# ISO 12215-5, Area pressure reduction factor, why Panel Area in denominator?

Discussion in 'Class Societies' started by Mariotti, Apr 2, 2021.

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### MariottiJunior Member

Hello, I'm new to this forum.

I'm trying to dimension a small sailing cat with the Iso 12215-5 rules and was surprised when I learned that in the formula for the area pressure reduction factor k_AR the panel area (l*b) is in the denominator of the formula.
That means for larger panels the factor gets smaller and thus the design pressure and the required skin thickness gets smaller too.
So if I increase the frame distance I can use a thinner hull skin???
The ISO 12215-5 version I'm using is from 2008. Unfortunately I don't have access to a newer version.

Am I doing something wrong?

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### TANSLSenior Member

What you say is true, in part, since KAR affects only one of the formulas that are given to calculate pressure but there is always a minimum value of pressure that does not depend on KAR. Therefore, even though PBMD and PBMP, for example, decrease with KAR, the design pressure, which cannot be less than Pxx MIN, does not have to decrease.
ISO 12215-5:2019 says : "this document considers that the local pressure diminishes when the area of a panel increases, as the panel is subject to an average of high slamming loads on small areas and lower sea loads on larger areas"

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There is a max and min value for k_AR.

Thus the minimum value ranges from 0.25 to 0.5, depending upon the location and vessel type.
Thus whilst the value of Ad may appear to increase, the caveat is restricted by the minimum value allowed.

Also note that Ad is also restricted, in its AR (aspect ratio). So you can't exceed an AR of greater than 0.4.

Like most rules, viewing one of the parameters in isolation, can lead to incorrect understanding. You need to play through the entire application to arrive at a set of values. Then play the sensitivity analysis of your values, and then you'll note where the caveats/restriction for each part of the formulation influences the final result, no matter how you arrange your frame/stiffener spacing.

Similar arguments are noted HERE and HERE, where the Op has only performed a partial calculation, and is getting caught up in his misunderstanding owing to the not following through the whole process; as noted, merely looking at one part in isolation.

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### jehardimanSenior Member

This topic (panel size reducing loads) came up a long time ago and I had an answer based on the physics of deflection as I was taught it.
As TANSL and Ad Hoc caution, you need to be cautious and consider the totality of the design. Larger panels will deflect more so you need to be sure to maintain sectional geometry and ensure that the panel doesn't damage anything inside as it deflects.

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### TANSLSenior Member

I do not know where you have been able to get this data and it would be interesting to know because the only thing that I have been able to find in ISO 12215-5 is that kAR will not be taken as <0 nor> 1 and, regarding the design area, AD, I have found what is indicated in the attached picture. No reference to the aspect ratio, as far as the kAR factor is concerned.

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From 7.5.2 and 7.5.3:

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### TANSLSenior Member

Yes sir, you are quite right and I beg your pardon for doubting your statement. I was looking at the 2019 rule and not the old one from 2008. Thank you for your kind clarification.
If you allow me to abuse your kindness, what can you tell me about the AR aspect ratio?

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In respect to what?

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### TANSLSenior Member

The relationship between the AR and the kAR factor, which is what we are talking about in this thread. You have introduced that topic in this thread and I would like to know why. You are probably right in saying that "Ad is also restricted, in its AR (aspect ratio)", but what that has to do with factor kAR escapes me. Once again, thank you very much for your response which, as always, will undoubtedly be kind and loving.

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The question (subject) is thus:

What is the driving parameter of k_AR?

Well, as Mariotti posits, the panel area.
So, with this, it therefore implies not just the "area" as an absolute", but the AR.

Since Ad - for plating - as this is related to the OP's query:
Thinner skins relate to which part of the structure - the plating!

And when investigating Ad, and the parameter that drives this is what... the AR, as noted in the - shall not be greater than - limitation.

That's it.

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### MariottiJunior Member

Thanks a lot for all the answers.
I’m aware that there are bounds for k_AR, the design pressure and the influence of the aspect ratio in the rules etc. But even taking all this into account they require smaller thickness for the plating with a larger frame spacing in some parameter ranges.
So the explanation (as pointed out) is the reduction of the slamming pressure for flexible panels. That’s an intersting insight. Thanks a lot for the help.

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### rxcompositeSenior Member

I think you are all mixed up.

1. There is no reduction of slamming pressure. Slamming means you are planing and there is an increase of pressure. If you are in the displacement mode, the pressure is reduced. kAR depends on the Taylors number or operating mode.
2. The pressure does not change whether the panel is large or small because it is a unit of measure derived from the DESIGN Pressure formula. What you get from the formula is the pressure influenced by the mode of operation.
3. There is no such thing as flexible panel. The rules guides you into the MAXIMUM panel size allowable. You may opt to design for a smaller panel size though. In panel design, you are satisfying the maximum allowable stress on the outer skin (use the tabular table H2) and maximum allowed deflection of the panel. You have to satisfy both.
4. The AR or panel aspect ratio table influences the k2, the panel aspect ratio bending strength. Less than 2 and there is a corresponding factor, greater than 2, it does not care. Except that for long aspect ratio, panels will deflect more and you have to satisfy 3. as stated above.

Last edited: Apr 3, 2021
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### MariottiJunior Member

I'm talking about the physics here and not the rules. A flexible panel reduces the slamming pressure. Slamming is water hitting your boat. This can be caused by waves or ship motion. You don't need to be planing for that.

Again I'm talking about the physical slamming pressure not the rules pressure,

Well all panels and structures are flexible.

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### rxcompositeSenior Member

Why did you post this?

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### MariottiJunior Member

Because with a larger frame spacing I got a smaller required plating thickness. It did not seem right and I could not make any sense of it. Thanks to this forum now I know why.

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