Structural design - some brief on approaches

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

  1. Alik
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    Alik Senior Member

    Did some write-up on structural design of composite boats. Just a summary and a 'must read' for those who 'take lamination schedule form another boat' or 'modify existing hull'.

    Structural design of boats in composite materials... So how does it work?

    1. First, we need to find what rules or standards the craft should comply structurally. Is it ISO 12215-5/6 or Class Rules? Or maybe we use direct engineering methods? Time to think and ask Naval Architect for advice. For most of smaller boats, critical would be local strength - ability of panels and stiffeners to withstand local pressures from sea, people and cargo. On small craft, other structural design cases (such as global longitudinal bending, twisting, etc.) are usually of secondary importance.

    2. Then, we need to define the design loads. Design loads depend on type of craft, its speed, geometry, operational sea state. Slamming impact loads also depend on size of panels and stiffeners - assuming the 'spot' impact load is distributed among them. So, if spacing between stiffeners is changed, the design loads will change as well!

    3. Define properties of composite materials used for construction. This is property of each layer, and in case of sandwich or complex laminate - the properties of 'stack' of layers.

    4. Then, we need to define criteria of strength/stiffness, and safety factors. For composite structures, there could be up to 4...8 criteria which have to be checked simultaneously, such as stresses in inside and outside skins, interlaminar shear strength, core shear strength, minimum skin thickness criteria, skin buckling criteria and others... Usually criteria and safety factors are given in the applicable standard. For direct engineering methods, those factors are defined by the designer.

    5. After that, we calculate every panel and stiffener for the design loads, to check the criteria if they are within safety factors. Not too easy? Yes, but this is how it works...

    Very often, amateur 'boat designers' do not realize the complexity of structural design, and they 'borrow' lamination schedule from other boats. This is a critical mistake, as any lamination schedule is only valid for what is it designed - speed, type of craft, weight, size of panels and stiffeners, etc.

    Or, those 'designers' take an existing hull, and convert it into different design - heavier, faster, etc. With change of craft's parameters, such as higher operational speed or increased weight, the design loads will increase as well... So, if the boat is significantly heavier (someone decided to put more passengers, more fuel, etc.), it is not likely to survive the operational loads - another critical mistake!

    And finally, problem is in the safety factors: say, safety factor for recreational craft is about 2.0; for commercial craft can be up to 3.3. In practice, this means that the 'recreational designed boat' will fall apart if used commercially, maybe in couple of years...

    The conclusion: then buying a boat, check carefully if it is certified by internationally recognized authority and/or if the structural analysis of the boat has been performed by qualified engineer. Structural analysis is a book of 50+ pages, for 15m boat - not a one sheet of paper! And finally, is the craft designed for recreational use, or can withstand loads for commercial (passenger) craft? Something to think about...

    Albatross Marine Design https://www.facebook.com/AlbatrossMarineDesign/posts/2750245498542639?__tn__=K-R
     
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  2. priyesh saxena
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    priyesh saxena New Member

    I hope boat builders in India read this and realise how important it is and not ignore the "calculation/validation" phase of structural design. thank you @Alik. I have always used your work as a benchmark.
     
  3. Dejay
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    Dejay Senior Newbie

    Thank you. So from what I understand there are two ways, either look at each individual part and determine local loads, or use complex FEA software that can verify and optimize the whole structure globally.

    Is the determination of local strength something that a novice can learn or would this be inefficient and dangerous?
    Does FEA allow to design more lightweight?
    Is FEA something that is affordable for a one off DIY boat design project?
     
  4. gonzo
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    gonzo Senior Member

    FEA needs to be approached with caution. It is not a shortcut for common sense and experience. Basically, if the data input is wrong, the output will be wrong too. Also, software often has assumptions that do not reflect the real materials. For example, that elongation continues to infinity without the material ever breaking.
     
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  5. Alik
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    Alik Senior Member

    For craft below 24m in length, the global strength is usually of secondary importance. Local strength is primary!
    FEA is an overkill for DIY boat projects. Better spend time to build boats.

    Note that in most of rules for composite craft safety factors are non dependent on calculation method. With FEA one will get more stress points (also dummy stresses due to simplifications in the model), but still need to satisfy the same safety factors. The resulting structure might not be lighter! The attempt to reduce safety factors for FEA is done in ISO12215, this was my proposal.
     
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  6. Eric ruttan
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    Eric ruttan Senior Member

    What magic in physics causes a change at 24m?
     
  7. bajansailor
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    bajansailor Marine Surveyor

  8. Ad Hoc
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    Ad Hoc Naval Architect

    Any vessel over 24m means it is "technically" a big ship.... thus you go from small boat rules to big ship rules.
    Unless the vessel is able to operate as a different type of vessel... such as a high speed catamaran ferry. Despite being over 24m Loadline, she can use the HSC code which is tailored for 'small' fast vessels, rather than big ship rules tailored for large steel ocean going vessels. So the rules for compliance in the HSC code are very different from a Big Ship passenger code. But the philosophy of safety is maintained as an equivalence.
    In a nut shell.
     
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  9. Eric ruttan
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    Eric ruttan Senior Member

    Oh, my mistake. I thought the thread was about structural design. Not legalites of some jurisdictions.
     
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  10. Ad Hoc
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    Ad Hoc Naval Architect

    You're correct. I was only responding to your question/quiry about 24m.

    Local strength requirements, in terms of the vessel, is only considered for vessels up to circa 40m for some Classifications societies and 50m for others. Beyond these values, global strength dominates and so you need to check global loads as well as local loads. The 24m is a misnomer in that sense as it only appears to be an ISO issue, which is predominately aimed at small craft.

    Does that answer your Question?
     
  11. Alik
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    Alik Senior Member

    :confused::confused:
     
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  12. Eric ruttan
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    Eric ruttan Senior Member

    Firstly, thanks for trying. Secondly, no.

    It seems you are suggesting, as did friend Alik, and also many Clasification societies, apparently, that there is a size in which some value of the structure changes from 'local' to 'global'. And also there is domination. And the actual point that this change occures depends on which group of people say it does.

    And that seems silly to me.

    Probably this is because I fail to understand some fundemental truth or axiom. I often see societies with these.

    Can it be explained why there is a point at which the domination of the structure changes? And why this point changes depending on who is looking at it?

    If not, thats fine.
     
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  13. Ad Hoc
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    Ad Hoc Naval Architect

    Agreed.
    But...

    Yes.

    If you take an I-beam, apply a load for a given span, it is easy enough to calculate the deflection and stress.

    If you now increase the span, but the same laod is applied, what happens? Well the the bending moment increases.
    For a simple supported beam, in this case, it increase by the span. Thus the greater the span the greater the bending moment.

    This then means, the greater the bending moment on the I-Beam.

    However, if the I-beam was "ok" on a smaller span, it clearly is in difficulty in a greater span, or may even fail.
    In addition the defection shall also be greater by a similar ratio to the span by cubed power, or L^3.

    Knowing this, the same is true of a vessel. If the I-beam is a vessel, increasing its span or in the case of a vessel, its length means = increase in bending moment.

    So as the length increases the bending moment on the I-beam, or vessel, increases and is no longer dominated by local loads.

    But why 40m or 50m is selected by Class, you'll have to ask them that. However, I suspect that they will cite in-service experience of many vessels in this range as a 'cut-off' limit between local and global loads.

    So it is all evidence based for Class societies that they use for this cut-off limit between local loads and global loads.

    In other words, it matters little what we think is right or wrong, it is based upon what they know, from previous vessels in-service.
     
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  14. Eric ruttan
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    Eric ruttan Senior Member

    I am not an expert in 3 point loads or I beams,
    Having covered them casually, I defer to your expertise. But I have never seen the concept of local and global and domination used to describe 3 point loads. Perhaps if I had, I would have paid more attention.

    It seems to humble me, you are just describing loads. And the ideas of local and global and dominiation is the kind of thing a guy sitting in a cube creating rules for other people would come up with, as opposed to the guy out building boats would come up with.

    Because even small boats need to be aware of the loads of the largest span beams for said boat, it would seem.
     

  15. TANSL
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    TANSL Senior Member

    Perhaps someone can deduce from all this (I say perhaps) that when the global loads of the beam-ship have to be considered, the local loads are no longer considered, but it is not correct, in addition to the global loads, when they are taken into account. , local charges must be taken into account.
    On the other hand, it should be clarified that there are boats less than 24 m in length in which the global loads on the hull must be taken into account.
     
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