Aka attachments to Nacra 5.2 hulls as amas

Discussion in 'Multihulls' started by nwguy, Oct 28, 2019.

  1. revintage
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    revintage Senior Member

    I also bought from a trampoline company. As they had a lot of outdated models, I got them almost for free.

    The struts is a good idea as this will give a more constant rig tension, independent of crew placement. But the struts seem placed a little high on the hull sides to be effective. I guess you will not sail with the center hull stern as the water line implies? Curious, how are the struts anchored in the hull? If worried maybe there are some wing formed struts to use. 6061-T6 Aluminum Streamline Tubing On Wicks Aircraft Supply http://aircraftproducts.wicksaircraft.com/viewitems/aircraft-aluminum-metals/6061-t6-aluminum-streamline-tubing

    About the shroud bridle, just wondering what height it is and if you consider it high enough?
     
  2. fallguy
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    fallguy Senior Member

    lurker here

    can't you calculate the loads?

    seems like one ought to be able to determine what the thing is capable of; it is basically a beam; no? It should be really easy to determine the capability; a bit harder to determine real forces
     
  3. nwguy
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    nwguy Junior Member

    Revintage, the waterline at the stern of the vaka is probably too high. In this boat's previous incarnation, the foiling T rudder it had weighed much more than the stock rudder now on it. The struts are bolted to a stainless steel strap that extends through the hull from side to side and is bolted onto a bulkhead I installed. It's plenty strong. The struts are made from 6061 AL pipe. In one prior version I did use faired hang gliding down tubes. The shroud bridle configuration has worked well when it was a hydrofoil boat, so I don't see any need to change that.

    Fallguy, I really have no idea how I'd calculate the loads. I guess the worse case realistically would be me and my sailing pal (about 200 lbs. each) sitting at the far back corner bouncing over a 3 foot swell in 30 knots of wind. I don't plan to ever be in those conditions, but you never know. Hard to say how much lifting support the rear part of the bridle provides. Maybe enough on it's own. Maybe switching to a water stay instead of a strut would suffice.
     
  4. upchurchmr
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    upchurchmr Senior Member

    You will find out how much the rear tube strut hits the water soon enough.
    Don't fix something you don't know is bad.

    In light air you are going to want to be forward on the tramp. That will raise the strut.
    In heavy air the boat is going to heel, raising the tube.
    There is going to be somewhere in between where a wave is going to slap the tube.
    Don't sweat a "problem" you are not sure about.

    Looks good, when are you going to be able to take it out?
    Have they outlawed being at sea by yourself?

    PS: personally I have invented lots of problems that never really materialized!
    And, the ends of the akas are going to hit waves on the leeward side anyway, IMHO.
     
  5. nwguy
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    nwguy Junior Member

    Upchurchmr, you are exactly right. I'm waiting on delivery of some sunbrella fabric and a zipper to make my jib cover, and I need to modify my main cover. Then I'll launch and get it moored to the buoy in front of my house. Hopefully this coming week. People are still sailing around here.

    Most of the time I'll be sailing in our fairly sheltered bay in summer northerlies ranging from 8 to 15 knots.
     
  6. revintage
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    revintage Senior Member

    So what is the height to the joint?
     
  7. upchurchmr
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    upchurchmr Senior Member

    Do you think you will be able to fly the main hull in 15kts of wind?
    You have so much beam, I really wonder. Especially with the crew weight out on the windward hull.
    That must be a lot of righting moment.
    BUT, I don't remember how much sail you have.

    PS: will you be wearing a mask while sailing? :rolleyes:
     
  8. nwguy
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    nwguy Junior Member

    The junction where the strut meets the hull is 9" above the bottom of the hull, and 4" above the scum line on the hull that shows the water line when moored. The bottom paint there is at least 3" higher than it needs to be, and may be even more with the lighter rudder.

    I reconfigured the struts this morning so they use the scraps of faired AL tubing I have. This should have less drag when they do hit swells.

    struts1.jpg struts2.jpg struts3.jpg
     
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  9. nwguy
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    nwguy Junior Member

    Upchurchmr, I don't have rudders on the amas, so I don't ever intend to fly the main hull. With the beam I have (boat is 17' wide), it's hard to imagine that I won't have enough righting moment to keep it heeled/level like I want it to be. I had nice sails made from aramid fiber by Eric Taylor in Sequim, WA that are about 30% bigger than the Solcat 18 sails I originally used. The Solcat's are 175 sq ft (16.3 m2) for the main and 45 sq ft (4.2 m2) for the jib. So my guess without looking at my plans from Eric are about 230 sq ft for the main and 60 sq ft for the jib.
     
  10. upchurchmr
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    upchurchmr Senior Member

    Flying the main hull about 1' would leave lots of rudder in the water.
    At those speeds it would probably be still effective.
    Sounds like fun.
     
  11. fallguy
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    fallguy Senior Member

    You calculate the beam strength; not the load.

    I'd use an unsupported beam calc as a starting point.

    all you need is some measurements of the beam and aka distance to the ama
     
  12. upchurchmr
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    upchurchmr Senior Member

    Actually, calculating the strength of the rear beam will not work at all using a unsupported beam theory.
    The thing is a supported beam, actually a truss.
    Changes the loads in the parts completely.

    NWguy - you are probably not going to get much support from the tubular strut, supporting a downward force (due to your weight).
    The tube is so slender that it is going to buckle at a very low force.
    It will work perfectly for an upward force, like the shroud tension on the windward side, or the righting force on the lee side.
     
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  13. nwguy
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    nwguy Junior Member

    Well, it worked when it was a foil boat. It's pipe, not tubing. The wall thickness is probably 3/16". Pretty rigid pipe.
     
  14. fallguy
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    fallguy Senior Member

    I accept my error, but it is certainly simple to calculate the beam's capability using a proper method.

    And my gut tells me the strut will have nearly zero effect as well. Kindest regards and great luck.
     

  15. upchurchmr
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    upchurchmr Senior Member

    Fallguy, it's not too hard, but you need the load to make it mean anything.

    nwguy, Buckling really depends upon the overall length, and the diameter of the pipe.
    Even the slighest bending will take out most of the capability.
    As you said, it already worked, so making a calculation doesn't mean much.
    The large pipe has such a large cross-section that I bet it would support the body weight, even without the shroud support or the pipe strut.

    Do you have any idea how much the boat weights?
    Is the rear cross beam a single piece?

    I'm done trying to muddy the waters. Looks to me like you are good to go. And I'd like to see it go. Do you think driving from Texas to Washington breaks any social distancing rules????
     
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