Coefficient of Scantling

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

  1. Rumars
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    Rumars Senior Member

    I feel we need to go back to the basics.
    Design spiral for weight, first round:

    1. Define SOR. The result directly impacts the dimensions of the boat. Maximum lenght, beam and draft are determined here. Draft is normally especially important because it impacts area of operation.
    2. Draw hull shape.
    3. Underwater hull shape + maximum draft = total displacement. From now on, if you want to adjust displacement, you modify the underwater shape.
    4. Determine sail area as a function of total displacement.
    5. Determine ballast weight as a function of sail area and draft.
    6. Surface area of hull and deck => structural elements weight.
    7. Systems weight and placement (including mast, rigging and sails).
    8. Interior weights and placement.
    9. Payload

    This is only the weight parameter, there are other factors with their own spirals, and each affects weight, but basicly total displacement is fixed by the designer when he makes the choices of lenght, beam, draft and basic underwater hull shape.
     
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  2. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Can't tell you how I arrived at that,
    it comes from years of emotional experience around boats.
     
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  3. Ilan Voyager
    Joined: May 2004
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    Ilan Voyager Senior Member

    Yes it's very emotional. It seems that everybody is wasting his time in this thread.
     
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  4. Annode

    Annode Previous Member

    noun: curmudgeon;
    1. a bad-tempered person, especially an old one.
    Ah. yes.
    Right. OK

    And I assume we would scale the ballast by the same amount?

    Displacement = 2.53 * 50,000 = 126,500 Kg - that seems much more like it.
    Ballast = 2.53 * 9000 = 23,000 Kg

    Is that good enough for a first rough approximation?

    Plating + Framework = 10,400 KG

    In which case:
    hull and framing = 12%
    Ballast = 18%

    everything else = 70%

    Doest that sound better?
     
  5. Annode

    Annode Previous Member

    1. Crusing sail boat. 3om x x6m x3m roughly
    2. Draw hull shape.
    DONE
    3. Underwater hull shape + maximum draft = total displacement.
    From now on, if you want to adjust displacement, you modify the underwater shape.

    EXACTLY

    This is where I am today. I am trying to calcualte the waterline. To do that I need the weight. Then I can adjust the underwater hull shape to get the waterline where it needs to be.

    4. Determine sail area as a function of total displacement. HOW ??? (simply please)
    5. Determine ballast weight as a function of sail area and draft. HOW ??? (simply please)

    6. Surface area of hull and deck => structural elements weight. THE SUBJECT OF THIS THREAD
    I need this now. Sail area affect ballast obviously, but I just need a ROUGH approximation now to get the underwater hull shape. I am designing hull now. I dont want to get sidetracked with scantling design, sail area calcualtions and masts and ballast calcs. I just need a rough approximation.

    7. Systems weight and placement (including mast, rigging and sails). 2nd or 3rd ITERATION PERHAPS
    8. Interior weights and placement. 4th or 5th ITERATION PERHAPS
    9. Payload nth ITERATION

    FYI, while I may end up with a shorter boat, the design process for a 30m steel object takes the process out of the realm of fudging design by using the hull plating to achieve stiffness as discussed at length in the thread on origami boat building. This is a design exercise as much as anything. So everyone please relax with the dire warnings.
     
  6. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Annode please, you have a shape, draw a line 3m from the keel, calculate volume, convert to weight. What's so difficult about it? Then you take this number and see if it's enough for you purposes and adjust accordingly.

    There is only one black art in boat design and that is getting the customer to spit enough meaningfull info to make a good SOR. If you don't know when you begin to draw if we are talking about a 100t or a 250t boat you need to ask yourself some questions.
     
  7. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    "2. Draw hull shape. DONE"

    Are we allowed to see this mysterious hull shape?
    It might actually help those who are trying to help you if you post a copy of what you have drawn so far.
    A copy of your initial general arrangement so far would be useful as well. Or is this classified?

    "This is a design exercise as much as anything."

    Are the odds of you actually building this boat fairly remote then?
    You do realise (I hope) that the cost of building it yourself is still going to be in the millions?
     
  8. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    We are not wasting time. Firstly, we are getting a vocabulary lesson on obnoxious terms. Secondly, we are participating on a quest to discover the magic number that bypasses all the pesky calculations us engineers and naval architects waste out lives on.
     
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  9. Annode

    Annode Previous Member

    noun: sarcasm;
    the use of irony to mock or convey contempt.
    Why a healthy dose of sarcasm makes you brighter and more creative https://www.independent.co.uk/news/science/sarcasm-how-the-lowest-form-of-wit-actually-makes-people-brighter-and-more-creative-10416281.html

    Maybe trillions because i want gold for the ballast :/
    [​IMG]

    THIS IS A WEIGHT ESTIMATION THREAD
    THIS IS A WEIGHT ESTIMATION THREAD
    THIS IS A WEIGHT ESTIMATION THREAD

    This is a steel hull sailboat like this
    https://sailboatdata.com/sailboat/jongert-21s


    Ah well, glad you asked. See boats are like dinosaurs.... thin at one end, thick in the middle, and thin at the other end!
    So while a longitudinal line at 3m below the intended waterline might seem to be the solution, this hull needs the bilge to rise at the stern and possible descend at the bow to the 3m level.

    Then there is the hull shape, rounded, V, chined, rounded chine... it all changes eveything, but then you know all this.

    the ACTUAL location of the waterline is not predicable (at least for me) from looking at the underwater hull shape. It needs to be calculated. that requires a weight estimation... hence this thread. OK?

    So back to the question of the minute:

    Displacement = 2.53 * 50,000 = 126,500 Kg
    Ballast = 2.53 * 9000 = 23,000 Kg

    Is that good enough for a first rough approximation?
     
    Last edited: Apr 12, 2020
  10. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Good grief Annode, can you not see that people on here are trying to help you?
    You accuse Gonzo of being sarcastic, yet almost every single post of yours has a too generous dose of sarcasm in it.

    You accuse me of trying to steer your thread off in a different direction when I asked if you could show us some (any) drawings of what you have produced so far, simply because it IS beneficial to those who are trying to help you.
    This only makes me conclude that you do not actually have any drawings yet, despite your assertion "2. Draw hull shape. DONE"
    If you have - show us. They will be useful. Yes really.

    Re your question of the minute above, if you are going to use the Jongert's figures of 50,000 kg displacement and 9,000 kg ballast, then I think your multiplier should be closer to 3 than 2.5.
    See my post #74 above.
    This is assuming that you do literally want a larger version of a Jongert - is this correct?
    If not, then please do tell us what sort of boat is your ideal dream boat?
    And if it is not your ideal dream boat, what is it?
    Again, a drawing or a photo would be useful here....... do you see where I am coming from?
     
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  11. Annode

    Annode Previous Member

    >Good grief Annode, can you not see that people on here are trying to help you?
    Well yes and no. We at page six when we should be on page one. This has been like herding cats. You are smart people obviously. To guess the number I am after to 2 dec places is nothing short of astonishing.
    The problem is the world is not big enough for all the egos an hurt feelings that result from my "improper" approach.

    >You accuse Gonzo of being sarcastic, yet almost every single post of yours has a too generous dose of sarcasm in it.
    True. But mine are responses., and sarcasm can be humerous which is a much higher tone than some of the train wreck threads I have read in this forum...
    You guys drew first blood. Not me
    Gonzo chose to conflate the start of the design spiral where wieght estimation IS valid, with the end of the design spiral where it is obviously not. This is trolling. Its designed to solicit a defensive response, a brush fire leading to an argument, and drag the entire thread into yet another debate about the validity of my question. no no no. My question IS valid.

    >You accuse me of trying to steer your thread off in a different direction when I asked if you could show us some (any) drawings of what you have produced so far, simply because it IS beneficial to those who are trying to help you.
    TWO, yes TWO of my threads have bee closed due to trolling that took them into a meaningless debate about why my questions was valid.
    You tried to nose dive this thread by suggesting that it would be too costly to build such a boat and therefore any answers are pointless. Give me a break.

    >If you have - show us. They will be useful. Yes really.
    This is another diversion down another rabbit hole that is not relevant to estimating weight. I dont want to debate my desing, I want to estimate weights. If I wanted to debate my design I would start a thread for that, which I may do next, If i can ever get some simple answers to really basic questions. instead of nitpicking details that are irrelevant at this stage.

    >Re your question of the minute above, if you are going to use the Jongert's figures of 50,000 kg displacement and 9,000 kg ballast, then I think your multiplier should be closer to 3 than 2.5.
    See my post #74 above.
    This is assuming that you do literally want a larger version of a Jongert - is this correct?
    YES! for the purposes of ESTIMATING weight, sure. Why not.

    >do you see where I am coming from?
    Yes. You are all professionals. You do it the "right" way. I am not. This is a cause of irritation which causes 6 pages of "why is that the question?" and making fun of me. Go ahead, but im going to respond.. civilly of course.

    I am not afraid of troll storms. Im not going to NA school. Im am probably not going to do it your way, and I dont mind making a fool of myself when I miss the obvious, like scaling length without considering the volume, an elementary mistake, made late at night.

    This is a steep learing curve with many software challenges, not the least of which is modelling surfaces accurately in CAD. But right now I just want to estimate weight! Please!

    So, using your multiplier of 3:

    Displacement = 3 * 50,000 = 150,000 Kg - that seems heavy
    Ballast = 3 * 9000 = 27,000 Kg

    Plating + Framework = 10,400 KG

    In which case:
    hull and framing = 7%
    Ballast = 18%

    everything else = 75%

    Doest that sound better?
     
    Last edited: Apr 12, 2020
  12. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    Growth Factor for buildings is <1, for vehicles its about 3:1, for aircraft 6:1, for spacecraft launched from Earth its 1000+:1.

    Growth Factor being "if you add an extra pound of weight to a design, how much incrementally bigger would the whole design need to become to compensate".

    I can't decide if a boat would be more or less than a vehicle. I'd guess for planing hull it will be a bit more, and for a displacement hull a bit less. I'd also guess the above quoted Growth Factors numbers decrease as size of subject increases. Sports car has higher # than dump-truck.

    Growth Factor is different from "Coefficient of Scanting", but seems relevant to this thread.

    First question I'd ask about CoS is "Is this new boat expected to handle same size waves or will be wave-handling size scaled up with boat size?"

    IMO, best way to get REAL answers and figures is put your design into Autodesk Inventor FREE STUDENT EDITION and run stress tests. If that is daunting, consider making a few quick and dirty similar structures and running stress tests on those, scaling up and down, and that should give you Instant Wisdom.
     
  13. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    One last time and then I'm done:
    The type of hull underwater profile (long keel, short keel, fin, fin with bulb, slack bilges, firm bilges, etc.) and above water shape should be clear to you before you even touch a pencil or keyboard, it comes from the SOR. It is important and needed because you don't design to weight you design to immersed volume.
    Example: if you want a walk in engine room with headspace and workbench you either put that room above the waterline or it will have enough buoyancy to float everything you put in there and then some.

    There is no way to estimate ballast or displacement by hull surface area. Ballast is determined by the needed righting moment, wich is a function a hull shape and weight distribution. Simply changing ballast material for example from lead to concrete will drastically change ballast weight (you need a lot more concrete). Ballast position has a similar effect, the lower it is the less you need.
    The only thing that can be estimated by the hulls surface area is the shell plating weight and framing weight. This implies one knows the aproximate thickness needed for the plate and has already been explained.

    Now if "THIS IS A WEIGHT ESTIMATION THREAD" where "WEIGHT" = total/lightship displacement if you want answers you give us the info about SOR and underwater shape.
     
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  14. Annode

    Annode Previous Member

    I think you meant to say that an Engine room below the waterline has enough buoyancy to float everything in it?

    SOR is whatever the SOR was/is for Joghert

    Well I am assuming that if Suhaili can handel the southern ocean in the 60s, then a boat like Joghert would be capable.
    Suhaili - Wikipedia https://en.wikipedia.org/wiki/Suhaili
    I woould expect the growth factor for full displacement steel hulls, in terms of raw materials ahould be less than cars, esp large steel boats with space inside.

    I have looked at Autodesk. Nice package, but I got into Rhino for a number of reasons and began using it for cars. At this point, I would need a very good reason to move to another CAD platform. Maxsurf offers a lot, but the interface is basic. I cant spend hours moving the model in perspective with sliders at the edge of the screen after Rhino.

    I understand that you would just do the framework design work quickly and redo it later, like most others have said. I am not able to do that yet, and I am working on the hull. As engineers, you begin with SOL and then create a boat, well, that leads to eyesores like this:
    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    These look like they were designed based on budget and SOR.
    All very well engineered boats no doubt, but they look like they were designed by an engineer.. and no, thats not a good thing.
    Lines, proportion, curves are the language of hull design, NOT an SOR


    now compare that to the lines on a classic 1930s CAMPER & NICHOLSON BERMUDIAN KETCH

    [​IMG]

    And before you tell me they are utterly different boats, I am discussing aesthetics.

    So if you dont mind, I am going to start with aesthetics and circle back around to all the realities you assert as first order of business, becuase, for me anyway, unless its enspiring to look at, I might as well not bother, or go buy a set of plans

    The underwater shape at THIS stage of design, is a function of the above water shape. Whats above water? well that depends where the waterline is doesnt it? To know that, you need to estimate weight.... Hence this thread.
     
    Last edited: Apr 14, 2020

  15. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    those boats might not be most svelte things on the water, but looks like they were what someone pretty salty finally decided to get.
     
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