defining structural design goals

Discussion in 'Boat Design' started by Annode, Mar 1, 2020.

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  1. Annode

    Annode Previous Member

    with respect to STRUCTURAL integrity (ignoring hydrostatic analysis for now), how string is "strong enough" defined for steel hulls around 25-35m for private use under sail/motor.

    I am referring to structural integrity for a blue water boat.

    I assume there are some standards set for commercial vessels like trawlers that carry large loads of fish, but these are built like concrete bomb bunkers and pay the penalty in weight and propulsion.

    With the ability of software to do analysis, this is no longer a problem of determining metrics, its now a problem of knowing what is strong enough, not strong enough and what is overkill.

    Are there coefficients that are applicable?

    Then there is this:

    As I've said elsewhere, you have to consider kinetic energy and momentum.

    Drop a 40 gr piece of lead on 5mm steel and it bounces off. Lay it on the surface and smash it with a large hammer and it smears out to a thin paste.

    Wrap it in gilding metal and accelerate it to 1000m/s and it punches a neat round hole all the way through.

    I have instrumented data showing a four-fold increase in load due to dynamic movement such as ship pitch & roll. It is this sort of thing that a simplistic statement about static loads and tensile strengths don't take into account.
    Last edited: Mar 1, 2020
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Well, if that is your opening gambit, then you'll never square this circle you have in your head.

    Class rules, cover all these vessels very well. Only those that feel they are over built, will constantly be looking for a way out. That "way out", comes very sharply to the front of your mind on a cold dark winters gale in the winter in the middle of the North Sea, when you hear the structure creaking and groaning and defection that cause you concern, because you didn't see the need to fit 5mm plate but assumed 3mm will be "strong enough".

    The "strong enough" is precisely that for Classification societies, based upon decades of in-service experience and failures. But if you elect to ignore this, that's your prerogative.
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  3. rxcomposite
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    rxcomposite Senior Member

    Well, there is a question of "Strong enough?" and "Stiff enough?". In Class Rules, both must be satisfied.

    I agree, Fishing boats or Commercial boats are built like a tank. Only because such vessels are subject to abuse of constant banging on the wharf and severe weather conditions. Unlike
    weekender boats or leisure yachts that are "babied", the scantlings are much smaller.

    But in Class rules, there is no such thing as overbuilt. It is built in accordance to Minimum Standards. The coefficients are there for every type of boat and the area of operations. Sometimes, there are special class rules for certain types. Say a workboat would have up to 1.4 X thicker scantlings than a leisure boat.
    Last edited: Mar 1, 2020
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  4. gonzo
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    gonzo Senior Member

    It is wrong to believe that computer analysis will do all the work. Software may do a lot of the drudgery, but garbage in gets garbage out. Also, you need to understand the results and decide if they are correct. For example, if you use Creo for FEA, it will assume materials elongate to infinity. You need understand the material limitations.
  5. Annode

    Annode Previous Member

    I can see I need to ask more specific questions.

    When is an answer not an answer?
    When it contains no useful information!
    AdHoc, listen I dont want to read any more of your long, semi accusatory, passive agressive criticism for even daring to ask the question in the first place.

    Im sure you are the best naval architect in the world, but dealing with you makes me want to run the opposite direction from every naval architect, and thats not fair to the ones that manage to provide simple, usefull information in a few well thought out sentences.

    So, The question is about coefficients and standards.
    >The coefficients are there for every type of boat and the area of operations.
    Thank you for that. Where do I find them please )
    Last edited: Mar 1, 2020
  6. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    "The coefficients are there for every type of boat and the area of operations.
    Thank you for that. Where do I find them please?"

    Regarding a 25m - 35m motor sailer, these rules might be useful -
    Rules and Regulations for the Classification of Special Service Craft

    Also these perhaps -
    Rules for Sail-Assisted Ships

    Re a controllable pitch propeller for your motor sailer, this will require more maintenance than a fixed pitch prop - Bruntons has this to say about them.
    I think that an Autoprop will probably need more maintenance than a standard CPP -

    You mentioned that this yacht would be for private use - are you planning on operating it yourself?
    Or does this just mean that she will not be available for charter?
    Be aware of the rules for vessels over 24 metres load line length re crewing requirements.
  7. gonzo
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    gonzo Senior Member

    It depends of from what height you drop it from. The potential energy transforms to kinetic energy when the object falls. Also, at 1000 m/s pure lead will punch a hole through the steel plate.
    If you don't want to start from first principles, using traditional rules of thumb is a conservative way of designing. However, it limits the designs to normal parameters and shapes. Complying with a class is another way of conservative designing, but parameters and shapes are also constrained. It appears to me that you want to bypass learning engineering/naval architecture and get a computer to provide a solution. Even though I am an engineer, I think that lofting with battens is a superior method. To model the curve one of my sticks makes, you would need a huge mainframe computer. I can simply modify the curve by moving a nail in my lofting table.
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  8. Annode

    Annode Previous Member

    Well thank you for the links. I scanned through all 1000+ pages. Obviously this is a very detailed spec designed for commerical craft that Lloyds assumes anything over 24m will be.
    I think this falls into the category of overkill.
    I am looking for hull integrity in terms of loads in torsion, cantilver and impact. I dont need tank level iron work that i have seen in commercial trawlers even in big seas.
    This is not for commercial use and I dont want to get drawn off topic in this thread with a discussion about crewing requirements.

    I understand that Lloyds has been at this a long time, but I doubt many boats that get certified meet all those requirements.
    I suspect its similar to road traffic laws and as any policeman will tell you, no matter how hard you try to obey them all, he can follow anyone for 10 mins and find a violation without much difficulty :/
  9. TANSL
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    TANSL Senior Member

    Absolutely wrong. But it is not worth discussing, and I will not.

    You would be surprised at the degree of accuracy with which ships comply with the regulations that are applicable to them: full compliance and, when or if something is not met, the reason should be justified by direct calculation.
  10. Annode

    Annode Previous Member

    Well I agree. 3rd order cubic spline code is trivial to execute as all the nurbs based CAD programs demonstrate. higher orders for class 1 surfaces are no necessary for boats.
    I dont mean to dismiss the old drafting techniques. There is something to be said for working with materials that a screen full of numbers can never convey

    Ive looked at a few hulls now, and I am sure none of them would pass all 1000pages of regs. They appear to just be overbuilt and no doubt the naval architect explained the trade offs at time of certification. I am sure they all met the major requirements, but that is what I am trying to ascertain from this discussion.
    Last edited: Mar 1, 2020
  11. bajansailor
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    bajansailor Marine Surveyor

    Annode, can we maybe go back a bit, and ask for some background info please?

    Are you looking to design yourself a steel yacht (it might be a motor sailer) in the size range 25 - 35 metres?
    Or are you just looking for general info on methods and procedures for designing such a boat?

    Are you primarily looking for methods to calculate the hull scantlings, ie the thickness of the shell plating and the bulkheads, and the dimensions of the transverse and longitudinal frames / stringers / girders?
    A 30 metre yacht is pretty 'large', relatively, compared to (say) a 10 metre yacht.
    However it is still very small when compared to a super tanker.
    Get the scantlings wrong on a super tanker and it will break in half much more easily (but it will still probably break in half if you don't load or discharge it 'properly' such that you don't exceed the maximum allowable bending moments).
    It will be rather difficult to 'break' a 30 metre steel yacht - relatively, a yacht this size will be much stronger than a big tanker.
    You might well find that the minimum theoretical scantlings re strength do not give an adequate margin for corrosion, so you want to increase the thickness (eg in the bilge especially).
    Or that the steel deck might be plenty strong (you are not going to 'break' it in a hurry) but it flexes, (which is not good) - so you need to add some stiffening.

    There does not appear to be any requirement for watertight subdivision in private yachts under 24 metres (commercial craft under 24 m do have rules though re watertight bulkheads).
    It would be prudent to employ the same standards of watertight subdivision (at the most basic, the minimum number of bulkheads) on your yacht as what the Rules (take your pick - there are many to choose from, not just LR but DNV-GL et al) prescribe for commercial vessels.

    Similarly re anchoring equipment - just because it is a private rather than a commercial yacht doesn't mean that you should have smaller anchors and cables - the displacement and the windage are still the same.
    The Equipment Numeral is used for this -
    Equipment Numeral Calculation for a Ship - a Guide - TheNavalArch

    I have missed out (deliberately) lots of other things that you can consider, but I just thought I would throw the above thoughts into the pot for consideration.
    There is still a LOT more to consider when even coming up with a very basic initial design.
    Last edited: Mar 1, 2020
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  12. Annode

    Annode Previous Member

    Yes many many details to cover. im trying to narrow the topic so i can get to some usable metrics for structural design for now.
    Yes it will be difficult to break a 30m steel boat with 8mm or 10mm plate because it is relatively small for its weight. this begs the question I am asking, how much is enough?
    What metrics could be reasonably used to design longitudinals and cross bracing and stringers.
    If Alluminum hulls can pass strength regs, it should be trivial for steel plate at this length without building a submaring tank.
    I need some numbers. Something. anything other than 1000+ pages of lloyds regs, to do a first aproximation so I can calcualte weight and THEN do the hydrostatic analysis.
    Just first approximation. Guys. thats all.
  13. TANSL
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    TANSL Senior Member

    You can do hydrostatic analysis wihtout knowing the weight.
    Enough is what ensures that the maximum permissible stresses for your material are not exceeded.
    If you give me the necessary data, I can calculate the thickness of the hull quite quickly.
  14. Annode

    Annode Previous Member

    Well yes you can do hydrostatics without the weight, but I still need to calculate it.
    Im trying to stay focused on Structural analysis. I appreciate your offer, but if you do it then I learn nothing :)

  15. Ilan Voyager
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    Ilan Voyager Senior Member

    NA, I mean NA for something bigger than a dinghy or a coastal small sailboat, is a job with lots a theoretical stuff to learn, a few thousands of pages of regulations to read and to understand plus some ability for maths. After a few years of working in a NA office is a must. Idem for a NE. That ask for studies in some university or institutions.
    Your very naive questions and your answers specially to Ad Hoc or TANSL show your ignorance in this matter. Ignorance does not mean stupidity, simply it means that you have not enough knowledge in the boat design field, and even less in structural calculations.
    All that shows that you are unable to make detailed structural calculations for a boat, or to consult similar plans or to visit in detail similar boats in shipyards. So you're looking desperately for scantlings.
    Your question is also incomplete. What is the use of the planned boat? Because I smell that will take passengers, as nobody makes a DIY personal family boat of this size. Almost surely it's out of your league, you will need the services of a NA, and you'll be obliged to respect the rules, all the rules and there are many, if you want to exploit the boat at least in first and second world countries.
    You're in UK so even for yachts you have to respect a big bunch of rules. If you want to insure you need a design following the rules plus a survey by a class society like the DNV, the Veritas or other. If you want to sell it with some value over the price of crap, you have to respect strictly the regulations.
    Over 24 m, all boats are submitted to at least some rules even for personal use. If you want to make charter you'll enter into the IMO rules and all the UK, European and so on regulations for commercial boats.
    Class rules are not for making overkill scantlings, they are for safety after many years of analysis of disasters or simply observation of deformations and a lot of broken material.
    Nobody with a working brain and common sense in this forum will give ciphers and scantlings. At fortiori without plans and a bunch of details and data. It's not greediness or wickedness it's elementary caution.
    And if you want to learn only for the beauty of learning, for your personal pleasure, sorry to have to learn boat structural calculations and to read thousands of pages.
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