Folding system loads

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

  1. bruceb
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    bruceb Senior Member

    Real numbers

    Thanks H, I think that is a good idea. I may still have to use the current model, and even if not, the attachment points and the beam length are going to be close to the same, so I don't think the loads on the lower pivot will be that much different. And as you suggest, it will be a good baseline to compare others to. I have already learned a lot from this exercise, and I haven't even drilled any new holes in my boat- yet.
    B
     
  2. hump101
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    hump101 Senior Member

    FEA Rev 0 results

    Hi Bruce, trust you had a good holiday/New Year. Here are results for the rev 0 model. This has 6000lb applied at an angle of 20 degrees from the vertical at the end of the beam, representing 20 degrees of heel. Stresses are plotted in MPa, yield for 316 is around 200, so I've set the upper contour limit to that. Everything that is grey is outside the range, so higher than 200MPa. The beam is removed for clarity.

    The very high max is a modelling anomaly due to combining shell and solid elements, so can be ignored, and also the pins are solidly fitted into the plates, so non-conservative.

    Clearly with this geometry the pins, and the adjacent plates, are overstressed. Particularly the outboard pin on the upper arm, which has the most bending moment in it. This pin would definitely require better support, even for this low load.
     

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  3. bruceb
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    bruceb Senior Member

    New year, better design?

    Thanks H, I hope you did too. I had a good holiday, and we are almost cleaned up from our celebrations. ;)
    I think it is time I re-worked this, the loads are just too high. The pin loadings are a bit scary. If I used any larger parts, the structure starts to look more like a piece of earth moving equipment :mad:
    I have been "messing", and I think I can eliminate the upper arm as a loaded element- as you suggested. I will try to have a drawing posted tomorrow.
    Thanks, Bruce
     
  4. hump101
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    hump101 Senior Member

    Yes, thanks, we're just getting back into normal service.

    A major problem with this geometry is the bending moment induced on the pins due to the gaps between the braces and the beam. If these were reduced to just a tolerance, then the pins would be in near pure shear, which would dramatically reduce the stresses in them.

    However, certainly the loads are very high in the pivots, bearing in mind that this load is only your static maximum, not including any dynamic element.
     
  5. bruceb
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    bruceb Senior Member

    New year, new plan

    :cool: H, when the beams/arms are actually finalized, I should be able to keep the parts much closer together, and I might be able to add support on both sides. I understand the need, but first I have to get the overall geometry working. Maybe just ignore the pin "bending" until the rest is right.
    This set of drawings are based on an "F boat" style beam with set of compression pads at the inner beam tip and very little load on the upper folding arm. It will require a kink in the upper arm for pin clearance when folded. There will not be much control when completely folded, and will probably need some control lines/tackle added. A longer upper control arm would have better geometry, but would require cutting into my cabin- possible, but I would rather not.
    I used a better metric tape, so I hope the dimensions are a little more accurate than last time:rolleyes: The lower arm and its position are correct, but the upper arm and its hole placement might still need some adjustment. The current prototype beam articulates very well, and keeps the float well positioned through out its travel.
     

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  6. tspeer
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    tspeer Senior Member

    A Farrier design has the floats passing their lowest point somewhat short of fully folded. The buoyancy of the floats then pushes them into the hull when passing this point. It's basically an over-center arrangement. A pin is then used to secure the hull in the fully folded position. But this is mainly protection against accidental un-folding due to wave action.

    Besides the loads on the system when sailing, the loads on the hulls when folded have to be carefully worked out. In a Farrier design, the amas are not above the waterline when folded. They contribute a small amount of buoyancy to the boat, lifting the main hull a little. However, they can't be too far down, or it becomes too difficult to fold. Folding the first hull is easier than folding the second hull because the boat can heel toward the extended ama. Folding the second hull feels distinctly different from folding the first hull because of the extra push needed to complete the fold.

    The trajectory of the amas in the final part of the folding cycle drives the vertical location of the beams on the main hull. There's more to it than just the extended and folded positions. If you get it all balanced correctly, no tackles or additional lines should be necessary to get the boat folded.
     
  7. bruceb
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    bruceb Senior Member

    Farrier folding

    Thanks Tom, You are correct, I do understand the Farrier system, I should have said I was mostly referring to the folding arm attachments and position.
    I am retro fitting this to my boat, and I am trying to use as much of the existing structure and mounting points as possible. Since my boat was never designed to fold, especially the float profiles, some of the Farrier geometry will not work as well on the Buc, but I think it will be acceptable. I have tried to keep the float immersion volume change to a minimum through the folding cycle, but I expect to to require some "assistance". Mine also goes over center at the top, but the upper strut does not have as great of angle of attack as on a Farrier, and my boat and floats are heavier. I have a construction laser set up to track the waterline position on my prototype, and the float immersion seems to be ok, but I am unable to calculate how much the boat will heel during folding. I don't have plans or drawings of the 33, so I am sort of guessing at some of this. I know I am close, but a few degrees of heel can make a large change in folding loads. Once I can mount my new model on the actual boat, I will have a much closer idea.
    Bruce
     
  8. bruceb
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    bruceb Senior Member

    getting closer

    Sorry for the delay, but our weather has been keeping me inside. I think I have my geometry finalized though, and I am posting the latest version. I have most of the prototype ready to try on the boat as soon as I get a nice day:)
    These dimensions were taken off my full scale model, and it seems to work well. I have only shown the critical changes from the last drawing- mostly the upper arm and more accurate measurements;)
    Bruce
     

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  9. hump101
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    hump101 Senior Member

    I'll draw that in CAD and see how kinked the top arm needs to be.
     
  10. bruceb
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    bruceb Senior Member

    Kinks in the system

    Thanks H, On my folding model, the upper arm needed about a 30 mm kink to clear the lower arm pivot pin when the float was completely retracted. The float/hull clearance was the limiting factor. At that position, the overall beam of the boat/floats is about 11'7" (3530 mm) , within our 12' wide trailer restrictions.
    This is almost a copy of an "F" style folding system, with a shorter upper arm that is not intended to carry sailing loads. I would have to cut into my cabin and the crossbeam for a longer straight upper arm.
    I plan to use 3/4" (19mm) upper pivot pins, the same size that an F-boat uses, and will probably need larger ones on the lower arm. I did not show all the beam structure in my drawings, but it is thicker in the area of the pivots with very little offset between the lower arm and the side of the beam, just enough for a bushing. The beam sides/web in the pivot area is over 20mm thick on the Scarab beam design that is the basis for mine. This design also allows a short C section metal channel to be added inside the beam for added support for the lower outer arm pivot pin.
    B
     
  11. bruceb
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    bruceb Senior Member

    New test

    I finally got some nice weather. The two beams seem to work fine together, and everything lined up. The beams are mounted at the correct height, but are positioned 9 inches outboard for the photos. I didn't want to cut the boat until I was sure everything fits. ;) At the proper mounting points, the boat will be under 12' high and wide with several inches margin, so legal to trailer with a permit.
    I am ready to take my first float home and start installing bulkheads, and the beam profiles in the photos form the two sides of the beam mold. Of course, they get flanges so they will have a little deeper section than in the pics.
    Bruce
     

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  12. outside the box

    outside the box Previous Member

    Legend BB looking great.
     
  13. bruceb
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    bruceb Senior Member

    Thanks!

    It was quite rewarding to finally get it mounted up and working. :)
    B
     
  14. hump101
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    hump101 Senior Member

    Looks good Bruce. Does this match the dimensions in the previous post? If so, I'll model it. I've been tied up with real work again but have some spare time coming up.
     

  15. bruceb
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    bruceb Senior Member

    Dimensions

    Yes H, jobs do have to come first, I can't work on my boat full time either. ;)
    The dimensions are the same as my last drawing, I used them for the proto in the photos. The beams swing/fold very well with no binding and seem well controlled.
    I still have a 36mm kink in the upper arm, but it is not intended to be loaded except for folding. I think 19mm/3/4" pins will be fine on it. I am not sure on the lower pin/pivot size and structure.
    I am hoping to use 1" pins on the lower strut, but I have left room for 1 1/4"-36mm pins with bushings. I can support the pins on both "sides" of the folding strut arms if necessary, but I need some idea of the load to size the pins and lower pivot structure.
    Corsair uses a 3/4" pin (sometimes titanium) on their 32's, but the 32 is a lighter boat and the their load geometry is somewhat better. Their pins are well supported, but I think I can do as well.
    Just post if you find any errors and I will try to find and correct them.
    Thanks, Bruce
     
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