# Structural design - some brief on approaches

Discussion in 'Boat Design' started by Alik, May 10, 2020.

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### Eric ruttanSenior Member

This just seems obviously true. Loads are loads. All of them need the math done. Why even have the local/global/domination stuff? And why say 24m/40m/??m. It just adds confusion, no?

Do they get paid by the word?

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### Ad HocNaval Architect

An I-beam or the hull of a vessel, structurally, is the same.

Take the simplest form of beam with simple supports and apply a mid-span load. The bending moment is WL/4

So if L=2m, you get stress = X. But if the span is now 4m, the bending moment is twice as large as L = 4 not 2m. So 4/2 = 2 or twice as large.

Thus, fro a given I-beam, or any other structural member, i.e with a given stiffness of I, the stress in the beam of L=2 will be half that of L =4.

So it is obvious that doubling the length = doubling the stress in the beam.

Similarly, the deflection is by L^3... so changing the span from L=2 to L=4 the deflection mid span is now (4/2)^3 = 8 times greater!

There comes a point where the span (length) becomes so large that the global stress and importantly the defection is too much.

Where is that point.... Class define it as they see fit... some, 40m, some 50m, and some even 100m.

Indeed.
It doesn't matter what size/length the boat is, whether using a global or local load makes a difference to how the structure responds to the load on a given span.

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### DejaySenior Newbie

Well for someone who only wants to build a 15m boat that is certainly a load off. A global load

Eric, I guess the idea of dominance is that once you get to the length where you must design the hull scantling to take those exponentially growing loads and then you already exceed most of the local loads.
So you'd switch to a different iterative design approach and focus first only on global loads, then just verify the local loads once afterwards.

Thanks, this has been quite instructive for a novice like me. Breaking down a design into smaller parts to focus on local loads makes the structural engineering seem much more accessible.

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### DCockeySenior Member

The physics does not change abruptly at 24 meters. Experience has shown for most boats under 24 m that if they are designed to meet local load requirements, the global strength is sufficient. There can be exceptions if boat is "extreme" or very unusual and for such boats good, knowledgable designer will consider whether global loads should be considered. In 1995 the America's Cup boat "One Australia" broke in two during a race. The proximate cause of the structural failure apparently were a winch beaking which increased the loads from the rigging on the ends of the boat and thus increased the global bending moment on the hull, and argurably the sea conditions exceeded the conditions the boat was designed for. It has been claimed that the global bending strength of the boat should have been greater. America's Cup: Sinking of One Australia >> Scuttlebutt Sailing News https://www.sailingscuttlebutt.com/2019/03/05/americas-cup-sinking-one-australia/

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### Ad HocNaval Architect

Indeed it does not.

That is generally the case. But since you're citing the 24m as a length, one assumes you're referring to ISO rules rather than any Class rules? Class rules, as I have previously noted have varying cut-out lengths for global strength compliance. It is only ISO that is limited up to 24m. As that is their scope.. small boats and monohulls only at that. ISO is only concerned with local loads on small craft.

This is the crux of the matter.
No matter which set of rules, for compliance one is using, the role of the structural designer is to investigate all possible loading scenarios. One cannot assume that being under a 'rule' for a global check is sufficient on its own merit. It is not. One must always satisfy oneself that the vessel will not suffer unduly from global loads whether taken from the rules one is using or, investigating load cases, such as you the one you cite above. That is the role of a competent designer, and not merely a button presser.

Rules are only ever for minimum compliance, not maximum.
Thus the rules are the starting position and not the end position. The designer must check every possible load case even ones that may seem extreme. Not doing so is either ignorance or negligence.

For example, someone who has one come from a small boat background and ventures into ISO, may be totally unaware of such checks that need to be made - why, because global checks on a small 9m plastic tube would appear to be an overkill. Yet having odd/extreme dimensions for a 9m plastic tube could initiate a 'global' failure anyway!! However, upon a failure their usual opening gambit is - it doesn't say so in the rules. Which of course is correct, but does not absolve them of their responsibility to perform such checks.

It is not a case of what is or is not in the rules, it is a case of having to understand the design one is creating, its duty its purpose and hence understanding where any possible failure may come from, no matter its possible source and no matter whether is considered "global" or "local".

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### Ad HocNaval Architect

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### TANSLSenior Member

No, that is not correct, ISO 12215 also specifies in which small boats an assessment of longitudinal strength and buckling must be made. For example, in a boat with waterline length of 9 m, projected according to ISO 12215 for a maximum speed of 20 knots, its longitudinal strength may be needed to be checked.
For a designer, it is very important to know well the rules that he intends to apply or about which he wants to discuss.

Last edited: May 13, 2020
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### Ad HocNaval Architect

Would you mind citing the rule please.

I don't use ISO rules, as these are for small boats under 24m. I'm not overly familiar with them anmd my quick initial checks does not show this. Hence i'd be grateful if you could cite the rule you are referring to.
Many thanks.

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### TANSLSenior Member

you're welcome : ISO 12215.
The one you are talking about.

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### Ad HocNaval Architect

Indeed it is.

But which rule???...as you are saying ISO requires longitudinal global strength checks. I don't see this anywhere in those rules.
Hence asking can you please cite the rule number/part you are referring to, many thanks.

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### TANSLSenior Member

Effort yourself a little more, read the standard that you have been talking about for a long time and, if in the end, you do not find where ISO 12215 talks about the need to check the longitudinal strenght of some boats, I will tell you. Do your part. I do it for your sake.

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### Ad HocNaval Architect

Thus:
Thank you, that is what I am asking for - for you to tell me the rule reference for this.
I don't understand why this is so difficult for you to answer if you know the rule?

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### AlikSenior Member

The topic is about composite boats. Most of them are far below 40m and global strength is of secondary importance for most of them.
We do global strength check for many boats (because it is formally required regardless of size, say, in Russia, for recreatioanl boats below 20m). And the safety factors for global strength are very high, so we can state: the strucutre of small craft is designed to local loads, and only checked for local loads in justified cases. The only small composite craft in our practice where global loads were defining was hydrofoil passenger boat, 21m in length.

My writing is aimed mostly at amateur designers or those who 'modify' the hulls and 'borrow' lamination schedules. And at their 'clients', too.

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### AlikSenior Member

ISO12215-6 does cover global longitudinal strength , which should be checked in justified cases only.

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### TANSLSenior Member

I do not understand why it is so easy for you to talk about a norm, explain things in relation to it, if it turns out that you do not know it. (Let me ask, how many times do you act the same way?)
Yes, just look at ISO 12215-6, Ch 6.2 and Annex D.

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