Catamaran Beam To Hull Attachment

Discussion in 'Boat Design' started by ElGringo, May 11, 2016.

  1. ElGringo
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    ElGringo Senior Member

    I have bought six or eight books and searched this website for information on attaching beams to the hulls. There is just not much written about it. I doubt that I will get to build my cat but, I am having fun trying to work through the different problems as I design it. I like the looks of small 24' to 36' sailing cats but would prefer outboard motors.

    This is what I have come up with to attach the beams. Will it work??
     

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  2. messabout
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    messabout Senior Member

    Those attachment points are high stressed areas. It is conventional to spread the load as much as is feasible. One method is to attach something akin to giant chain plates that are attached to the outer skins of the ama. The load points are at the "enormous chain plates" which places the stress points farthest from the center of the ama.

    The separate amas set up a complex set of forces that include wrenching loads in both pitch and roll sometimes simultaneously with some lateral twist thrown in for good measure. The attachment methods are therefore a matter of serious concern.

    Maybe our resident cat designer, Richard Woods, will weigh in here with more authoritative information.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    It is hard to tell from your sketch, but i assume that is a plan view on a single beam?

    You need to identify the load path..in this case it is all vertical. Thus the webs of your beam are the members that transfer the load. Therefore proper attachment of the webs to the hulls frame/WTBs and also to the inboard and outboard sides of the hull are the areas you need to address.

    The odd looking bits sticking out that you show, will fail quickly. It creates out of plane bending on short members rather than the pure vertical load path you need.
     
  4. ElGringo
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    ElGringo Senior Member

    Those are blocks I would glue to the beams and bulkheads to lock the beam in position. Two straps would hold it down. The only good picture I could find was on Woods website showing a beam joined in about the same way without the blocks. It was on his Shadow model boat built by one of his customers.

    So, if I remove the blocks and just clamp the beam down it would be better? I don't want to glue it because I would need to take it apart to transport it.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    So you want a removable beam?

    You'll need to provide dwgs and/or much better sketches of your arrangement to provide more qualitative replies.

    You can easily bolt it together but, it depends whether you have sufficient load paths and the structure to take the loads on the beam and the hull too. Anything is possible...but you need to design it to behave that way rather than just hope and guess.
     
  6. ElGringo
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    ElGringo Senior Member

    What I want is a 24' long X 12' wide, two hulls, beams and a deck. Like a wide pontoon boat but, I don't want a pontoon that takes a 150 hp to push it. I'm thinking more like a pair of 20 hp outboards. I know the basic shape of the hulls that I want from looking at hundreds of pictures and websites. I think 3/8 plywood for the bulkheads, 1 X 2 stringers, 1/4" skin, and a deck of possibly 1" sheet foam with 1/4" plywood on each side. I am an average carpenter, a good guitar maker, and have used many gallons and rolls of fiberglass. After the motors and fuel are installed I would need no more than 1500 to 2000 lbs load capacity. Here are some sketches I have done as I thought of how to build it.
     

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  7. ElGringo
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    ElGringo Senior Member

    I want to bump this up in hopes that Ad Hoc finds it.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    You haven't answered the question....you want to have removable beams? Answering Qs then you get replies. Not answering leads to confusion and misinformation.

    From your child like sketch it perhaps appears so. Thus, on THAT assumption:

    Box-Beam.jpg

    Just glass in thick webs either side of your box beam. Then have a complaint frame that matches the thickness, with equally glassed up webs and then through bolt them.

    Make sure you do the calc's to work out the required bolt size, torque for the bolts, their min spacing and glass thickness for the beams and frame webs to transfer the load adequately etc.
     
  9. Richard Woods
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    Richard Woods Woods Designs

    Catamaran beam design is highly complicated subject, akin to monohull keel design. So it would take far to long to explain it all here

    I assume you have read this first lesson simple primer

    http://sailingcatamarans.com/index.php/faqs/15-general-questions/88-crossbeam-design

    Things to consider include:

    How identical will the two hulls be when you build them. How many people will assemble the boat. Will it be in a boatyard with help or on a beach. What material will the beams and hull be. How long a life (rot, rust, corrosion), speed of assembly. Flexibility of structure - will it squeak when underway? what trailer will you use. And many more

    I do wonder whether you might be better buying a set of proven designs from your favourite designer so you have a basis that you know works even if you then modify them

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  10. John Perry
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    John Perry Senior Member

    El Gringo, looking at your first sketch is seems that you propose to use four bolts at each hull to beam connection, the axis of each bolt being aligned fore and aft. The main loads on these bolted connections will act vertically rather then fore and aft or transverse, this is an assumption that is generally true of multihull structures, although it should not be assumed totally without question.

    So, the chances are that your four bolts at each hull/beam connection will be carrying mainly shear load - i.e. load tending to slice the bolt across its axis rather than pull it apart lengthwise. It is a general rule, albeit a rule that quite often has to be overridden, that heavily loaded bolted connections are best arranged so that the bolts are in tension when loaded - i.e. stretched lengthwise. I could list quite a few reasons for this, but a primary one is that with the bolt in tension the maximum load each bolt can safely carry is a value available from a reputable manufacturer of screw fasteners. With a bolt in shear arrangement the load each bolt can safely carry is not only lower than with the same bolt in tension but is also rather unpredictable being dependant on the torque applied to the bolt on assembly, the coefficients of friction between mating parts, clearances between bolts and holes etc. Coefficients of friction are notoriously unpredictable - they always seem to be high when you want things to slide over each other and low when you want things to stay in place!

    I think that if you look at existing multihulls which have bolts connecting hulls to crossbeams you will see that the bolts are usually aligned vertically, not horizontally as your sketch, although I am sure there are quite a few exceptions to this. Where the cross beam is simply a length of tube, you may see two vertical bolts at the end of each beam, each of these bolts passing transversely through the tube and into some kind of anchorage at one of the hull gunwhales. Another common arrangement is to have four vertical bolts, each pair of bolts clamping down a saddle that passes over the cross beam. And I did see a catamaran designed by the multihull designer who contributed the previous reply to this thread where each pair of bolts and the saddle were replaced by just a length of studding bent into a U bolt shape.
     
  11. ElGringo
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    ElGringo Senior Member

    Ad Hoc, I'm sorry, I didn't mean to upset you. I thought I had answered the question in post #4 when I said I would need to take it apart to transport it. It would be wider than the 8.5 feet allowed by law without special permit

    I will try to do better on my sketches
     
  12. ElGringo
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    ElGringo Senior Member

    Richard Woods, Yes I have read almost every word on your website and have studied every picture. In post #6 I said what materials I plan to use. The shape of the hulls is a composite similar to your Skoota, the Mertens, I think, plans found on boatbuildercentral.com, and several other websites. I measured the angles and scaled the photos, used what ideas worked for me and borrowed from other sources as well.

    None of the available plans meet what I want. If you, or the others, had plans for just the hulls and beams, I probably would have bought them.
     
  13. ElGringo
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    ElGringo Senior Member

    John Perry, I will post a better sketch of how I planed to do it. I spent 35 of my working years as an aircraft toolmaker so I do understand shear and tension. What I don't understand is why there is so little usable information available on beams and hulls for catamarans. I also seem to have the ability to make people mad when I ask questions and I know that to those of you who have spent a lifetime designing and working on boats that my questions, and sketches are child like.

    I'm thinking that it might be easier to just build some box beams bolt them on, wear a life jacket, and go for it. But, I have always wanted to know why things work the way they do. So, I'll keep trying to find the answers.
     
  14. ElGringo
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    ElGringo Senior Member

    John Perry, here is the end view of the beam without the blocks that I drew on the first one. There is a line on the sketch running parallel with the bulkhead that was added by the scanner
     

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  15. John Perry
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    John Perry Senior Member

    El Gringo - OK, so the bolts are vertical, not as I first imagined. Presumably each beam to hull connection has four bolts, two of which appear in your section sketch. This arrangement looks fine to me, in principle anyway - so what is the problem?

    For selecting the bolts and designing the other parts remember that the beam will be loaded mainly in bending. Contrary to what some people think, multihull cross beams are not generally subjected to high torsional loading. Consider the simple case of a catamaran with just two cross beams and a stayed sloop rig. A severe load case occurs when the boat is close to a diagonal capsize with high rig forces and the lee bow digging into a wave. The structure is then being twisted, the lee hull trying to pitch bow upwards due to the bouyancy force on the deeply immersed lee bow and the windward hull pitching stern upwards due to the tension in whatever stays support the rig - i.e. the windward aft swept shroud and/or backstay if it is a typical multihull sloop. At the same time that this is happening, the mast compression is bending whichever cross beam(s) supports the mast base. A somewhat similar situation could actually occur on dry land if a boat yard chocks the hulls up unevenly, or if the end of each hull is supported on one of four chocks, then one of the four gives way, although in that case the mast compression would probably be less significant. Considering such twisting situations, if the hulls and cross beams had comparable torsional stiffness then both the hulls and cross beams would carry comparable torsional moment. However, because cross beams are usually much smaller cross section than hulls, it is likely that the torsional stiffness of the hulls will be very much greater than the torsional stiffness of the cross beams. For that reason the hulls are forced to carry much larger torsional moment than is carried by the cross beams but the cross beams are required to transmit that large torsional moment onto the hulls which means that they are subject to significant bending, this is in addition to any bending caused by mast compression load. So, I would suggest that a safe way to design the bolting arrangement shown in your new sketch is to assume that this arrangement should be able to transmit onto the hull the full bending moment that the cross beam is capable of withstanding.

    There are a few other load cases that perhaps should not be ignored. Just as one example, suppose the boat is sailing full speed and one hull snags on a mooring chain or something - that loads the structure in a completely different way to the normal sailing situation and is I think the reason that if you look under the trampolines of some of the Farrier trimarans you see a pair of steel cables diagonally accross the rectangle bounded by hulls and cross beams.
     
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