A Frame Masts

Discussion in 'Multihulls' started by Gus7119, Jan 20, 2016.

  1. Gus7119
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    Gus7119 Senior Member

    That sounds interesting. Id love a wiged rig but seen a few diffrent designs. One that was a wing like structure but looked like a junk rig with two sides to thee sail.
    There was a design that was put up Ill try and find it, for the ultimate rig to reduce stress it was a frame with all four points like a tripod braced on the hull with a triangular frame with mast on top.
    Anyway thats a little diffrent but your idea sounds interesting. Theres heaps of really smart people and experts on rigging on this site draw up your design on a piece of paper and let us have a look. Id love to see it.
     
  2. Gus7119
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    Gus7119 Senior Member

    Possable comparison on two 14ft cats worth it or not......

    Hey fellas
    Jeff hadnt noticed the rig on the test.
    And I thad the same thought. Ive found a 14ft cat that the owners happy foe me to take if I junk it. But the I found 2 14ft cats from an old hire company in really bad shape but hulls all good for 250 Australian. So Im seeing if they are still available. But with all the discussions about scaling is it worth doing it on this size cat? And how do I make it so uts a true test, just tak'em out in 20knots and see how they go?
    Im thinking thatd work. But while this will show speed Im still struggling to find the answer of ease of use and load dispersion a knot here nor there while cruising to no time table don't bother me. As passages arw usally planned.
    But boy itd be great ro see them side by side wouldnt it.

     
  3. catsketcher
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    catsketcher Senior Member

    Gday Gus

    One problem with the A frame mast is that it HAS to be heavier than a single mast because of simple engineering. The compression load on a column is determined by the second moment of area of the mast section. If you make the mast section just a little bigger it can take much greater load. This makes it much more efficient to put compression into ONE compression member rather than two. You can't get around this. So the A frame mast will be heavier - read more pitching.

    Also don't get too interested in the leading edge advantages of the headfoils. Every now and then someone will talk of the problems of a sail being behind the mast and getting bad airflow. YET the fastest boats all have small or non existant jibs. Normal mainsails must be pretty fast or racing boats would have big genoas and small mains. Even boats that can't rotate their masts (monos) have small jibs and big mains. Headstay sag and inability to bend makes a headsail less efficient.

    Also the worry about reefing downwind is IMHO overstated. I can reef and unreef my mainsail on a square in 25 knots.

    If you are serious about building a boat then take the advice of those who have built their boats. There is no magic design and every boat will cost you lots - in time and effort. You will almost certainly be unhappy and pissed off with the project at some time. It will take huge amounts of effort and persistence to finish.

    cheers

    Phil
     
  4. hump101
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    hump101 Senior Member

    Just a note on this, the compression load is governed by the cross sectional area of the mast section, the buckling and bending loads are governed by the second moment of area of the mast section and the length between supports. Thus the section can be made sub-critical for buckling by reducing unsupported span, by lateral bracing of some description, and not necessarily by increasing the section size. It's also worth bearing in mind that the compression load in a typical A-frame arrangement is half of that in a stayed single mast due to the doubling of the width at the base.

    Nevertheless, as Phil notes, it will be heavier, around 10% minimum in my calcs, and even more so if my current rig was optimised. However, if compared to a biplane unstayed mast arrangement, which is probably closest in terms of easy handling, then the weight difference is small, the windage similar, and the single larger sail potentially more efficient.
     
  5. UpOnStands
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    UpOnStands Senior Member

    With the A-frame, any problem with the two hulls twisting relative to each other?
     
  6. TANSL
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    TANSL Senior Member

    Maybe I'm misinterpreting things or making a rookie mistake but I always thought that the compressive forces, in the longitudinal direction of the mast, are those that can lead to buckling problems. So I do not understand the phrase I have drawn from your post.
    Where I'm wrong?
     
  7. hump101
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    hump101 Senior Member

    You are correct that the compressive forces cause global buckling collapse, but this is a different mode to compression and my point was that the critical buckling load is not just a function of second moment of area, but also unsupported length, which can be modified using spreaders, stays, etc.

    If you are designing a section like this you would need to check it separately for compression, global buckling, local buckling, and bending, in all directions. Normal design practice is to ensure that buckling collapse is not the critical mode of failure, but without redundancy if any lateral support element fails, then the mast section will then become critical in buckling.
     
  8. TANSL
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    TANSL Senior Member

    Thank you for your explanations.
    I understand that it is a three dimensional problem but the methods, let's call them, "manual" analyze separately compression, bending about the transverse plane and the longitudinal plane. It is a simplification that makes calculation more affordable, and, as the results are unfavorable, it is usually accepted.
    Of course buckling load is not just a function of second moment of area, but also unsupported length, mechanical properties of the material, ... Well, to be more exact, the buckling loads do not depend on these parameters, what is up to them is the buckling strength of the element under consideration.
     
  9. babu
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    babu Junior Member

    FEA study necessary?

    @hump101,TANSL,catsketcher

    Very interesting discussion, thank you for your valuable insights.

    Question: Does the semi-rigid coupling typically found in the masthead (joining the two spars) complicate the dimensioning and load conditions so badly that reasonable calculations from first principle are impossible? (I imagine that for example twisting the heavily interlinked triangle of the two spars and the hull beam may be very complicated to analyze)
     
  10. Gus7119
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    Gus7119 Senior Member

    These are all really valued points. It seems that the cost associated with the callculations listed above for a one of rig of thos sort would be very restrictive in regards final design. I am still thinking a side by side test usimg two boats of the same size and make would provide interesting information. Not data as I wouldnt have the capacity nor know how on how to do that.
    With the rigs that were built as was asked above what effect if any of twisting motion was felt by the hulls if anyone knows? Or where this boats developed around the rig?
    Cheers for the info
    Gus
    Ps hqve built a few boats. But for others with the exception of an NS14 that I built with a mate at 15 over a weekend. We did this because we had a rig but the boat had flowen of a car on the trip back from WA and disintegrated. Took the new one out to test it in a race the following weekend and beat all the foam boats it was a great feeling.
    So I know I get pissed scream and shout at it and sometimes feel like setting a match to it. And Im sure half way through Ill find a boat exactly like the one I want for a give away price. But I have to do it as its time Ill waste my money and cry during the build but when Im on a down wind run in the middle of the Pacific for the third day straight itll all be worth the pain. And I can say dam I built this.
     
  11. hump101
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    hump101 Senior Member

    Personally I wouldn't bother with simplistic calculations for a composite structure like this, much more accurate to produce an FEA model, and for geometries like this the modelling is relatively quick and easy. We do FEA modelling commercially and it is very competitive, cost wise, compared with a thorough job using beam theory, and much more accurate.

    If you do use a more simplistic method you need to be very careful to address all the different buckling modes, not just the obvious ones, as local buckling of a sandwich skin may end up being the most critical (as we've recently seen in the Spindrift2 mast during the JV). The beauty of an FE model is that it can check all modes, local and global, using a non-linear buckling technique rather than the (non-conservative) Euler buckling method.

    It is worth bearing in mind that most comparisons between using FEA and "hand" calculations are considering code-based design, where the code already captures the structural response, and hence the accuracy difference cannot be fully exploited. For a "clean sheet" design like this the classification society rules would not be much assistance, and you would need a much more thorough analysis, so FEA becomes a no-brainer.

    Babu/Gus: The coupling design can be very highly loaded, depending on the staying arrangement. If stayed from four points at the ends of each hull then the rig will be preventing racking and thus will have to resist these loads. Two stays fore and aft to centreline (or a tripod) won't attract any racking load, but you then need some structure on centreline to resist the rig loads.

    The design of the connections would benefit from using FEA, but you can design anything using first principle calculations if you have a full understanding of the loads and load combinations. It would probably take longer than doing an FEA model, and would result in a less optimised solution, but it is possible.
     
  12. Spiv
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    Spiv Ancient Mariner

    Compression only???

    Never heard anybody considering the "bending" forces a main imposes to the mast.
    Remove the diamonds, the spreaders and the lower shrouds, then calculate the section and weight of the mast and then compare it with two masts on an "A" frame and a sail the same size but on a furler, hence compression only. (remember to add the weight of the two main shrouds to the much bigger, fat, single mast).

    I bet there will be less weight in the A frame.

    5y ago, I designed an A framed cat and had some preliminary calculation done by a respected composite engineering firm, did not complete as I ended up buying a production cat.
    I have now sold that and am back thinking.....

    I dislike the tone of some of the responses by people who have nothing better to do with their lives than write countless answer to countless threads. Go away, go somewhere where you are useful please, I already have a couple in my ignore list.:p

    Anyway, I just stumbled on this thread and I would like to suggest that anyone seriously interested in the development of the A frame rig, to get in touch with me by PM and I will organize an invitation only blog in which we can discuss this topic without having to spend countless hours defending our reasoning to the uninterested.
     
  13. brian eiland
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    brian eiland Senior Member

    I might suggest some reading back thru some portions of this subject thread, from which I will excerpt a few quotes:



    Spiv's A-frame rig thoughts....


    There is a good many more postings on that subject thread from which i took these quotes. Its just making this posting of mine too long. So I would suggest you all have a look thru them if you are thinking of an A-frame mast.

    One thing that has always troubled me about A-frame discussions is many people are imagining in their mind that the two legs of the A-frame are only going to be under a compression load, and thus spliting up that load into two legs seems like a good idea.

    But think about it. As an example consider a std single masted sloop rig under sail. The mast is under compression, BUT what is keeping it from being blown over the leeward side. That windward shroud that is under a big tension force. What is going to replace that shroud?....just the lone leeward leg of an A-frame rig? Is it possible that the windward a-frame member goes from compression to tension loading under heavy sailing conditions??

    ....for example consider the calculations noted here:
    I believe the photo posted at these 3 links will show some of that uneven loading of the a-frame masts..

    Here is a VERY interesting A-frame rig
    A-Frame Rig on 'Catbird Suite'
     
  14. brian eiland
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    brian eiland Senior Member


  15. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

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