsets to 1/16" (i.e. eights with a + or -) and the largest error was 3/8" where the body was rapidly tailing off. Even using CAD offsets to 4 decimal places, expect 1-3 eights errors when you try to run a batten through them depending on the b-spline method used by the program.

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### tom28571Senior Member

You can't but there are ways to get those fractions on paper. You draw length and beam in different scales. By compressing length scale, expanding beam scale and drawing only half breadths you can easily read 1/8" offset variation on 11" by 17" paper, even for a 30' boat. This is one method of lofting without doing it full scale. Unfair lines jump right out at you.

You don't have to draw the whole boat, only the line or lines you are interested in.

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### longliner45Senior Member

you need to contact a real mathamatition ,,,,may I suggest dr holyoak of yellow springs ohio.I will see if it is possible

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### Eric SponbergSenior Member

jehardiman and Tom Lathrop answered your questions about reading a scale. The fore-shortening method by Tom is used frequently and produces fair lines.

As to scale, I like working at D-size, either by hand or by computer, and for a 34' boat, I would likely draw it at 3/4" to the foot. That makes 34' = 25.5" long on the paper. This gives you room for a body plan at one end of the paper. On my Moloka'i motoryachts, I have to draw them at 1/2" or 3/8" to the foot to get them on D-size paper.

Eric

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### longliner45Senior Member

sorry gentlmen ,,I re read my last post and it looks a little wrong ,,I did not mean to take anything from anyone buy using the word (real mathamatition) please dont conscrew this into anything ,,just trying to be helpfull ,,thankyou ,,longliner

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