Structures - Composite Boat Framing System

Discussion in 'Boat Design' started by ToMeK, Feb 18, 2022.

  1. ToMeK
    Joined: Nov 2006
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    Location: Zagreb, Croatia, Europe

    ToMeK Young naval architect


    To warm up weekend atmosphere a bit I am starting discussion on framing system in small composite pleasure boats. I did quite a research in several books and articles published around, and I am very interested in your opinion, experience and comments on this topic.

    Whats is your opinion how should good structure look like?

    I made a simplified concept model of two systems I find interesting to implement in composite planing motor yachts. In this thread I will refer to composite boats (sandwich and single skin bottom) 35 - 60 ft in length.

    Transverse (mixed) framing system


    Longitudinal framing system


    What I would like to hear from you, which system do you find better for which purpose?
    When designing do you respect hierarchy?
    How do you implement this to whole boat concept? Do you have clashes with interior and how do you solve it? Do you implement this hierarchy in structural liner as well?

    Why I started this discussion? - for those who are more interested in this problematic :)

    As naval architect working mostly on structural mposite design, and even more as skipper sniffing around every boat I worked on, I noticed that most small boat structures differ A LOT from what I was thought in university classes as clean structure. Most of the boats were modular built using glued-in structural liner, module liner and ceiling. Classic for charter.
    All of those boats had only two bulkheads: collision (anchor) BKHD and engine room BKHD that were laminated or glued to hull and deck (or in some cases ER BKHD was glued on top of transvers frame. Rest of the BKHDS were interior plywood bkhds (15-20mm) sitting (and glued with sika) in module liner and ceiling liner grooves. Sometimes those grooves were not even glued to deck, but rather "staying in the air" between ceiling liner and deck (Liner was localy glued to deck, but not near bkhds grooves. I believe this cannot be considered as structural bkhds as they are not firmly connected to hull and deck in order to transfer loads and stiffen hull for torsion. Also most of the times, this bulkheads had very large openings leaving only 20-30mm of plywood between door opening and ceiling. Lately, analysing GA's on internet it is evident that bkhds are not even aligned in transversal direction, but rather going "zig-zag".
    Considering Framing System, most of the systems I managed to find in internet (besides maybe Safeheaven Marine, and only few of others) consist only on bottom stringers net, with no hierarchy at all. Sides are usually left to be stiffened by interior bkhds only in the front part of the boat, Engine Room topsides are usually left with no frames at all (on bigger boats sometimes aft engine room bkhd can be found if shaft propulsion is used.). Bottom stringers usually are not straight in top view but going "zig-zag" around tanks and other systems (Pascoe [3] call them "dog legs"). Also In profile stringers usually change height abruptly (crown is going "up-and-down")
    Considering deck structure in small boats, deck is usually so corrugated (benches, steps, ... are already moulded into deck geometry) that it is even impossible to implement any transverse structure besides bkhds. In my opinion this is ok situation if 4 -5 transverse structural bkhds are used through the boat length. Gerr [1] mentions "ring frame" which includes strong beams on bottom, sides and deck, all aligned and connected with brackets forming ring. I never came across place on the boat where I could implement one.
    Another thing I found a bit unusual, is alignment of vertical structure of e.g. superstructure and/or flybridge. In "standard structural design" and logic it is a MUST to support superstructure pillar that is holding flybridge structure by structural bkhd or at least "ring frame" (achieve alignment of flybridge pillar with deck girder and topside frame that are connected by bracket).
    Between decks (or flybridge decks) and ceiling liners is noticed some small plywood pieces that are are laminated or glued to both deck and liner and are supposed to have function of "structure". Similar "structural pieces" I used to find connecting hull bottom to module liner in both longitudinal and transversal direction and acting as stringers or frames. Usually 20-30 mm thick, overlaminated to hull.

    There are many more things I have seen that are not described here but seem quite unusual and not capable of acting as structural members on boats on 10 - 20 m. I would like to hear your experience and comments on this and similar examples.

    In my structural classes we were taught 3 basic principles every structure should comply with:
    1. continuity (no sudden changes in geometry - heights and widths)
    2. alignment (all members should be aligned and following each other)
    3. capability (capable to withstand applied loads)

    Rest of the literature I studied about this topic that led me to upper posted pictures:
    1. Gerr - Elements of Boat Strength
    2. Scott - Fiberglass Boat - Design and Construction
    3. Pascoe - Surveying Fiberglass Power Boats (Marine Surveying : Hull Design Defects - Hull Failure Part I - Boats and Yachts Surveys
    4. ISO 12215
    5. LR SSC rules

    I know post is too long, but seems important :) Looking forward to your comments and experience!! :)

    Kind regards,
    Last edited: Feb 18, 2022
  2. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect


    Structural design is – if taught and understood – straight forward and simple.
    Why?..Because no matter what material you are using the first principles are always the same.

    Primary, secondary and tertiary.

    How you arrange the structure, which is your choice as the designer, shall ultimately dictate which becomes the: primary, secondary and tertiary member.
    In simple language, what is supporting what?

    This can only be done once you have decided/selected what is the span and area of support. As this governs everything and the arrangement follows from this analysis.

    Take an Engine Girder, for example.

    If you have an engine of weight X, how do you support it in the ER?

    Well, you have girders, or do you??...the girders could be, could be, the primary support, or they could be, could be secondary support. It all depends upon how you decide to arrange the structure to support it, which could be dictated by factors, other than, simple spans. Head room, or chines, or some other feature.

    If that engine of weight X, is on a longitudinal girder, what is supporting that girder?

    So we can analysis this in the simple classical case of a built-in beam with a point (or small udl) load.
    The span, in this case, will be from WTB to WTB. The WTBs are the support for that 'beam'.

    If the modulus, that you calculate for strength and defection is excessive, then this may lead to an increase in weight and/or take up too much physical space/room, that prevents other items being installed in the ER.

    In which case, you then change your method of support.

    So, how do we do that… simply by making the long.t girder to be supported by the transverse frames?
    In this case the transverse frames become the primary member. And you analyse accordingly.

    It is all about understanding the spans and area of support, no matter how you arrange the structural and whatever material you use.

    Once that is done, you analyse your structural arrangement against the basics criteria to satisfy of: strength, shear, deflection and fatigue....all the time whilst being fit-for-purpose.
    fallguy, baeckmo and bajansailor like this.
  3. ToMeK
    Joined: Nov 2006
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    Location: Zagreb, Croatia, Europe

    ToMeK Young naval architect

    Ad Hoc,

    thank you for your reply. Of course, as you mentioned, calculation is straight forward. It is method that is learned and straight forward in theory.
    What I wanted to discuss is implementation of this method, from theory to practise.
    As I mentioned, many many times I have seen that structure is way out of what is written in rules. And somehow, boat gets certified for structure. In example, I have seen structural calculations carried out in Hullscant where transverse members are half height of longitudinals, but in calc report calculation length l for longitudinals was taken between those two transversals. In general, all spans (long and transv) were taken between cross sections, nevertheless of hierarchy. To make it more interesting, one of transverse members was splitted in two pieces midways between keel to chine (because module liner was only around 1 inch from hull bottom). Is this someting that is normal and common (good) practise, thus meaning I am too strict and not flexible since I am trying to do it 'like nerd'? :D
    Considering rules, ISO gives this guidelines for calculation of stringers using hierarchy, but for example I haven't seen anywhere giving any suggestion on how many bulkheads to use along length, and what can be considered as structural bkhd. In my opinion, not every vertical plywood plate can be considered structural bkhd. And many times I have seen similar examples.
  4. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    The "modern" high production line methods of building don't violate any structural rules. The structure needs to conform to the calculated loads, but the way it does this is subject to manufacturing limitations. These can be technological or financial, and are not always obvious or constant.

    Example: You need to design a framing system to satisfy load x. The expected number of boats to be buildt is 10. The manufacturer determines that it is not economic to develop tooling for grid liners and interior modules. Consequently the framing will be buildt over commercial foam formers and laminated in by hand and the bulkheads and interior will be CNC cut flat panels. In order to cut down on tabbing time the interior will be only spot glued to the hull, not fully tabbed in. This forces you to a certain approach, for example you can not use the furniture as stiffeners and your framing needs to extend to the topsides. The amount of fiberglass in the framing will consider the type of foam used as a core and since the formers come only in certain standard sizes you must also accommodate this. Depending on the imposed build sequence the longitudinals can be continuous with the bulkheads notched over them, or discontinuous.

    Let's say the builder determines 50 units will be buildt. Now you need to redesign the framing to use liners, since this is the cheaper build option. Grid liners are air cored stringers, so the fiberglass content changes because there is no foam, but you are not tied to predetermined sizes anymore. The longitudinal to transverse framing intersection needs redesigning with a large radius and more glass because only the top flange will be continuous. The manufacturer wants most bulkheads glued in after the liners, so you need to do something to compensate. The furniture modules are now fully glued to the skin and can be used as stiffeners, there is no need for framing on the topsides, so the furniture is buildt with different scantlings, etc.

    In both cases the installed structure satisfies load x, but in different ways and with different materials. A premade grid liner glued in with plexus or similar will be just as good as the wet laminated foam core stringers and frames. The grid liner will weigh more and will also cost more in materials (more glass, resin, plexus, plus tooling). But the reduction in work hours to make and install the grid liner vs. wet laminating the framing will make the grid several times cheaper, since man hours cost a lot more then materials. This is even more so when you need a good cosmetic finish, with a mold you only sand once.
    There is of course a difference between the two approaches when it comes to repairs, accessibility and other such things, but those are only considered if they are mentioned explicitly in the SOR. Otherwise it is perfectly acceptable to sacrifice them, the business model is similar to the automotive one, if the repair exceeds the insured value the boat is scrapped and the customer buys a new one.
    Material choices are similar, lifetime expectations determines what gets used. For example brass trough hulls wich need to be changed every 5 years, or the use of stainless steel instead of monel or titanium for fasteners and fittings, but also less obvious things like cushion fabrics.

    The goal is always to satisfy the customer SOR, not to make the ideal boat with the ideal material and building technique. If the customer only has fiberglass, polyester resin and balsa, you can not specify epoxy, carbon and foam, even if those would make the boat "better".
    baeckmo, Ad Hoc and DCockey like this.
  5. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    There is. Look at the minimum panel length rule. "l need not be taken > 330 x LH, in millimetres." If you decide on the spacing of the bulkhead and the plate length shows you are violating the rule, then you must subdivide the spacing of the bulkheads by adding transverses as in your illustration 2. A bulkhead is a primary structure. Transverses are primary structures as long as it supports the longitudinals.

    In your illustration 1, the longitudinals are supported by the bulkheads, thus this longitudinals becomes primary. The transverse becomes secondary. It is there just to stiffen the plates.

    Your opinion will not matter because ISO says the rules are for structural bulkheads (watertight or non watertight) and gives thickness requirement (11.8). The rule states that "non structural bulkheads are outside of the scope of this part".

  6. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    And, so what?
    If the structure is arranged that can be justified to support the applied loads and transfer the load from member to member and be below the stress, shear and deflection limits, so what?
    Why MUST it conform with what is written exactly in the rules of how structure is arranged?....that merely shows a lack of understanding of the theory and how to apply the theory in practice.
    Thus one becomes a rules and rules only based designer...plug in the numbers and out pops the answer... anyone can do that!

    Exactly..and fully concur with Rumar's comments.
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