# Moments of Inertia from spreadsheet

Discussion in 'Boat Design' started by Charly, Jun 24, 2014.

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### CharlySenior Member

Guys, this question springs from the aluminum thread in metal boatbuilding.

I am about to purchase two aluminum tubes, about 9.5 feet long for the compression tubes on my 36' cat build. The plans specify moments of 4x6, but no other specific size or wall thicknesses. It is up to me to figure out if whatever size tubes I use meets the moments criteria.

I still barely understand what moments of inertia are (is).

The designer sent me an excel spreadsheet for calculating moments with tubes. I can enter the different values, ie, wall thickness, ID, and OD. and it spits out numbers for the answers in different columns.

The column labels for the answers are:
SM=

I=

area

What I don't understand is how to interpret these numbers.

The "SM" is expressed in inches to the third power
the "I" is expressed in inches to the fourth power and the
"area" is expressed in square inches.

The plans call for compression tube moments of 4x6.
I plugged in an OD of 3.5 inches, an ID of 3 inches and a thickness of .25 inches, but I have no clue as to what the answers mean. I do understand how to get the third or fourth power of a number, but that is about it. I still don't know what the number itself means. What is "SM"? what is "I" ?

Thanks for any help

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### Richard WoodsWoods Designs

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Thanks.

so if the moments are 4x6 does that mean I multiply four times six and the inertia number needs to be more than that?

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### Richard WoodsWoods Designs

I assume since you are in the US the inertia is in in4. I don't know what that is converted to metric

Normally inertia is given for two dimensions Ixx and Iyy, basically horizontal and vertical.

So 4 would be horizontal, 6 vertical. Generally the bigger the overall tube size the stiffer it is. Your designer can tell you what wall thickness to use. I don't know what your 9.5ft long compression tube is used for, presumably not a kingpost, unless you have a very tall cabin. So I don't know if you will, for example, be riveting or bolting things to it, which might require a thicker wall than is optimum for stiffness.

However, to me it sounds like a structural item, so your designer should be telling you what to use, or at the very least give you some options. Certainly if he is a US based designer (which it sounds like from the 4 and 6) he could easily give you some metal suppliers local to you

Richard Woods of Woods Designs

www.sailingcatamarans.com

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Thanks.

So if SM means section modulus, and I means inertia, and area means area, how do I compare these numbers to "moments" which are 4x6?

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### philSweetSenior Member

You don't Charly. You find someone who can put this all into some kind of engineering context. The designer of a homebuild should not be supplying requirements in this manner. Different matter if it were to a big yard with in-house engineers. It's not as if this is a big deal to answer the basic question, but to any experienced engineer, alarm bells are going off about now. It's not the sort of question you answer inside a neat little box. The person who answers this is going to want at least a sketch of the big picture.

Having said that, you would do well to check the wiki and engineering toolbox sites on beams and columns and the terms listed - section modulus, moments of inertia, and their axes and how they relate to each other. It's college freshman engineering, so not that hard if you just do it the simple algebra way. Basic calculus helps. That way you can better understand what ever answer you get actually means.

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### NavalSArtichokeSenior Member

"4x6" is just the nominal size of the tubing. It should not be construed as a "moment" of any kind. It's like going to a lumber yard and asking to buy so many "2x4"es or "6x6"es.

BTW, a 2x4 piece of wood no longer measures 2" by 4", but something slightly less.

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Thank you Phil. your reply helps me to understand a general reluctance by professionals here to be more specific. I think part of the problem also is my assumption that everyone knew the boat and designer- a Kurt Hughes Daycharter 36 cat. Since I now use two usernames here, one for office nd one for home, that may have muddied things up a bit.

I have had a generally happy relationship with Kurt Hughes. He definitely is not lazy, even if he is sometimes difficult to understand. I bought his stock plans at a discount four years ago, and have been reluctant to pester him with too many calls or emails. I feel like I have gotten more than my money,s worth so far. Plan support must be a huge drain in the design business.

this site has been a very valuable resource and I really appreciate everyone's input.

I have forwarded a link to this exchange to Kurt. eventually I am sure I will come up with the proper size.

Please excuse the typos it's this danged ipad

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The original post said that these would be "compression" tubes. A compression member is usually a column loaded item and the engineering specification is a bit different from the specification needed for a beam. Perhaps you used the word, compression without meaning to describe the item that way.

The tubing may be intended for joining the two hulls. The size seems a bit small for a 36 footer if used for that application. There must be additional structural elements involved if the purpose is to connect the hulls.

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Hey mess about, the plans call them compression tubes. they are the connectors that go fore and aft, connecting the bow beam with the main beam. a drop down ladder is deigned to go between them down to the beach or whatever. just scaling them off the plans they look to be about 3.5 inches OD. wall thickness is not specified.
Still waiting to hear back from KH. (I told him RW called him lazy)

http://www.boatdesign.net/forums/me...tion-aluminum-paint-anodize-weld-46143-2.html

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### Richard WoodsWoods Designs

So I assume, to be fair, that you also told him I did not know your boat or your designer

Richard Woods

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I sent him the whole thread.
At the time I thought you did really know, since I have been posting regularly here about the build for the last four years.
do "moments of inertia" consist of "section modulus" and "inertia" with the number 4 (as in 4x6) corresponding to SM and the 6 corresponding to I?
thanks

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### philSweetSenior Member

Probably not, but in truth, I have no idea how to interpret your posted specs. There maybe something being lost in translation.

I too was unaware you had an ongoing build thread here.

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