Catamaran Beam To Hull Attachment

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

  1. ElGringo
    Joined: Mar 2014
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    ElGringo Senior Member

    John Perry, I think the only problem was my poor sketch, and not showing an end view. If you look at the 1st post where I tried to show the blocks on the beam and in the pocket where the beam sets, I was wondering if this would work. I would be putting it together by myself and if I did that to the 1st or fore and the last beam I could drop them in and it would hold the hulls in position while I installed the others. So, a yes or no on the blocks would be helpful.
     
  2. John Perry
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    John Perry Senior Member

    Ok, so what I think you are proposing is sets of 'teeth' on each side of each cross beam that 'mesh' with matching 'teeth' on the sides of the pockets which the beams drop into. Then when the beam drops into the pocket the teeth engage and ensure that the hull spacing is correct - have I got that right?

    I dont see anything wrong with that in principle, should work OK although I am sure there are plenty of alternative ways that you could achieve the correct hull spacing.

    From your first sketch it seems that you have two 'teeth' on each side of each cross beam and these engage three teeth on each side of each pocket. That does seem more compicated than it need be - why not just one tooth on one side of each cross beam dropping into a corresponding slot in the side of the pocket. Maybe it would help to have the tooth and mating slot tapered so that the hulls are forced into the correct spacing as the beam is pushed down into the pocket. .
     
  3. ElGringo
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    ElGringo Senior Member

    Yes, that is what I tried to sketch, two teeth engaging three and I do think you are right. One engaging two would be easier to get engaged and the taper would help as well
    You have been a great help and I do thank you for taking the time to get me going. The attachment has been bothering me for a long time.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    No, not at all. You merely didn’t confirm the question asked post #4. Engineering/design is not about guessing, it is about facts. Hence the Q, otherwise everyone is guessing on guesses!

    A strap across the beam is not ideal at all. And the reasons are shown below:

    Box-Beam-Bolted joint.jpg

    In image no.1 the beam is experiencing a vertical load on the port side hull. This puts the strap into bending and shear. Thus, this needs to be very stiff and with plenty of shear strength. Since when this strap bends this shall affect the bolts as it will try and shear off the heads as well as putting the bolts into bending. (Scale exaggerated for reference/illustrative purposes).

    However, when the port side hull is being loaded, so must the stbd side to remain in equilibrium, image no.2 As you can see what is then preventing the box beam from displacing vertically downwards….no bolts, just the glue and lower strap. Weak joint.

    Whereas when the arrangement is as per no.3, the bolts are constantly loaded whether vertically up or down and transfers the load via shear. The sides of the box beam are easy to increase in thickness locally to allow the transfer as would be the frame/WTB that it is attached too as well. Since the box and its associated structure is sufficiently stiff to prevent localised bending, ergo shear only.

    For such an arrangement one of course would only buy bolts from a reputable source, not Joe Bloggs down the road trying to be cheap. And you ensure you have a good factor of safety to allow for unknowns. Simple.
     

  5. ElGringo
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    ElGringo Senior Member

    Ad Hoc, Thank You. That was a very good explanation and I now understand what I need to do.
     
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