Coefficient of Scantling

Discussion in 'Boat Design' started by Annode, Apr 3, 2020.

  1. Rumars
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    Rumars Senior Member

    I am interested to know where this formula comes from, and what exactly "OKA scantlings" are.
  2. Annode

    Annode Previous Member

    Oh for goodness sake please... Misguided is irrelevant. Are you going to train me? Are you going to work through a scantling design with me? are you goign to buy me software? (still unnamed)
    No you are not. So your process will not work for me. I have to work with what I have got and the limited experience I have. Im not going to hire an NA. You are not going to do this for me so, I have to do it.
    The software wants a number to calculate the water line. A NUMBER. not an opinion.
    I cant enter "intuition" because it wants an integer. see the problem?
    So, please, stop apologizing and POST SOME NUMBERS

    There are some great answers on this thread. Informative and thought provoking. Please keep them coming
  3. Annode

    Annode Previous Member

    Mike, Thanks so much. What length was that boat please?

    The sofware calcualted 6.5tons for the hull plating ONLY with 6mm mild steel, and 8.6 tons wih 8mm steel.
    My initial guestimate was that the scantlings would be a factor of 2 heavier. But I have nothing to back that up. No experience, no rough calcs nothing. hence this thread.

    There are surely many people on this forum that could provide a number based on their experience building larger steel boats.
  4. Annode

    Annode Previous Member

    OKA = otherwise known as
    plural noun: scantlings
    a set of standard dimensions for parts of a structure, especially in shipbuilding
    This is a coefficient I invented as a way to estimate weight based on the assumption that scnatling weight and hull weight are related in a directly proprtional way to some degree. That degree is the subject of this thread.
    I am willing to bet that if you collected data from all steel boats in their respective size classes, an average number would emerge really fast.
  5. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    Okay, I get it now.
    I'd say 1 : 1.28
    Can't tell you how I arrived at that though
    because it comes from years of emotional experience around boats.
  6. Rumars
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    Rumars Senior Member

    Now I understand what you want. So it's not weight of structure minus scantlings. What you are after is the percentage of the structure out of the lightship displacement (I asume this is what "weight of hull" means)?
    It's not really possible to quantify for sailboats because ballast varies between 30-50% of displacement. Then there are the problems of hull shape and intended use, a wedge shape, double handed sloop racer is completley different from a full keel brigantine cruiser for 20 guests.

    Your best bet for structural components weight estimation based on lenght is to use one of the "cookbook" rules from the first half of the 20th century. There you find tables like the one you linked wich say for 6mm plate we want XĂ—Ymm stringers and frames on Zmm centers. From there you determine weight/sqm and multiply by total hull area.

    This still leaves you in the dark about ballast proportion and lightship displacement, that's why people begin to design to a predetermined arbitrary (means more or less fixed by SOR) aproximative displacement.
  7. TANSL
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    TANSL Senior Member

    And why not?. I could do it if you needed to. It wouldn't be the first time I've done it. And I could give you a free copy of a software that allows you to calculate the scantlings according to ISO 12215-5, and another that would allow you to develop the naval architecture calculations for your boat and another that would help you to make the construction drawings of the hull. But you would need to be able to understand them. You cannot get an idea of how many things I would do with someone who humbly asked for help. I do not want to train you or disrupt your training plans, I only offer help. I do not know when I have offended you or how I have done it, but I apologize.
  8. MikeJohns
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    MikeJohns Senior Member

    19.8m LOD 17m LWL 5m Beam 40 Tons Ketch rigged 6mm hull plating.

    Looking up the Rudder and shaft, that was 250kg for a hollow NACA 0012 foil. Interestingly almost exactly neutral buoyancy.

    No where near twice as heavy.
    Between 6 PL and 8 pl weight depends on the extent of the thicker plating . It also really depends a lot on the structural layout. You can always replace 6 with 8 with the same framing and weight will only increase by a factor of 8/6 for the plating. But the framing and stiffeners can often be lighter depending where they sit in the available section sizes.
  9. rxcomposite
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    rxcomposite Senior Member

    The WBS system might be too much for you. The breakdown is from Structure/Propulsion/Steering/Anchoring/Navcom/Fittings/.....ect. Each one has a coefficient that interacts with each other. It is a software in itself.

    Since you want to zero in on scantlings, the weight of the hull structure in relation to the displacement of the vessel should be between 40% to 60% depending on the Cb or blockiness of the boat and its intended use.

    Scantlings is the arrangement of the plates and stiffeners. Generally, the stiffeners will be from 15 to 28% of the structure plates. The plates takes up most of the weight. Plates will include anything that is of large expanse designed to hold off water and that include transom,deck,superstructure, and bulkheads.

    There will be two types of Stiffeners. The primary structures such as non watertight bulkheads, Frames, Girders. It is basically a large plate with smaller local reinforcement welded into the webs. Stiffening frames would be about 28% of the calculated individual structure weight. Say a finished frame would be about 100 kg total. 72 kg would be the amount of plates need to define shape and 28 kg would be the weight of the additional materials that would be added.

    A plate that will be be stiffened with (L bar or t bar) longitudinal or transverses (secondary structures) will be about 15% of the plate weight.

    Thus if you can calculate the Plate weight of the total structure, add about 28% for the primary stiffening members and about 15% for small stiffeners. The total of the weights should be about 40% of the displacement (optimal design) to about 60% of the displacement (non optimal, heavy). This is a general rule for small boats.
    bajansailor likes this.
  10. Annode

    Annode Previous Member

    Ta Da... Winner!! Thank you. So simple, reasonably general and based on EXPERIENCE. PERFECT!!
    This is exactly what i wanted, a first approximation for the first step in the design proces.
    And thank you for being brave enough to post a number that no doubt some will disagree with (yet not post a number that they believe is more accurate). You won 10 forum bravery points.

    yes all good points. Lets look at prismatic coefficient for a moment:
    That last sentence is all I need for now. If I said give me a number, what would it be?

    WEIGHT OF THE HULL PLATING = weight of the steel plates that make up the exterior surface of the hull including the deck plating, keel plating, but NOT including any stiffeners, fittings, ballast, keel member.

    Picture the boat in a cad program. As you rotate it in 3D, the hull is everything you can see.
    The scantlings is everything you cannot see that is used to stiffen the hull plating

    OK. So with all your experience (which I dont have) <removed> if I said give me a number, what would it be?

    Thank you. Can you refine that to guestimate to a quotient where the hull plating is the denominator?
    By hull plating I mean plating that is on the exterior of the boat.

    OK. Thank you for those numbers ) Since the displacement is also a function of many other things, I am trying to remove all those other things from the equation and express this in terms of the hull plate that the stiffeners are designed to stiffen. Can you refine that to guestimate to a quotient where the hull plating is the denominator?
  11. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    Well, if you liked that then you're going to love this refinement:
    It's still an estimate but much more accurate.
    bajansailor likes this.
  12. Annode

    Annode Previous Member

    and now I suspect you are pulling my tojjer. I dont know you well enough for that, and boat building is a serious business!
  13. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    What do you think?
  14. Annode

    Annode Previous Member

    well since you asked, I think that boat building presents an interesting challenge somewhere between a car and a plane, yet for some reason, there is an eliteism that exists in the world of marine engineers (or Naval Architects) that goes back to the days of Napoleon.
    I took your number seriously, and now you undermined your credibility. You can only do that once )
    In car forums or even experimental plane forums there is usually interest in others projects, a sense of exploration and camaraderie between the participants sharing information with joy. I dont get that sense here. Two of my threads were nose dived into the ground by one troll in particular, and this one has been a difficult road.
    There are some very good engineers here who did not study boat design, yet I get the distinct impression that they are retiscent to post anything because of the tendency for those that have, to ridicule them as amateurs if they dont know some obscure reg or factoid that is not taught in college, but gleened from years of experience.
    Well, I am not afraid to look like an amateur, because I dont have a lifetime to do over as a boat designer, and even if I did, I have better things to do than aquire an intimate knowledge of lloyds specs, written because of the failure of boat builders to get it right

    I dont know if this comes from the salty rough hewn nature of the marine industry, or if it is professional eliteism, and I dont care. I also do not mind being a novice boat builder, and not having all the answers. To me that is the point of a forum like this.
    Unlike the medical profession, no one is going to die if the boat isnt perfect, and before you tell me that boats are dangerous, one professionally designed and built boat sinks every four days in the professional marine world.
    So, go ahead and make fun of me, sneer and deride. That just makes the completion of this relatively simple project all the more sweeter.

  15. gonzo
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    gonzo Senior Member

    Your arrogance and airs of superiority will not get you better advice. If you want a cook book approach, look at a similar design for approximate values. In fact, if you don't want to learn how to design a boat properly, buy a set of plans,
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