# ABS guide to cross-deck structure of catamarans

Discussion in 'Class Societies' started by Alik, Nov 4, 2023.

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1. Joined: Jul 2003
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### AlikSenior Member

Came across ABS guide and looking to apply it to power catamarans in aluminum, as the way to save time instead of FEA.

Question: what is Ii value here? Is it defined around x, y or polar coordinates?
To me seems like it should be defied around Y, at x=0?

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### TANSLSenior Member

I understand that the effect of the "Fvertical" of the figure is being studied. According to this, it would be the bending moment with respect to the X axis, in the longitudinal direction of the hull. Therefore, in a section of each transverse beam along a longitudinal plane, it should be used the moment of inertia of said section with respect to the X axis that passes through its neutral axis.

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### AlikSenior Member

Thanks TANSL, actually there is table D.1 in ISO12215-7. Similar method.
The ABS is using same approach, but neglecting GJ rotational stiffness.

As a background: we try to walk away from FEA for simple alu designs. We know that the only purpose for global strength calc is to satisfy the Class. So if we take some simplifications it will work.

And the boat in question is 19m alu passenger cat.

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### TANSLSenior Member

A beautiful boat. Congratulations. It reminds me of one of my designs.
Although the ISO has many drawbacks, I am convinced that for small boats it has its advantages. Whenever I can, I use ISO 12215.
I already know that there are people who, without knowing it, criticize it, but I prefer to take into account the criticism of those who do know it for having handled it, as they help me understand it better. The others do not contribute anything positive.
Cheers.

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It is just referring to the member (or members) at the location in question, on the x-axis, taken from the FP.
So if you had just 2 box beams, one at the FP and the other at the AP, the Ii would be the inertia of the box at the FP and/or AP.

It is rather similar to the LR simple calculation too, where each member is identified as contributing:

After doing endless FEAs over 30 years on raft structures, I also came up with a quicker simpler method, on smaller cats like these.

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### AlikSenior Member

Thanks, what part of LR is that?

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