Questions about Dave Gerr's book The elements of boat strength

Discussion in 'Boat Design' started by Arvy, Oct 21, 2007.

  1. Arvy
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    Arvy Senior Member

    Hi all,

    I recently acquired the book The elements of boat strenght to start working on the structural construction of my design (40-foot sailing yacht, to be built in either aluminum or perhaps steel) and while I was working through the book some questions came up to me.

    Perhaps someone can explain to me what Dave means with the Sternframe and the Hornplates on page 248 and 249. I am not native english but dutch and untill now I have only worked with dutch literature. From his description I cannot make up what he means with those 2 terms. Any help would be appreciated.

    Then I do have a question about the scantlings too, after calculating them you will get a lot of scantlings like 7,19 mm or things like that. How does one decide to round them up or down, for some structural parts Dave is very clear about it, but for most not. With material availability of lets say 6 mm thick and 8 mm thickness and a scantling of 6,24 mm thicknes will it be safe to round it down to 6 mm or do I have to round it up to 8 mm? I have done quite a lot of structural work on a lot of designs (mostly van der stadt designs) but that was some 10 years ago so my experience on the topic has left me a little bit.

    And my last question, here in The Netherlands most designers use a frame spacing of 400 mm (with longitudinals). Now after using Dave's calculations I get either a frame spacing of 970 mm (longitudinal framing) or 325 mm (transverse framing). The calculated webheights and thickness are respectively: 88 x 7,19 and 49 x 7,19. Now when I want to make the frame spacing 400 mm, is it safe to interpolate between those values (while keeping the longitudinals of the longitudinal framing scantlings) so make the height 55 mm and the thickness at 7,19? And do the same with the flange sizes?

    I hope some of you can give me some clarity about these questions.

    Grtz,
    Arvy

    ps. I created an excel worksheet for calculating the scantlings almost automatically for given LOA, LWL, Max Beam and Depth of the hull, for hull speeds below 15 knots and in Metric units. You will still need the book tho, but it can help one save time with the calculations, most of them are in the sheet, but same scantlings come from tables and there are references in the excel sheet to the pages where one can find the tables. I am not sure if I can upload them, due to perhaps some copyright infringements when publishing it, but if that isn't the case, I will upload it for all to use.
     
    Last edited: Oct 21, 2007
  2. Arvy
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    Arvy Senior Member

    Sternframe is clear now.

    I should have read further in the book, there is a picture of a sternframe on page 278, making it very clear.
     
  3. Eric Sponberg
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    Eric Sponberg Senior Member

    Arvy,

    Some of the decimals you see there are conversions from imperial units. For example, 6.24 mm is exactly 1/4". Generally, when figuring scantlings, calculate in the units you intend to build in, and that will be the most accurate. Always, scantling rules results are calculated minimums, so you should almost always round up to the next available size. You can go heavier if you want to, but you should not go lighter.

    Frame spacing can be whatever you want it to be, and there are practical minimums. I find on my motoryachts that a working in multiples of 8" (200 mm) works best. Actually, this starts with a frame spacing of longitudinals of 16". It can be 24", 32", etc., any multiple of 8". Stair tread heights 8" are very convenient, and multiple units of 8" for a set number of stairs will determine the 'tween deck height. Keep your frame spacing at a unit that will be easy for the builders to work with, and if you are using flatbar and angle bar structures, use the most convenient and easily available sizes, the next up from what the scantling calculations show. Boat building is supposed to be easy--don't make it difficult with odd sizes of structure.

    Eric
     
  4. Arvy
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    Arvy Senior Member

    Hi Eric,

    Thanks for your reply.

    I think you are right about boat building supposed to be easy, I totally agree, and this is also why I want the framespacing to be 400 mm (about 16") and round off the plate and profile thicknesses to the next readily available sizes.

    Where I run into tho is that when using Dave Gerr's book I get a hull plating thickness of 6.2 mm and when I round this up to the next available thickness, I will have to use 8 mm thick plates (for a 40ft yacht this seems an awfull thickness) as 7 mm isn't readily available (I am looking for different suppliers now). When I would be using steel, the calculated thickness is 3.91 so that would round up to 4 mm, but in that case I would probably use use 5 mm for the extra mm to rust away.

    For frames and longs I don't mind putting in an extra mm, but the plating is such a large part of the structure that it will add significantly to the weight of the structure.

    I think I will look up the lloyds class rules and iso 12215 and see where those calculations lead to.

    In Gerr's book there is also a lightweight aluminum section and following those calculations (with same frames and longs), the bottom plate thickness required is only 5,45 mm so 6 mm would do. I just find this kinda strange as the earlier scantling was already the minimum..
     
  5. Eric Sponberg
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    Eric Sponberg Senior Member

    Dave Gerr's book is not intended to be a definitive engineering guide. It uses much simpler methods of determining scantlings that are not as fundamental or as accurate as the classification society rules (ABS, Lloyds, etc.) which are based on more fundamental engineering principles. So when Gerr's book gives you a thickness of 6.2 mm, you can take it with a grain of salt, and round down as well as up.

    In classification society scantling rules, the strength and modulus of elasticity of the materials figure directly into the equations for plating thickness and frame size. Frame spacing, frame length, and plating panel sizes are all parts of the equations, so you have much more control over the determination of scantlings than you do with Gerr's book. It's much more number crunching, but it is also much more accurate, and you get to the minimum required scantlings with a much better degree of accuracy.

    Eric
     
  6. chandler
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    chandler Senior Member

    Eric
    Thanks for the response on carbon rig design.
    What do you think of Gerrs vs mcnaughtons scantling rules. Do you think either should be used?
    Chandler
     
  7. Eric Sponberg
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    Eric Sponberg Senior Member

    Chandler,

    I have not read McNaughton's scantling rules, so I cannot comment on them. As for Gerr's, they are good for a first pass in preliminary design, but I would not be assured that they would stand up to engineering scrutiny, particularly when taking the design through compliance with classification society rules or through ISO/EU standards. The only way to be assured of that is to actually go through those types of calculations later on in the design process.

    Eric
     

  8. Arvy
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    Arvy Senior Member

    Eric,

    You are right, I now started the number crunching using the Germanischer Lloyd class rules, and it sure is a lot of number crunching. But as a mechanical engineer, I am rather pleased that the GL rules work with the section modulus of the structural parts. Using the section modulus instead of just some rule with unexplained coefficients (is this also an english word?) gives me more confidence.

    But for hull plating thickness using standard mild steel the scantlings happen to be identical between GL and Gerr's (3,91 mm, so use 4 mm). When using aluminum al5083 there is more than 1 mm difference (GL: 5.2, Gerr: 6.2, so I would go for 6).

    But at the moment I am still number crunching, and I will post the differences from my calculations, this might be interesting for other people as well.

    Greets,
    Arvy
     
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