Folding system loads

Discussion in 'Multihulls' started by tamas, Nov 11, 2013.

  1. bruceb
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    bruceb Senior Member

    structures

    Hump, I agree those are thin bushings, but they are right off the Command 10 plans, that is why I asked. I have no idea if that is the way people built them, but there are a number of them still sailing. The folding design that I am copying loads it joints more,(shorter "base"), uses larger pins (30mm), and as designed does not use any bushings.
    Once I get my geometry finalized, I do want to get someone to review the loads as I am not qualified, but the problem is whom? Most engineers, even marine specialists, have no idea what is common practice when it comes to small multihulls. I have the size specs for the pivots and fasteners for almost all the common production tri's under 35', so some reverse engineering is possible- but it would really be nice to know what is best.
    B
     
  2. hump101
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    hump101 Senior Member

    With such thin bushes the bush is only providing lubrication, it's failure would not compromise the joint integrity, just its function, and hence why it is designed so thin. It sounds as though your proposed design follows a similar philosophy, discarding the bush altogether.

    I appreciate that finding someone with small multihull engineering experience is not easy, but if you are providing the design, you just need someone suitably qualified/experienced to check your calculations/scantlings. That person would not necessarily have to have specific small multihull design experience, as long as they are able to derive suitable loading regimes based on the data provided and sound industry practice.
     
  3. bruceb
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    bruceb Senior Member

    qualified/experienced?

    And there is the problem. If I supply the design and the numbers, I have really only hired a calculator that will probably just repeat any mistakes I make. Not good, and certainly not what is needed.
    The real question is what vertical load to assign at the end of the beam. All the rest of the forces fall into place with that number, but all I have ever seen is speculation. For example, if I used your 5x factor on my 6000 lbs volume floats, the lower water stay load would be over 300,000 lbs- and I don't know of any 33' tris designed/built that strong. On mine-a very well tested design, the lower attachment is two 3/4 course thread stainless bolts. I estimate about 35,000 lbs safe load max. I/we are quite possibly using bogus load numbers, and they are definitely not to be trusted.
    I do need a real tri designer, nothing less, that can say I/we use X in that calculation. It will be well worth my expense.
    B
     
  4. redreuben
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    redreuben redreuben

  5. catsketcher
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    catsketcher Senior Member

    I feel your pain

    Gday Bruce

    I understand what you are going through in reverse engineering the folding system. It would be nice if old designers sat down and let us know all of their assumptions before we lose their empirical data. I have asked more than a few successful designers and I do think there is a little bit of fudging going on with many of our multis.

    I would reverse engineer a very successful system using what you know - Beam, weight, angles etc. Then you will be able to work out an appropriate factor of safety. Although the loads may be massive they may be reduced by
    - a veed bottom on the float to reduce wave impact
    - less overall beam
    - using solid section rather than wire with terminals
    - redundancy
    Ther are not many parts of the boat where you have redundancy and underwires/struts is one area that does. My old Twiggy broke an underwire about 2 years after I sold her. Nothing happened but the owner sailed her home sedately and replaced it.

    I would be wary of finding someone (unless it is Farrier, Irens engineer - Leveck, Carbon Peter or VPLP) who could properly redesign the part for you from real first principles based on empirical trimaran data. So do the maths on tris that work and go with the safety factors on the boats that don't break.

    If you put some data up here - we could give it a go. Take the names away and I can do my best at working out failure loads. I am not good at fatigue loads near bolts or shear on bolts though but we can work out loads on the struts and stress on successful boats.

    One boat that always gets me intrigued is the Searunner 31 - the alloy beam version has tiny struts to hold it together. They certainly do not have a safety factor of 5.

    cheers

    Phil
     
  6. hump101
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    hump101 Senior Member

    "Design" was probably not the right term to use - I was assuming you would provide the overall geometry (beam length, stay position, etc.) and then an engineer can give you the stresses in each component.
    From the numbers above you are suggesting that the geometry you have will give a load at the lower water stay that is over 8 times the force on the end of the beam. If you only have 35k lbs capacity at that joint, then the max force on the end of the beam can only be approx 4000lbs, which I suspect is less than your max static load, so something is not adding up.

    As Catsketcher has said, if you can give us your actual geometry we can all have a stab at it. It won't replace your designers advice, but will be another point on the graph.
     
  7. bruceb
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    bruceb Senior Member

    Real data

    Thanks guys, I think we are getting somewhere. I can't drive yet, so the final measurements will have to wait till next week.The system that comes closest to a direct fit is off the "new" Scarab 32. I purchased a set of his plans- and they are nice! The beam length, boat weight and inner beam base are very close to the Buc 33s. It is an untried size, but his boats have done fine so far as I know. I have already mounted a full scale beam side and pivots/arms to the beam bases on my boat, and it is a good fit. I have a slightly taller beam base so I am planing to use all of it with the arms and beams adjusted to fit. I will have to construct an adapter "bracket" to fasten to the Buc's existing beams- a 5"x8" alloy pipe at the top, and a 1/2" x 2 1/2" stainless bar at the base, 14" center to center. They go across the boat, and are extremely well attached. I calculate that the loads are going to be about 30 % greater than the Buc's original design, with the lower arm attached to the beam at about 50% where the standard water stay attached at about 80 % of beam length. (Hump, that is the difference in loads- I think it puts the number closer to 7000 lbs although I don't have an exact measurement). I think the cross structure is plenty strong enough for the increased loads, but the attachments will have to be designed correctly.
    IMO, the boats I have measured seem to use a strength factor (vertical load) between 1 and 1.5 of the boats displacement or float volume, which ever is less. Strictly reverse engineering, I have no facts to work from:(, but I have measured every tri I could find.
    For this to work, I have to build a one piece bracket that has the upper and lower pivots eyes built in- I think best done in fiberglass, so I do have to get the loads correct.
    Thanks for the advice so far, and I am still hoping for a designer to comment:cool:
    Bruce
     
  8. catsketcher
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    catsketcher Senior Member

    Could you do a diagram of what you want to do and another of the other boat you know works well? Then I can have a go at the loads.

    One thing that may be hard is to change the Bucc beams from full length waterstays (where the beam acts only in compression) to half length - which is a much different kettle of fish. Then again if the other tri is the same we can go from there.

    Phil
     
  9. bruceb
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    bruceb Senior Member

    drawings

    I will try to post something more accurate soon, but I have to get a couple of measurements first. I am planing on using the scarab design fiberglass beams so strength shouldn't be an issue, but the pivot hole placement and the arm length will be a little different. The metal beams would not work. I need new float bulkheads anyway, mine were rotten, so placing them as necessary for the Scarab design won't be a problem or any extra work. My floats are almost the same size as the 32's.
    B
     
  10. tamas
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    tamas Junior Member

    I thought I would share the strut system size I have. The boat will not be taken too far off shore, generally passage and bay conditions and we don’t race if the wind is over 25knts. That will give you an idea of the loads and stresses I plan to encounter. Working from catsketcher figures I think I have about 7t load (to be shared by both arms), if I read that right it will be about 4t front and 3t back, load on the beams?

    The strut and bracket size is greater than a F22. I have compared the both. My brackets and struts look big and probably are for the size of the boat. See some pics here. http://husky6.blogspot.com.au/

    I still can’t work out the inward load on the 12mm bolt I have that holds the Vaka end of the arm down. The compressive loads I have been told (as per first post) are great. Would the bottom strut take up some of the loads to prevent a lot of the inward pressure? My top control bracket (arm whatever it is called) is not structural but will take the ama weight as it folds. So is a 12mmm bolt enough. I can easily work out a system so the load is transferred across the boat to the other beam. Would that be recommended?

    Regards and many thanks
     

    Attached Files:

  11. bruceb
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    bruceb Senior Member

    compression

    Tamas, do the inner end of your upper beams "butt" into anything to take a compression load? I know I don't see anything at the rear. If not, your upper arm will be taking most of the compression- quite a large load, particularly at the front. I "think" the 12mm bolt just locates/ holds the upper arm in place, as it would not be large enough for the shear loads. It shouldn't be hard to add a compression pad if necessary, although your arms look pretty robust as they are. The lower struts are about the same size as an F-24s, a much heavier boat and should be plenty strong. I have no idea on the brackets and the upper beam.
    B
     
  12. ChineTri
    Joined: Mar 2013
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    ChineTri Junior Member

    Thanks

    For some time i've been working on the design of a 23 ft trimaran, something like f-22 / scarab-22. It will be a multichine foamcore build. I modelled all hull and interior panels in freeship, and made a 1:5 model to check if there are any thinking errors, see attached photos.
    The current design phase is working out folding strut geometry and calculating strengths of parts. I was just making my mind up if I should ask a question on this forum, when I found Tamas' question and catsketchers answer with load analysis PDF in particular; exactly the anser to my question. Thank you all for sharing your knowledge on this forum.
    I try to document my design process, but find little time. What I have is on https://sites.google.com/a/tammingas.nl/trimaran-design/home
     

    Attached Files:

  13. bruceb
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    bruceb Senior Member

    model

    That is a nicely detailed model and it sounds as if you have thought it out well. A couple of thoughts- A "family" and your supplies, even for day sailing, is going to be a very full load on a 23' tri, (750 lbs ?) so really watch your loaded weight and structure calculations. It is hard to keep the weight out of the one-off build, and you really do need to plan extra layers in the bottom. A "ready to sail" C-24 weighs about 1800 lbs, and it will be hard for you to build even 500 lbs lighter. ( Or leave the kids at home :rolleyes:)
    I think I would mock up a full scale interior with cardboard, you may find you want to keep it more open.
    Start building! :cool:
    B
     
  14. ChineTri
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    ChineTri Junior Member

    Earlier this year I measured the lower folding strut of a homebuilt F-22: it is composed of 2 parallel aluminium rods of 14 x 23 mm. On wikipedia I found the yield strength of aluminium to be 110 MPa (6061-T6) or 55 MPa (6061-O).
    I interpret this to mean that you can apply a force of 110 N per mm2 without permanent deformation in the material. I would guess that Farrier used the stronger version. This would also be the safest choice when reverse engineering.
    Multiplying this 110 N with the measured area of the struts (644 mm2 for the two rods) gives a maximum load of 70840N for the lower folding strut.
    This is approximately what catsketcher calculated, but without the proposed factor of safety.
    Is my calculation sound? And what is your opinion on the calculated value with regards to factor of safety?
     

  15. hump101
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    hump101 Senior Member

    You've not included the vertical distance between the beam where it hits the main hull, and the lower strut. We need this distance to do the calcs.

    Also, when calculating stresses in your strut, you can't use the strut total cross-section, you need to use the cross section remaining where your 25mm stainless bar passes through, with a stress concentration factor of about 1.5 to account for the effect of the hole (the stress will not be evenly distributed across the remaining section).

    Additionally, whilst it is clear your 12mm bolt in shear will not cope with the loads on its own, you also need to consider the structure of the main hull that is going to absorb this load. Just bracing across to the other beam is not the only requirement, as the force is not being generated just by the outer hull on the other side, but by all the mass of the boat, and its cargo, so the main hull will need some longitudinal structure to follow this load path as well.
     
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