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

    Hey nwguy, no curved main sheet track will be needed if you keep the wishbone boom. I like wishbone, will you keep it?
     
    Last edited: Oct 31, 2019
  2. trip the light fandango
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    trip the light fandango Senior Member

    W17 designer talked about any floatation that is already displaced was effectively wasted when it comes to amas/floats , I didn't really understand what he meant but he's a very experienced trimaran builder.
     
  3. revintage
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    revintage Senior Member

    Might be the volume of the amas that are below waterline when the boat is at rest?
     
  4. trip the light fandango
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    trip the light fandango Senior Member

    Yes that is displacement, but surely the submerged area provides support, control and balance with chop and wave action, not just drag. I suppose he's saying the drag cancels out any advantages particularly if outright speed is the goal.
     
  5. upchurchmr
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    upchurchmr Senior Member

    What do you mean by support?
    Isn't that what displacement means in a multihull?

    How is control affected?

    He's saying that when you start sailing - wind in the sails causing an overturning moment - that displacement at rest on an ama does not balance any wind pressure. You have to depress the ama more to get the support against the wind pressure.
     
  6. trip the light fandango
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    trip the light fandango Senior Member

    I'll have trouble explaining this so I shouldn't have mentioned it I suppose, but I'll give it a go, what I mean by support is variation in water surface, the pit and crest of a wave and how the extra hull surface that was displacement reacts to the changing conditions of the surface. Control is affected by the extra area adding a stabilising factor. I understand your description of the support against wind pressure, thank you.
    There is some sacrifice in spilling wind from the initial overturning/tipping/ heeling moment that counters the extra drag /displacement, it may not be much.
     
  7. upchurchmr
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    upchurchmr Senior Member

    I think I understand the situation you describe, but I don't know what difference it makes to giving a benefit of any kind.
    Are you saying a raft does not rock back and forth, sideways? I guess I'd agree if your concern is when sitting at rest, but not if you are actually sailing.

    Control of what?
     
  8. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    I suspect the W17 designer was referring to wasting wetted area on amas at rest rather than using the vaka for buoyancy.
    It's perhaps the simplest explanation and it certainly makes sense... to me anyway...
    As you were, carry-on.
    <smile>
     
  9. trip the light fandango
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    trip the light fandango Senior Member

    I suppose I'm saying simply by having more wetted area there is a stabilising factor, but then in some conditions that is possibly detrimental.
    A raft that lifts one side out of the water then the other would feel like a less stable platform and therefore less in control when it turns nasty..close chop.
    I don't think it is of particular importance to the OP, it's more of a cruising issue.
    Just bye the bye, I would glass a smaller pair of cat end sections[with their saddles] on to the nacra hulls, and you can lower the vaka by packing the saddles.
     
    Last edited: Nov 9, 2019
  10. nwguy
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    nwguy Junior Member

    I'm keeping the wishbone bone. No need for a traveler or vang then. It'll be the 3rd tri I've built using one.

    So I decided to stick with using the forward saddles for the front aka. The height is really close, and I might be able to raise the amas an inch via tension on the shrouds coming from the mast. As someone else mentioned, I can always shift my weight to change the roll angle. Prior to this decision I sanded off the gel coat on the sides of the amas around that saddle. So I laminated various layers of carbon and fiberglass to beef up that area, then faired with fairing compound before repainting the hulls with topside paint, bottom paint and nonskid for the decks. Installed new inspection ports and 4 drink holders using ABS pipe. The drink holders have drain tubes made from fiberglass ski poles that drain out the sides of the amas. :)

    For the rear aka attachments I reinforced the transoms with 2 layers of heavy unidirectional carbon fiber oriented at 90 degrees to each other, then 2 layers of biaxial fiberglass. Then I fabricated vertically adjustable aluminum brackets with 4.5" tubes that the akas will slide into. Need to have my welder friend TIG weld the tubes onto the vertical plates. I used 3/6" aluminum plates on the insides of the transom (see pic through inspection port) to act as big washers. After I determine the optimal pitch angle for the amas I'll trim off the excess aluminum plate so they don't extend down in the water and so they'll be lighter. See pics below.

    IMG_20191118_122919943.jpg

    IMG_20191118_122927129.jpg

    IMG_20191118_122934015.jpg

    IMG_20191118_123536897.jpg

    IMG_20191118_124936986[1].jpg
     
    BlueBell likes this.
  11. BlueBell
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    BlueBell Ahhhhh...

    Yippee-ki-yay!
     
  12. upchurchmr
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    upchurchmr Senior Member

    Great, when's the launch? :)
     
  13. nwguy
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    nwguy Junior Member

    Not sure. Holidays coming. Then I'm off to Baja for a couple of months to windsurf. Maybe before then if I'm lucky. Otherwise March.
     
  14. nwguy
    Joined: Feb 2010
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    nwguy Junior Member

    A friend let me use his industrial sewing machine and I finally got my tramps made and installed. Used mesh from this company:

    Trampoline Fabric Manufacturers and Supplier https://www.trampolineproshop.com/trampoline-fabric.php

    The nice lady who answered the phone said plenty of people have bought their fabric for multihull tramps. Way cheaper than sailrite or seattle fabrics. So cheap that I got enough to do double layer. They attach to the vaka with boltropes, and then to either slug slides in rails or wound around the crossbars at the front, rear and outer sides. I used 1/8" dyneema for my lacing rope. It's got a 2,800 pound breaking strength. Stood on the tramps for the first time today making my new lazy jacks. They're really tight and feel great. I had the choice of getting calendared or uncalendered cloth. After some reading, I got the uncalendered, mainly for the strongest possible cloth since calendering can weaken the cloth. The tramps are huge. Will definitely try to keep the boat level so as not to let much air under there. Sewed some pockets for jib sheets and halyards at the front/inside areas of the tramps. I used the gray plastic, UV-resistant electric conduit that Home Depot sells for the rods in the seamed, folded over edges at the front, rear and outer edges. That conduit is really hard to break, and seems to work fine for tensioning the tramps. All I have now is to finish modifying my mainsail cover and to make foresail covers (I have 2 jibs). Trying to make it as easy as possible to set up and take down and store sails.

    So, question for you who may have thoughts on this. On trimarans I've made in the past that have 2 akas (crossbars), I've used 1x19 stainless steel cable for the waterstays under the front aka, and have used rigid AL pipe for struts under the rear aka. My thinking being, the rear aka is less supported for downward forces than the front aka, since the mast shrouds are anchored closer to the forward aka than the rear. So the compression stress of 2 people sitting at the far, outer back corner may apply downward forces on the aka that aren't handled by the shroud attachment point on the tops. A strut fixes that. However I think the strut may drag in the water, and I'd like to eliminate it if I can. I know, just try it without the strut and see if things warp or twist or god forbid break. Just wondering if I'm out to lunch on this thinking.
    -Jim

    tramps3.jpg tramps2.jpg tramps1.jpg
     

  15. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    You could do the same thing above the beam, it may be useful as a hand hold.
     
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