Small trimaran ama angle?

Discussion in 'Multihulls' started by smallyachtsailr, Jul 4, 2012.

  1. smallyachtsailr
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    smallyachtsailr Junior Member

    I'm currently designing a 14-foot trimaran--a larger version of my current single-person 10' 4" trimaran. As with the smaller, the new tri will have a centerboard. All hulls will be single-chine plywood construction. I'm going for handiness and a fair amount of speed--for a 14-footer.

    The amas will be suitably canted outward to allow for heel, but I'd like some opinions on whether to position the amas parallel to the main hull, or angled in a degree or two in planform--essential "toe-in". There seems to be two schools of thought on this. Will angling them slightly inward actually help the craft to point a bit better, or is this irrelevant considering I'm putting a centerboard in the main hull? I'm tempted to simply put them parallel to the main hull, since the boat will be drifting leeward while going to windward anyway, which should allow the leeward ama to generate a bit of lift anyway. My old smaller tri has a bit of toe-in and seems to work fine (it also has a main-hull centerboard), but I have no way of really telling if the toe-in actually helps or hurts.

    Opinions?
     
  2. garydierking
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    garydierking Senior Member

    And just to make the question more interesting; sailors of double outriggers in Indonesia (where trimarans were born) swear that they are faster when the bows of the amas are canted out. And if you think about the way the hulls move when making leeway to windward, it does align the ama with the actually direction of movement.
     
  3. RHough
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    RHough Retro Dude

    That makes sense. The ama toed out reduces the angle it moves through the water at. All lateral resistance comes from a relatively efficient dagger board instead of trying to use the leeward ama to share the load.

    R
     
  4. Doug Lord
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    Doug Lord Flight Ready

    Or you could use a gybing daggerboard in the main hull, or a daggerboard in each ama toed in a bit. With the ama angled out at the bow you still have the main hull moving a little sideways. Frankly, I don't think it makes enough difference on a tri to fool with-unless your interested in nth degree racing.
     
  5. smallyachtsailr
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    smallyachtsailr Junior Member

    Interesting bit about the Indonesian outriggers. I often sail in shallow waters (sometimes 0-1 ft!), so in my mind daggerboards are out of the question. I plan on relying on the main hull centerboard for most of my lift to windward, and I'm not planning on (at least formal) racing, so I plan on no boards in the amas. I guess my real question is what angle on the amas would be the best compromise between windward performance and speed.

    RHough's point about most of the windward performance coming from the CB is a good one. Since I had also recently decided that in the interest of buoyancy I was going to go with single chine amas instead of triangular sections, I think I won't try for windward lift with the amas since I probably won't get much from them anyway. I think I'll just put 'em on parallel as a compromise. That way my crossbeams can probably be made identical and I won't have to figure out which goes where each time I assemble the darn thing either!

    Thanks for the comments!
     
  6. Silver Raven
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    Silver Raven Senior Member

    Gooday all. WOW - what a subject. This will get my blood pumping - for sure.

    How about - a serious - in length - in depth - discussion about "a gybing daggerboard in the main hull" - Please oh please.

    I'm not sure I know enough about this subject from a hands-on however very technical point of view - BUT I sure do want to learn.

    I - am - going to make the c/b in the tri - a 'tacking/gybing' c/b' - I'm also going to extensively modify its length (depth), foil shape - cord etc etc & as well - all at the same time - make it 'tack/gybe' to at - what I'd thought would be the 'correct' (ha ha ha) angle of 4 degrees - make it easy to alter from the helm position - when sailing 2 or 3 up rather than having to have a full crew of 5 to 6 on board.

    So - now - - What say you all ??? Thanks for all the info & feed-back I'm going to get. Ciao, james
     
  7. Doug Lord
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    Doug Lord Flight Ready

    =======================
    Gybing boards are usually daggerboards but they don't have to be. The idea is that as you tack the board changes it's angle of incidence relative to the hull from say +2 degrees on port tack to + 2 degrees on stb tack automatically. You can make that work on a centerboard when it is all the way down , probably without affecting its ability to kick up. I don't know of an example off hand but some of the big tris use a variation-like Banque Populaire V, I think(but I'm not positive) that it uses a movable trailing edge flap on the main daggerboard.
    Again, though, probably not worth the trouble on a daysailer.
     
  8. cavalier mk2
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    cavalier mk2 Senior Member

    I'd suggest keeping the amas on centerline but make them to some degree asymmetric..
     
  9. smallyachtsailr
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    smallyachtsailr Junior Member

    Now you've done it...I like the idea of a gybing centerboard AND asymmetric amas. Unfortunately I'm trying to keep the 14' tri very light both to car-top and to keep the power-to-weight ratio up. Asymmetric amas I can do easily. A gybing C/B? That's exactly the kind of excessively gadgetly thing I'd love to put on a little tri--just to impress. I love a challenge--now I know what I'll lie awake thinking about tonight--and the rest of the week. I can hear my wife now..."Why are you staring at that wall??"

    Thanks again for some good ideas.
     
  10. Doug Lord
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    Doug Lord Flight Ready

    If you use a gybing board you don't need asymmetric amas.....
     
  11. RHough
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    RHough Retro Dude

    Just kidding here:

    Gosh, give up the stellar upwind performance of a Hobie 16?

    It seems that asym hulls give about the same upwind performance as a good square rigger ... :D
     
  12. smallyachtsailr
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    smallyachtsailr Junior Member

    Working on the problem of a gybing C/B, but I have to be able to sail with it in pretty thin water, so it also needs to be able to be tucked in partially or mostly some of the time. Of course most of the time it'll be fully extended, so maybe I'll concentrate on that.

    Having been an RC sail racer in the past, I also know the great benefit of a good foil shape on windward performance, so that'll have to be factored into the design. No flat board for me if I can avoid it. Unfortunately it can leave some larger gaps in the bottom of the C/B slot. Yet another problem to be solved.
     
  13. smallyachtsailr
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    smallyachtsailr Junior Member

    Following the other thread on gybing boards, it sounds to me like no matter how you swing it (the C/B), your efficiency upwind is still dependent mainly on rig efficiency and board shape (plan and profile), and that perhaps simply a good foil shape might be the best thing, especially on a tiny (14') tri.

    Lots of interesting discussion...I'll keep my ear to the ground...
     
  14. Doug Lord
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    Doug Lord Flight Ready

    ===============
    Unless you're racing all out( or a techno freak like me)-keep it as simple as possible. All the little complex details pick at your time like termites knawing wood. While they can be fun to work on, maybe too much trouble for daysailing?
     

  15. oldsailor7
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    oldsailor7 Senior Member

    Doug is right.
    At that size it really doesn't matter.
    Use the KISS principle.
    Symmetrical hulls.
    Keep hulls parallel.
    Have 5 deg of dihedral on the akas.
     
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