Outrigger to beam connection

Discussion in 'Multihulls' started by erikhaha, Apr 22, 2012.

  1. erikhaha
    Joined: Sep 2007
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    Location: san clemente, ca

    erikhaha Junior Member

    I have searched this forum and I can seem to find any info about various methods of attachment of a tri outrigger to the main hull.

    I am building a 20ft tri based on Gary Dierking's Ulua Proa.

    I have increased the main hull dimensions by approximately 30% and the 2 outriggers that I have built are each 16ft in length and based on my calculations each outrigger has a 580lb displacement capacity fully submerged.

    I am at the stage that before I can glass the outside of the 1/4" ply ama's I need to design the attachment connection between the main beams and the ama. I am interested in the benefits of a rigid connection versus a connection that will allow for some movement, such as a lashed connection or something in between.

    Since this is a relatively small boat and the forces are not that great I have been thinking that a flexible type of connection would be better than a rigid connection, but I am not sure.

    Any comments and or experience with either would be appreciated.
     
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  2. teamvmg
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    teamvmg Senior Member

    There are 1000s of 16-20ft beach cats sailing, they must have figured out the connection by now, taks a look at some of their solutions.
     
  3. Corley
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    Corley epoxy coated

  4. erikhaha
    Joined: Sep 2007
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    erikhaha Junior Member

    Thanks for the responses but the outriggers that I have built have a lot more capacity then the ama's on the Ulua that you refer to.

    I specified the displacement in my initial post although not in metric units. So for the people posting here from the UK and anywhere else that use the metric system:

    Outrigger displacement = 263kg
    Outrigger length = 4.85meters

    Since the boat the I am building is a trimaran and not a single outrigger, and the sail area is greater then the proposed area for the Ulua's original design, I believe that I need to re-think the connection of the beam to outrigger.

    I am using 3" square x 1/8" thick T6061 Aluminum beams so I can't get the same type of connection as the originally designed Ulua.

    Thanks though.
     
  5. Corley
    Joined: Oct 2009
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    Location: Melbourne, Australia

    Corley epoxy coated

    The method below is very strong I used it on a 18ft off the beach tri the only difference was I built strip plank beams but it was a square section so comparable to what you want to do. Your aluminium beam will have to be long enough to go into nearly the full depth of the float.

    If your using a square ally tube this is what I'd suggest. Build into the ama a double bulkhead. Space them so they are a snug fit forward and to the rear of your beam. Tab, fillet and tape the bulkheads into the float hull. Drill a hold through your beam and bulkheads fore and aft, fit a sleeve into the beam and fit a ply doubler where the pin comes through the bulkheads. If the float is intended to demount make the pin accessible by fitting a spinout plastic hatch forward of the beam/bulkhead area. You will want to fit some sort of rubber section or spray skirt at the beam/deck area.
     
  6. buzzlymutt
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    buzzlymutt Junior Member

    we need more information! Are the beams straight,or curved? Overall beam? Planned height of beam from waterline?Demountable ? I feel 1/4' ply is to heavy for displacement of ama.NO NEED TO GLASS! Did you consult this forum before?
     
  7. erikhaha
    Joined: Sep 2007
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    erikhaha Junior Member

    Corley, thanks for the info. Actually that is the same attachment method that I had already planned on using.

    I posted here because I wanted to get other opinions as to the pro's and cons of a rigidly attached outrigger to the beam versus a more flexible connection.

    Buzzly, why do you think that 1/4" ply is too heavy and that I shouldn't glass them?

    Since I will be using this boat in the ocean I do not want to take any chance of salt water entering into the ply. I am glassing the 2 outriggers with 6oz cloth on the outside and 3 coats of epoxy inside.

    I am interested in hearing your thoughts.

    Attached is a pdf of the design.
     

    Attached Files:

  8. buzzlymutt
    Joined: Jul 2008
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    buzzlymutt Junior Member

    For the displacement of Ama's 4mm{5/32'}ply would be best.One could even use 1/8" in the compounded/tortured method.Weight is the deciding factor, Glass tape the keel only.No more as this will add to an already overweight hull.Its strong enough already.3 coats of epoxy or untill you have an obviously thick covering,no dry patches.Applied wet on wet,each coat in 1/2 hour intervals,or so.Allow a lot of time for this operation.Good luck
     
  9. Ziphius
    Joined: Oct 2014
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    Ziphius New Member

    Simular problem

    Hi!

    My name is Magnus and I'm currently rebuildinging my mono hull to a trimaran here in Sweden.
    Before anyone points it out, I know that the performance wont be great :D
    I'm doing it because I got to kids and they need a different kind of boat than the one i got (Its a Maxi 68). Also, it gives me a hobby.

    I'm currently constructing the outriggers and trying to figure out the dimensions I need to use in the beam to outrigger connection. I'm thinking of using a square pipe (aluminium/steel) bolted into the double bulkhead in the outrigger. That sticking out of the outrigger, I then thread the beam over the part sticking out like a sleeve and bolt it into place. Now for my question.

    I figure that the maximum static load on one "beam-outrigger"-connection is the weight of the boat. However the dynamic load will of course be much greater under the right circumstances. I figure my boat will way about 1000kg (approximately 2200 pounds). What could be the maximum dynamic load on one beam-outrigger-connection? Probably the most forward one.

    I don't know if this is the right forum for it but i ask anyway and hope for the best.

    Cheers / Magnus, Sweden
     
  10. Tom the rower
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    Tom the rower Junior Member

  11. Ziphius
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    Ziphius New Member

    @Tom the rower. Thank you for your very quick answer. The solution you are showing isn't far from the one I had in mind. However, my question was what range of dynamic load i could expect in the beam, in worst case scenario, I already have the basic construction figured out. The problem is what range of force i have to dimension it for.

    Cheers Magnus
     
  12. teamvmg
    Joined: May 2008
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    teamvmg Senior Member

    This guy wishes he had thought this through too.
    He spent 11 hours clinging to his wind gen mast with no floats or rig one night last week
     

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  13. hump101
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    hump101 Senior Member

    Classification rules for a displacement vessel would give a typical dynamic amplification factor of 1.5, but in reality this factor is a function of scale, with smaller vessels have higher DAF's. You need to have confidence in your static figure (1000kg) by calculation, then think about a DAF. If you don't anticipate flying the boat off waves or hitting something, then 2.0 may be adequate, but think about resilience in an accident, and off-axis loading as well.
     
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