# ABS Section Modulus Units

Discussion in 'Boat Design' started by Mitch1990, Jun 3, 2020.

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1. Joined: Feb 2020
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### Mitch1990Junior Member

Hi all,

This is real quick and more of a confirmation, the units for section modulus in the ABS are m-cm2. Based on a moment being shown as N-m in the US, I have assumed that m-cm2 is m.cm2 and therefore to convert my section modulus which is in m3 to m-cm2 I need to divide by 10,000? For interest, why is it shown as m-cm2 instead of m3?

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correct.

No.

m3 = m x m x m
m.cm2 = m x cm x cm

100cm = m

m.cm2 = m x 100(cm) x 100(cm) = x 10,000

The size of the numbers and to make it feel like a real number.

If you have a section modulus of a say tanker... and use cm's only... the numbers will be huge.
If you use m's only.. the numbers will be very manageable, but seem too abstract.

Thus using m.cm2 yields numbers that are 'larger' and you can get a feel for... it also comes from the days before excel spreadsheets.
Doing midships section modulus calc's in just cm's... takes up too much paper!!

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### Mitch1990Junior Member

I messed that up, it was definitely x 10,000. Thanks for the insight, I find ABS the most difficult of the class societies to deal with. That is not saying much as I haven't used any of them professionally.

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### jehardimanSenior Member

Yes, and ease of calculation...longitudinals and plates that make up the section only have areas in sq cm's......but are meters apart. So leaving the section area in sq cm and the lever in meters makes the actual calculation much simpler in the old spreadsheet days. Same with in^2-feet.

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### Mitch1990Junior Member

0.000005 m^4 isn't overly straight forward to deal. On the subject of ABS, is there a diagram that shows their definition of structural components?

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I did far too many calc's like that on paper.... thank heavens for excel!!!

What do YOU mean by - structural components?

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### Mitch1990Junior Member

Everything in part 3 Chapter 2 for steel ships under 90m in length.

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I think you need to buy a book on ship terminology then.
As there is nothing odd at all... the rules, any rules, use terms which are common to all.

Thus if you're finding it difficult to understand what is that... go buy a book on ship terminology.... that was the first advice I got when wet behind the ears...

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### Mitch1990Junior Member

Ok, thanks. I will continue with the books

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### Mitch1990Junior Member

Actually, can you recommend one? the books I have are very much broad overviews of marine engineering and naval architecture. As an example they do not define a longitudinal side frame

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There are many, even good free ones such as this:

https://www.wartsila.com/docs/defau...cyclopedia/wartsila-o-marine-encyclopedia.pdf

Well technically that is a misnomer.
Because frames run transversely - athwartships.
A "frame" as a member that runs longitudinally - is a girder, not a frame!

What does that book say:

same!

So perhaps tailor your search to structural books.