Leaving the frames in? How wide?

Discussion in 'Fiberglass and Composite Boat Building' started by John Smithson, Sep 7, 2021.

  1. John Smithson
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    John Smithson Junior Member

    I'm planning out a 28 foot sailboat in plywood with fiberglass construction.

    But running into trouble with the details of the frames / ribs / stringers.

    I was intending to do 3/4th inch plywood ribs doubled to 1.5 inches. And making them 9 inches deep (leaving room to add insulation and then attaching the interior to these ribs).

    Then spacing them 18 inches apart all down the boat (too close?)

    For stringers, I was thinking 3 inches deep and 1.5 inches wide.

    Then doing 1/2 inch plywood over the top for the skin.... and finally glassing everything together into one.

    From what I've seen, this is pretty unusual for a fiberglass construction? More towards that of a full wood construction?

    Am I wrong in thinking this would just mean more weight but give a much stronger hull in the end?
  2. bajansailor
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    bajansailor Marine Surveyor

    What is the basis for your frame sizes and spacing John - are you using a set of rules for guidance, or maybe a similar sized plywood boat (eg one where you can buy plans for online) - or are you simply 'winging it', with a seat of the pants approach re if it looks ok, it should be strong enough?
    Or maybe a combination of all of the above?
    Can you post any sketches re your thoughts so far please?
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  3. TANSL
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    TANSL Senior Member

    You must propose a structure in its general lines, connecting all the transversal and longitudinal elements in a way that forms a three-dimensional grid as perfect as possible. The spacing between frames or longitudinal members will be a consequence of the scantlings calculations, although you can copy similar ships to begin with. But everything is a harmonious whole and some things depend on the others. You must make the scantling of the structure that could be divided into two stages:
    - design the structure
    - calculate the scantlings of its elements.
    The most appropriate thing is that you be guided by some of the regulations that, for such purposes, have been published by various companies. Given the characteristics of your boat, it is best to calculate it, in terms of scantlings and stability, in accordance with the ISO standards for small boats.
    If you are looking for the lightest hull possible, abandon the idea of wood and focus on FRP.
    You will be amazed at the amount of weight and labor that good calculations can save you.
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  4. BlueBell
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    BlueBell . . . _ _ _ . . . _ _ _

    "Talk's cheap until you get the bill."
    Drawings or sketches would be very, very helpful here ...
    ... to help you ...
    Otherwise, threads can talk around in circles ... forever ... and get nowhere.
  5. John Smithson
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    John Smithson Junior Member

    I've been looking for some time at other boats (as much as I can finding free information at least). But many are wooden boats Petrel 28| family cruising sailboat | small boat plans http://www.modernwoodenboat.com/mwb_project/sailboat-homebuilder-petrel-28/

    The spacing I propose seems roughly in line with drawings of catboats I've seen.


    Though the depth of the ribs seem quite a bit deeper (I haven't found many boats that give much consideration to insulation while giving enough room to attach interiors).

    1/2inch plywood with glass on top for the skin might be a bit overkill though? 5/8th seems to be for pure plywood skins, so maybe less thick?

    But, overall, I'm just cobbling together what information I can find to design a boat.
  6. Ad Hoc
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    Ad Hoc Naval Architect

    What is the full displacement.. and where do you plan to sail her?
    Without the answer of these...it all depends!
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  7. John Smithson
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    John Smithson Junior Member

    Unsure on the displacement (since how it's built will have such an impact on how heavy it is)

    The sailing goal is to take it down the mississippi river and then off to the caribbean and who knows where. Which is why the design is more like catboats (with shallow drafts), but a few changes to give it a greater angle of vanishing stability (such as a drop down bulb keel).
  8. Rumars
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    Rumars Senior Member

    Plywood boats are normally not designed to have meaningful fiberglass skins, the strenght is in the ply. Modern ply boats have all furniture bonded to the skin acting as stiffeners, reducing the need for separate framing. There is a relation between skin thickness and framing scantlings, thicker skin = less framing.
    Buy a book on how to calculate the stuff, it's not something you can simply decide on by looking at other boats. Even better, find a design to your liking and buy the plans. Any modern shallow bilge boat can be converted to a variable draft keel.
    This site even hosts free plans for a boat that might suit you, the Kavalier 800 KAVALIER 800 Designed by Albert Nazarov, Maxim Kovalyov, Dmitry Dolinsky https://www.boatdesign.net/nyd/K800/ all you have to do is build it.
  9. bajansailor
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    bajansailor Marine Surveyor

    John, that Petrel 28 is light years removed from a traditional catboat - and it's construction arrangement is even more so.
    If you try to build a Petrel with catboat scantlings, it will end in tears.

    How did you arrive at 28' long?
    Have you sketched out a general arrangement yet for what you want, and 28' is the minimum length needed to achieve your Statement of Requirements?
    I presume that you have written out a comprehensive SOR?
    If not, then that should be your first priority.
    Along with a GA drawing.
    And you will need a rough lines plan, to get an estimate of displacement - are you planning on doing this by hand (pencil on paper), or do you have a CAD programme? If the latter, what do you have?

    Do you really need to have shallow draft capability for going down the Mississippi? I wouldn't have thought so, after seeing the huge ships, tugs and barges that trundle up and down it.
    To sail off to the Caribbean from New Orleans will involve a lot of windward work - unless you follow the coastline all around the Gulf and Florida, and then go through the Bahamas, in which case yes, shallow draft would then be useful.
    Apart from the Bahamas, pretty much most other anchorages in the Caribbean are not too worried about depth for yachts - many are quite deep even.
    Drop down bulb keels are probably more complex than fixed keels to build and maintain, and if you do not absolutely need one, then maybe plan on keeping your design as simple as possible.

    Look at as many online sites selling boat plans as you can find, and see if any of them have a boat that appeals to you.
    As Rumars said above "Even better, find a design to your liking and buy the plans"
    The cost of the plans will be a miniscule fraction of the cost of building the boat - and the experience that went into drawing those plans is invaluable.

    One site that I love browsing through is Sam Devlin's - there is just so much variety here.
    Devlin Design Catalog https://devlinboat.com/about-devlin-design-tech/devlin-design-catalog/
    And there are a few sailing yacht designs around 28' in this link that might be of interest to you.

    And there are so many other yacht plans now available online as well.
    If you can give us a more detailed SOR as to what you want, you can be sure that somebody on here will find an online plan that comes pretty close to it.
  10. John Smithson
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    John Smithson Junior Member

    Any longer seemed excessive for the amount of room I need single handing. Following the catboats wider beam gives it a lot space inside.

    I'm using fusion360 to plan it out

    I'm starting in the kansas river, then into the Missouri river... then ultimately into the mississippi. So draft is a problem (especially down the kansas).

    Because of the unusually wide beam, the keel will need to be fairly deep when at sea to give it a large enough angle of vanishing stability. The only way I could figure of achieving this with the shallow draft was a long drop down keel.

    I haven't really seen any boats that fit all the criteria I'm looking for. I'm not sure why a catboats hefty beam hasn't been taken into an ocean sailing design yet with a few modifications for the harsher conditions (such as a keel that gives it more safety at sea while still allowing it to have the tiny draft, or a sail plan and mast that is more suited).
  11. Blueknarr
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    Blueknarr Senior Member

    The marine highway 29 is currently increasing its minimum depth from nine to twelve feet.
    No 28ft sailboat will come close to bottoming out.
  12. fallguy
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    fallguy Senior Member

    Based on your questions and assumptions, I'd say you ought to avoid doing the design. And I say this as a builder and a boater and someone who wants you to succeed.

    First of all, a plywood boat generally develops its strength from the plywood, not the skins. Rumars points this out and it was my first point of concern.

    Second, if you are asking a question about how far apart to place bulkheads; you also do not belong in the design business. And as Rumars also suggests; you've got some reading to do...

    I encourage you to shop for a plan. It'll be a lot more fun. Designing sailboats is a complex business...with lots of room for irreversible errors. Think keels falling off..masts falling over..groundings damaging the hull....not to mention poor performance...
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  13. John Smithson
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    John Smithson Junior Member

    Which book would you suggest for finding the way to calculate the frame spacing and size?

    I'm happy to do some reading, but finding sources that go into it has been difficult. Do you have a site or book that details how to calculate the frame spacing and size?
  14. John Smithson
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    Location: Kansas

    John Smithson Junior Member

    Here is a work in progress so far of what I was thinking for the ribs (still needs stringers), with the spacing 2 feet apart. The upper left is an example of a professionally designed sailboat of similar-ish size (though the one I am doing is a fair amount wider, towards the dimensions of a catboat).

    Obviously they won't all be so hollow, I still need to sort out the interior with them which will be added to the corresponding frame intersections.

    Overkill? Hard to say. I don't believe it's as built as an older fashioned solid wood boat, but it does seem more built than the similar professional craft.


  15. fallguy
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    fallguy Senior Member

    John, designing a sailboat is the thing of four year degrees or years of experience. I mean no disrespect, but truly find a designer and use your work to help you find a boat already specified. There are too many potential problems.

    We had a fellow here about 5 years back design his own catamaran. Well, there were things he did that later he was not pleased about. And they were hull related. In your case, what if the boat doesn't heel well, for example. Does that mean you only go out in 10 knots or less?

    Ad Hoc is a naval architect with years of schooling. Bajan is a surveyor whose cv I don't know. Rumars cv I also do not know, but based on his postings on the forum; he has a considerable grasp on boats. Me, just a builder. I fancied designing a small scow pram tender, but found a design and the designer gave me the plan and asked me to showcase it. Zero cost.

    I bought one book, Gerr's 'the elements of boat strength'. But I only bought it to sort of better understand the needs of hulls; not to design.

    I'm pretty sure you have more bulkheads than needed. This would drive up your center of mass and increase your vessels weight considerably. You may think you can get away with less longitudinals this way or they may not be drawn, but I doubt this is right.

    Anyhow, I really think you ought to use what you have as a springboard to find plans that are similar. Buy a study plan or two. Email the designer if you want to change riggings. Maybe even buy a plan.

    I bought a LB26 plan years ago and I still love the boat, but decided it was a bit wrong for me.

    If nothing else, you ought to find a few commercially sold plans that are similar to discuss here.
    bajansailor likes this.
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