Folding system loads

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

  1. bruceb
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    bruceb Senior Member

    dimensions

    H, I will recheck my measurements. I wish my cad skills were better, but I have no other need to use cad, and I make too many input mistakes. I took the measurements from my full size folding mock-up, and it does work, but there is some slop in the pivots that might account for the errors. I have even hung it on the boat to try it. My first drawing did have an error, but I thought I corrected everything on the last one. I will improve the pivots today and see if anything changes, and redraw the diagram.
    I apologize for the lack of accuracy.
    B
     
  2. hump101
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    hump101 Senior Member

    No problem, just don't want to spend time analysing a geometry that is not valid. It is very sensitive to dimensions, so some slop will greatly affect the results. If you can give me some accurate dimensions of the float and waterline relative to the pivots I'll plot them so you an see the required dimensions.
     
  3. catsketcher
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    catsketcher Senior Member

    I don't get it

    Bruce

    As I see the structure you don't need the struts after the arm is extended. Why have them load bearing at all in the extended configuration? Could you not bolt the top flange (at least) on the top of the stub so that the upper arms are taken out of the equation when sailing? With a compression pad and bolts to hold the beam down it should be a very tight fit.

    I would make the compression pad by coating the beam end in wax and then squishing the beam end into a socket filled with epoxy glue.

    I would also consider altering the straight line at the end of the stub to have an angle or wiggle in it so that the shear force (lifting the beam up) would be resisted by the way the stub and beam end come together. Again mould these ends in situ so they fit exactly. This is what I do in my folding cat with its geometry and I am happy with the tight tolerances this produces.

    I would be worried with the current geometry that the beam will slide up the stub with any slop in the system (and there will be slop). Then you will get the struts taking greater load and maybe end up with a failure mode from out of tolerance loading.

    cheers

    Phil
     
  4. bruceb
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    bruceb Senior Member

    New model

    Definitely don't waste your time trying to analyze my mistakes. :( I will get it correct. I am building a new mock up with fresh pivots to take out as much slop as possible. I should have it ready in a day or so and I will post then. I have been using my construction laser as a waterline, so I will have that dimension also at any point of the fold.
    The B-33 only has about 24" under the wing to the water, and the floats are 30" wide at max beam, so a substantial part the the floats will be in the water if the boat is completely folded. Of course, they would not go that far without some force, but that is ok. Even if it only folds to about a 14' beam in the water, that will allow it to dock in most slips, or it could be winched tighter. There is plenty of height on the trailer for it to drop the rest of the way for transport. I have about 40" from the bottom of the wing to the top of the trailer fender.
    B
     
  5. bruceb
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    bruceb Senior Member

    upper beam bolts

    Phil, on the S-32, the beam does bolt down to a pad on the top of the upper arm, making the upper arm/beam one part for load purposes. (except for any slop) My geometry allows the bolt pad to be almost over the upper inner pivot, and it would certainly be bolted when sailing. The only movement should be at the pivot pin, and it can be a good fit.
    Practical considerations make the pad/socket system a little harder to execute on my boat. My upper inner pivot is in the end of the oval 6"x8" alloy cross tube (pipe) on the hull side and the arm fits in easily, but as designed, the beam end has to be cut away for clearance so there isn't a good place to engage the beam mount, and the bottom of the beam has to be open for the folding arms. The alternative would be a much more heavily built beam end with a light arm only intended to take folding but not sailing loads. The fasteners/bolts would have to be BIG. On the S-32 design, the upper and lower swing arms are of equal strength, so I guess they are expected to have about equal loading.
    I haven't started building anything yet, just mock-ups, so any details can be changed for the better.
    Bruce
     
  6. catsketcher
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    catsketcher Senior Member

    Gday Bruce

    I am not an engineer but I have designed and built a couple of folding cats. My new one is getting very close after years of work so I get folding and I love tris.

    If the struts are the same scantlings we can safely say that they can only take the same load in tension or compression. It seems to me that the bottom strut can only be in tension whereas the top one may be in compression. Going into compression will totally alter the design criteria for the strut and will make it much much bulkier although maybe not that much heavier. So I think the struts are designed this way for ease of manufacture or over designed BOTH for compression. (Compression is usually the earliest failure mode)

    I still don't get why the upper strut has to be designed for the sailing compression load if it gets bolted. Don't worry about explaining, I will wait for some more drawings and see if I can follow it through. If you could make a model in thick cardboard (use fine pattern grid paper glued on) or even thing plywood you could see if you can close up the beam ends and strut ends so that the shear load is dealt with by a close fit and some interesting shapes.

    I have an idea that may be useful. Imagine a protuding nose (a bolt or pin) on the bottom of the folding beam. As it finally folds into the stub end the nose fits right into a corresponding socket. The nose then takes the tension of the lower flange and if husky enough take some shear load the the join. This will be very easy to build if you make the nose out of tapered alloy and the socket out of reinforced epoxy (again coat the nose in wax and squish together). If the boat is not going to be folded often I would jump in the dinghy and put a nut on the nose - picture a bolt with a tapered plug on its shank. Really the nose should be a wider wedge with a bolt inside (wedge would entail lower stress at the join)

    I can draw it if you like.

    cheers

    Phil
     
  7. bruceb
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    bruceb Senior Member

    Folding needs

    While I have the 33 on my local lake, I don't expect to fold it very often since I have a wide enough slip. However, I do plan on trips to the coast with it, and then folding easily becomes a much higher priority. On most of the US eastern seaboard, dock space is at a premium, and wide boats are not very welcome, (at least ones that don't have big fuel hungry engines) so folding to dock opens up many more practical marina options. As the S-32 system is designed, I think folding should be less than 5 minutes a side, (remove 4 bolts from the top and slack the outer stay), and I consider the ease of folding one of my most important goals. Remember, I am just copying a system that already works.
    Since the upper inner pivot and arm becomes an extension of the beam when bolted, I don't see why there is a need for a "socket/pad" that just duplicates the upper pivot's load path. The inner end of the beam can't be any longer either as its folded height becomes excessive.
    I am not trying to re-engineer the S-32 system, as it appears to work fine, but to determine the loading and changes necessary to fit it on the B-33, and then design and build the hull "brackets/pivots" that adapt the system to the B-33 existing structure.
    Of course, as with most things on a boat, nothing is ever very simple:(
    B
     
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  8. bruceb
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    bruceb Senior Member

    new numbers- and pics!

    There was a dimension mistake on the lower arm:mad: I will try not to do that again. I am only using inches, I realize the rest of the world is ahead of the US, but my smaller local machine shops only think inches and feet, and all materials around here are SAE, so hopefully this will save some errors.
    I have removed almost all slop in the prototype joints, so this is fairly accurate geometry. However, I have found that the system will still work with errors as much as a 1/4 of an inch. It is somewhat forgiving.
    My float mounting system will allow several inches vertical and horizontal alinement during final assembly, so final beam position is not too critical either. I realize any load calculations are based on a "snapshot" of guesstimated loads, but the lower pivot seems to be the most important from a construction standpoint.
    The blue tape in the pictures represents the waterline. Completely folded, the beam will be less than 12', important for transport. At just under 14', the float decks (and hatches) will be clear, and should be fine for docking and motoring. My pictures also show mid fold and extended. The float profile is at the forward beam, the maximum draft is more aft, so some of the float will remain in the water. (the same as designed)
    If it ever stops raining, I will mount this on the boat for a final test.
    I hope this works, I am ready to start building;)
    Bruce
     

    Attached Files:

  9. hump101
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    hump101 Senior Member

    I've plotted this into CAD, and there are small inconsistencies of between 6.5mm and 8mm, so not critical from an analysis point of view. With this arrangement the brackets can toggle in the fully folded position. You need to decide how far you want it to toggle as this will dictate the required bow in the upper arm (though you could modify the connection detail and avoid any bowing, which would be better in a compressive member).
     
  10. bruceb
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    bruceb Senior Member

    progress

    Thanks H. If you could note where you are seeing the mistakes, I will try to fix/adjust for them. I have a "Cinderella" CAD program that I can't draw very well with, but I can use it to open some Cad files if that would help.
    I am prepared to make a set of light weight alloy arms and an accurate base plate, which should take out almost all play and then recheck dimensions. The extended measurements are not going to change enough to matter for load calculations.
    With the current geometry, the upper arm "barely" clears the outer lower arm pivot when it is folded/toggled in all the way, or at least compressed enough to get the total folded beam under 12'. As you can see, small changes to the pivot locations can amount large differences in folded width, and my prototype may not be accurate enough yet. As is, it appears that a single alloy square section might work best for the upper arm, with the pivot holes offset slightly to the bottom, a straight section would work. 6061-T6 heavy wall square and rectangle alloy is readily available here, could be very robust for a small weight penalty, and might be the easiest/best material choice.
    Bruce
     
  11. bruceb
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    bruceb Senior Member

    proto pics

    This was my first attempt, the pics on the bench are later with different length arms, but this should give an idea of how it works. (This is the same beam)
    B
     

    Attached Files:

  12. hump101
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    hump101 Senior Member

    Attached is my CAD file. You can see the discrepancies at the upper holes. They are drawn based on the vertical offsets, and the beam lengths, which then gives an error in the distance between the two upper holes. As noted earlier, I don't think this is an issue in respect of the structural analysis.

    I'd like to do the FE model in the next couple of days, so if you have a decision about what sections you want to use I can include them.
     

    Attached Files:

  13. bruceb
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    bruceb Senior Member

    more choices

    Thanks H, I think I understand the discrepancies, I will see what I can tell on my prototype. I know there is an issue there, but I can't tell where/how much. Can your CAD program show numbers? I can only open it but not measure/modify the drawings. I also agree, I don't think they matter for your calculations.
    Lets try a 3"x5"x 1/2" wall 6061 T-6 aluminum rectangular tube for the upper arm- (5" vertical) instead of two steel links. It would fit better and seems about right for the bending load. At least a starting point.
    The lower arm still needs to be two separate links for clearance and pivot mounting. The S-32 specified 12mm plate, x 60mm, with 100mm ends in the pivot area, (a pair) with a welded spacer in the middle, in 316 stainless. That is a fairly expensive arm to build, I am open to suggestions.
    The pivots are assumed to be 1 1/4" stainless bar, with the fixed supports as close as practical.
    I realize you can only calculate based on static loads, but I think in the real world when the boat is rolled up on its side, both upper and lower links will see both compression and tension as the boat moves through waves. I would prefer a comfortable safety factor :)
    B
     
  14. hump101
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    hump101 Senior Member

    Here's a copy with dimensions on it. Unfortunately I draw in mm, so have converted all your imperial to metric. I've also attached my conversion sheet.

    We can calculate based on dynamic loads as well, including inertial loads, in irregular seas, so once we have a model we are not limited to static loads. However, dynamic analyses are computationally expensive, time-consuming, and to be meaningful would require a fully defined problem, which I doubt you will be able to develop or benefit from. Simpler to use a base load case, with sensible dynamic load amplification factors, and to check all components in compression/buckling and tension. We would do this, in any case, prior to doing a dynamic analysis, so it is a sensible starting point.
     

    Attached Files:


  15. bruceb
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    bruceb Senior Member

    Loads of beams

    Ok, I am sure I can fix my measurements with that. :)
    Static is fine, and way more than I ever would have had on my own.
    H, I really appreciate your effort on this. I have been on several multihull forums for the last five years, and I think this is the first qualified engineering beam analysis I have ever seen posted. I am sure that that there are several people watching the results. :cool: Sorry about the imperial measurements.
    I will spend the rest of the day rebuilding/adjusting my prototype, and see how it turns out, but I think it is ready for analysis now.
    Thanks, Bruce
     
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